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Structural Engineer’s Pocket Book

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Structural Engineer’s Pocket Book
Fiona Cobb

AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO

Elsevier Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 200 Wheeler Rd, Burlington, MA 01803 First published 2004 Copyright ª 2004, Fiona Cobb. All rights reserved The right of Fiona Cobb to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988 No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publisher Permissions may be sought directly from Elsevier’s Science and Technology Rights Department in Oxford, UK: phone: (þ44) (0) 1865 843830; fax: (þ44) (0) 1865 853333; e-mail: permissions@elsevier.co.uk. You may also complete your request on-line via the Elsevier homepage (http://www.elsevier.com), by selecting ‘Customer Support’ and then ‘Obtaining Permissions’ British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress ISBN 0 7506 5638 7

For information on all Elsevier Butterworth-Heinemann publications visit our website at http://books.elsevier.com Typeset by Integra Software Services Pvt. Ltd, Pondicherry, India www.integra-india.com Printed and bound in Great Britain

Contents
Preface Acknowledgements 1 General Information Metric system Typical metric units for UK structural engineering Imperial units Conversion factors Measurement of angles Construction documentation and procurement Drawing conventions Common arrangement of work sections Summary of ACE conditions of engagement 2 Statutory Authorities and Permissions Planning Building regulations and standards Listed buildings Conservation areas and Tree preservation orders Archaeology and ancient monuments Party Wall etc. Act CDM 3 Design Data Design data checklist Structural form, stability and robustness Structural movement joints Fire resistance periods for structural elements Typical building tolerances Historical use of building materials Typical weights of building materials Minimum imposed floor loads Typical unit floor and roof loadings Wind loading Barrier and handrail loadings ix xi 1 2 3 4 5 6 8 10 11

13 14 17 18 19 21 24

25 26 29 30 31 32 34 38 41 43 44

vi

Contents

Selection of materials Selection of floor construction Transportation Temporary works toolkit 4 Basic and Shortcut Tools for Structural Analysis Load factors and limit states Geometric section properties Parallel axis theorem and Composite sections Material properties Coefficients of linear thermal expansion Coefficients of friction Sign conventions Beam bending theory Deflection limits Beam bending and deflection formulae Clapeyron’s equations of three moments Continuous beam bending formulae Struts Rigid frames under lateral loads Plates Torsion Taut wires, cables and chains Vibration 5 Geotechnics Geotechnics Selection of foundations and retaining walls Site investigation Soil classification Typical soil properties Preliminary sizing Trees and shallow foundations Contamined land 6 Timber and Plywood Timber Timber section sizes Laminated timber products Durability and fire resistance Preliminary sizing of timber elements

46 47 48 52

55 56 60 61 64 65 66 67 68 69 76 78 79 81 84 88 89 91

92 93 94 95 96 100 109 113

117 119 120 122 125

Contents

vii

Timber design to BS 5268 Timber joints 7 Masonry Masonry Geometry and arrangement Durability and fire resistance Preliminary sizing of masonry elements Masonry design to BS 5628 Masonry design to CP111 Lintel design to BS 5977 Masonry accessories 8 Reinforced Concrete Reinforced concrete Concrete mixes Durability and fire resistance Preliminary sizing of concrete elements Reinforcement Concrete design to BS 8110 Reinforcement bar bending to BS 8666 Reinforcement estimates 9 Structural Steel Structural steel Mild steel section sizes and tolerances Slenderness Durability and fire resistance Preliminary sizing of steel elements Steel design to BS 5950 Steel design to BS 449 Stainless steel to BS 5950 10 Composite Steel and Concrete Composite steel and concrete Preliminary sizing of composite elements Composite design to BS 5950 11 Structural Glass Structural glass Typical glass section sizes and thicknesses Durability and fire resistance Typical glass sizes for common applications Structural glass design Connections

127 135

141 143 147 148 152 166 168 170

175 177 179 180 182 185 205 207

208 210 239 242 246 249 261 269

275 277 281

284 287 288 289 291 293

viii

Contents

12

Building Elements, Materials, Fixings and Fastenings Waterproofing Basement waterproofing Screeds Precast concrete hollowcore slabs Bi-metallic corrosion Structural adhesives Fixings and fastenings Cold weather working Effect of fire on construction materials Aluminium Useful Mathematics

295 296 299 300 301 302 304 307 308 310 314 320 331 336 339

13

Useful Addresses Further Reading Sources Index

Preface
As a student or graduate engineer it is difficult to source basic design data. Having been unable to find a compact book containing this information, I decided to compile my own after seeing a pocket book for architects. I realised that a Structural Engineer’s Pocket Book might be useful for other engineers and construction industry professionals. My aim has been to gather useful facts and figures for use in preliminary design in the office, on site or in the IStructE Part 3 exam, based on UK conventions. The book is not intended as a textbook; there are no worked examples and the information is not prescriptive. Design methods from British Standards have been included and summarized, but obviously these are not the only way of proving structural adequacy. Preliminary sizing and shortcuts are intended to give the engineer a ’feel’ for the structure before beginning design calculations. All of the data should be used in context, using engineering judgement and current good practice. Where no reference is given, the information has been compiled from several different sources. Despite my best efforts, there may be some errors and omissions. I would be interested to receive any comments, corrections or suggestions on the content of the book by email at sepb@inmyopinion.co.uk. Obviously, it has been difficult to decide what information can be included and still keep the book a compact size. Therefore any proposals for additional material should be accompanied by a proposal for an omission of roughly the same size – the reader should then appreciate the many dilemmas that I have had during the preparation of the book! If there is an opportunity for a second edition, I will attempt to accommodate any suggestions which are sent to me and I hope that you find the Structural Engineer’s Pocket Book useful. Fiona Cobb

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Acknowledgements
Thanks to the following people and organizations: Price & Myers for giving me varied and interesting work, without which this book would not have been possible! Paul Batty, David Derby, Sarah Fawcus, Step Haiselden, Simon Jewell, Chris Morrisey, Mark Peldmanis, Sam Price, Helen Remordina, Harry Stocks and Paul Toplis for their comments and help reviewing chapters. Colin Ferguson, Derek Fordyce, Phil Gee, Alex Hollingsworth, Paul Johnson, Deri Jones, Robert Myers, Dave Rayment and Andy Toohey for their help, ideas, support, advice and/or inspiration at various points in the preparation of the book. Renata Corbani, Rebecca Rue and Sarah Hunt at Elsevier. The technical and marketing representatives of the organizations mentioned in the book. Last but not least, thanks to Jim Cobb, Elaine Cobb, Iain Chapman for his support and the loan of his computer and Jean Cobb for her help with typing and proof reading.

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1
General Information
Metric system
The most universal system of measurement is the International System of Units, referred to as SI, which is an absolute system of measurement based upon the fundamental quantities of mass, length and time, independent of where the measurements are made. This means that while mass remains constant, the unit of force (newton) will vary with location. The acceleration due to gravity on earth is 9.81 m/s2. The system uses the following basic units: Length Time Luminous intensity Quantity/substance Mass Temperature Unit of plane angle m s cd mol kg K rad metre second candela mole (6.02  1023 particles of (Avogadro's number)) kilogram kelvin (0 C ˆ 273 K) radian

substance

The most commonly used prefixes in engineering are: giga mega kilo centi milli micro nano G M k c m m n 1 000 000 000 1 000 000 1000 0.01 0.001 0.000001 0.000000001 1 Â 109 1 Â 106 1 Â 103 1 Â 10À2 1 Â 10À3 1 Â 10À6 1 Â 10À9

The base units and the prefixes listed above, imply a system of supplementary units which forms the convention for noting SI measurements, such as the pascal for measuring pressure where 1 Pa ˆ 1 N/m2 and 1 MPa ˆ 1 N/mm2.

2

Structural Engineer's Pocket Book

Typical metric units for UK structural engineering
Mass of material Density of material Bulk density Weight /force/point load Bending moment Load per unit length Distributed load Wind loading Earth pressure Stress Modulus of elasticity Deflection Span or height Floor area Volume of material Reinforcement spacing Reinforcement area Section dimensions Moment of inertia Section modulus Section area Radius of gyration kg kg/m3 kN/m3 kN kNm kN/m kN/m2 kN/m2 kN/m2 N/mm2 kN/mm2 mm m m2 m3 mm mm2 or mm2/m mm cm4 or mm4 cm3 or mm3 cm2 or mm2 cm or mm

General Information

3

Imperial units
In the British Imperial System the unit of force (pound) is defined as the weight of a certain mass which remains constant, independent of the gravitational force. This is the opposite of the assumptions used in the metric system where it is the mass of a body which remains constant. The acceleration due to gravity is 32.2 ft/s2, but this is rarely needed. While on the surface it appears that the UK building industry is using metric units, the majority of structural elements are produced to traditional Imperial dimensions which are simply quoted in metric. The standard units are: Length 1 mile 1 furlong 1 yard (yd) 1 foot (ft) 1 inch (in) Area 1 sq. mile 1 acre 1 sq. yd 1 sq. ft 1 sq. in ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ 1760 yards 220 yards 3 feet 12 inches 1/12 foot 640 acres 4840 sq. yd 9 sq. ft 144 sq. in 1/144 sq. ft ˆ ˆ ˆ ˆ ˆ 2240 pounds 112 pounds 14 pounds 16 ounces 1/16 pound

Weight 1 ton 1 hundredweight (cwt) 1 stone 1 pound (lb) 1 ounce Capacity 1 bushel 1 gallon 1 quart 1 pint 1 fl. oz Volume 1 cubic yard 1 cubic foot 1 cubic inch ˆ ˆ ˆ ˆ ˆ 8 gallons 4 quarts 2 pints 1/2 quart 1/20 pint

ˆ 27 cubic feet ˆ 1/27 cubic yards ˆ 1/1728 cubic feet ˆ 6080 feet ˆ 600 feet ˆ 6 feet

Nautical measure 1 nautical mile 1 cable 1 fathom

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Structural Engineer's Pocket Book

Conversion factors
Given the dual use of SI and British Imperial Units in the UK construction industry, quick and easy conversion between the two systems is essential. A selection of useful conversion factors are: Mass Length 1 kg 1 tonne 1 mm 1m 1m 1 mm2 1 m2 1 m2 1 mm3 1 m3 1 m3 ˆ 2.205 lb ˆ 0.9842 tons ˆ 0.03937 in ˆ 3.281 ft ˆ 1.094 yd 1 lb 1 ton 1 in 1 ft 1 yd 1 in2 1 ft2 1 yd2 1 in3 1 ft3 1 yd3 1 lb/ft3 1 ton/yd3 1 lbf 1 tonf 1 lbf/in2 1 tonf/in2 1 lbf/ft2 1 tonf/ft2 1 lbf/ft 1 tonf/ft 1 lbf ft 1 lbf/in2 1 lbf/in2 1 in3 1 in3 1 in4 1 in4 y F ˆ 0.4536 kg ˆ 1.016 tonnes ˆ 25.4 mm ˆ 0.3048 m ˆ 0.9144 m

Area

Volume

ˆ 0.00153 in2 ˆ 10.764 ft2 ˆ 1.196 yd2

Density Force Stress and pressure

1 kg/m3 ˆ 0.06242 lb/ft3 1 tonne/m3 ˆ 0.7524 ton/yd3 1N 1 kN 1 N/mm2 1 N/mm2 1 N/m2 1 kN/m2 1 kN/m 1 kN/m 1 Nm 1 N/mm2 1 kN/mm2 ˆ 0.2248 lbf ˆ 0.1004 tonf ˆ ˆ ˆ ˆ

ˆ 0.000061 in3 ˆ 35.32 ft3 ˆ 1.308 yd3

ˆ 645.2 mm2 ˆ 0.0929 m2 ˆ 0.8361 m2

ˆ 16 390 mm3 ˆ 0.0283 m3 ˆ 0.7646 m3

ˆ 16.02 kg/m3 ˆ 1.329 tonne/m3 ˆ 4.448 N ˆ 9.964 kN ˆ ˆ ˆ ˆ

145 lbf/in2 0.0647 tonf/in2 0.0208 lbf/ft2 0.0093 tonf/ft2

0.0068 N/mm2 15.44 N/mm2 47.88 N/m2 107.3 kN/m2

Line loading Moment Modulus of elasticity

ˆ 68.53 lbf/ft ˆ 0.03059 tonf/ft ˆ 145 lbf/in2 ˆ 0.7376 lbf ft

ˆ 0.0146 kN/m ˆ 32.69 kN/m ˆ 6.8  10À3 N/mm2 ˆ 6.8  10À6 kN/mm2 ˆ 16 390 mm3 ˆ 16.39 cm3 ˆ 1.356 Nm

ˆ 145 032 lbf/in2 ˆ 61.01  10À6 in3 ˆ 61.01  10À3 in3

Section modulus Second moment of area Temperature

1 mm3 1 cm3 1 mm4 1 cm4 x C

ˆ 2.403  10À6 in4 ˆ 2.403  10À 2 in4 ˆ [(1.8x ‡ 32)] F

ˆ 416 200 mm4 ˆ 41.62 cm4 ˆ [( y À 32)/1.8] C

General Information

5

Measurement of angles
There are two systems for the measurement of angles commonly used in the UK.

English system
The English or sexagesimal system which is universal: 1 right angle ˆ 90 (degrees) 1 (degree) ˆ 60H (minutes) 1H (minute) ˆ 60HH (seconds)

International system
Commonly used for the measurement of plane angles in mechanics and mathematics, the radian is a constant angular measurement equal to the angle subtended at the centre of any circle, by an arc equal in length to the radius of the circle. p radians 1 radian ˆ 180 (degrees) ˆ 180o 180o ˆ ˆ 57o 17H 44HH  3:1416

Equivalent angles in degrees and radians and trigonometric ratios
 6 30 1 2 p 3 2 1 p 3  4 45 1 p 2 1 p 2 1  3 60 p 3 2 1 2 p 3  2 90 1 0 I

Angle y in radians Angle y in degrees sin y cos y tan y

0 0 0 1 0

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Structural Engineer's Pocket Book

Construction documentation and procurement
Construction documentation
The members of the design team each produce drawings, specifications and schedules which explain their designs to the contractor. The drawings set out in visual form how the design is to look and how it is to be put together. The specification describes the design requirements for the materials and workmanship, and additional schedules set out sizes and co-ordination information not already covered in the drawings or specification. The quantity surveyor uses all of these documents to prepare bills of quantities, which are used to help break down the cost of the work. The drawings, specifications, schedules and bills of quantities form the tender documentation. `Tender' is when the bills and design information are sent out to contractors for their proposed prices and construction programmes. `Procurement' simply means the method by which the contractor is to be chosen and employed, and how the building contract is managed.

Traditional procurement
Once the design is complete, tender documentation is prepared and sent out to the selected contractors (three to six depending on how large the project is) who are normally only given a month to absorb all the information and return a price for the work. Typically, a main contractor manages the work on site and has no labour of his own. The main contractor gets prices for the work from subcontractors and adds profit and preliminaries before returning the tenders to the design team. The client has the option to choose any of the tenderers, but the selection in the UK is normally on the basis of the lowest price. The client will be in contract with the main contractor, who in turn is in contract with the subcontractors. The architect normally acts as the contract administrator for the client. The tender process is sometimes split to overlap part of the design phase with a first stage tender and to achieve a quicker start on site than with a conventional tender process.

General Information

7

Construction management
Towards the end of the design process, the client employs a management contractor to oversee the construction. The management contractor takes the tender documentation, splits the information into packages and chooses trade contractors (a different name for a subcontractor) to tender for the work. The main differences between construction management and traditional procurement are that the design team can choose which trade contractors are asked to price and the trade contractors are directly contracted to the client. While this type of contractual arrangement can work well for straightforward buildings it is not ideal for refurbishment or very complex jobs where it is not easy to split the job into simple `trade packages'.

Design and Build
This procurement route is preferred by clients who want cost security and it is generally used for projects which have economy, rather than quality of design, as the key requirement. There are two versions of Design and Build. This first is for the design team to work for the client up to the tender stage, before being `novated' to work for the main contractor. (A variant of this is a fixed sum contract where the design team remain employed by the client, but the cost of the work is fixed.) The second method is when the client tenders the project to a number of consortia on an outline description and specification. A consortium is typically led by a main contractor who has employed a design team. This typically means that the main contractor has much more control over the construction details than with other procurement routes.

Partnering
Partnering is difficult to define, and can take many different forms, but often means that the contractor is paid to be included as a member of the design team, where the client has set a realistic programme and budget for the size and quality of the building required. Partnering generally works best for teams who have worked together before, where the team members are all selected on the basis of recommendation and past performance. Ideally the contractor can bring his experience in co-ordinating and programming construction operations to advise the rest of the team on choice of materials and construction methods. Normally detailing advice can be more difficult as main contractors tend to rely on their subcontractors for the fine detail. The actual contractual arrangement can be as any of those previously mentioned and sometimes the main contractor will share the risk of costs increases with the client on the basis that they can take a share of any cost savings.

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Structural Engineer's Pocket Book

Drawing conventions
Drawing conventions provide a common language so that those working in the construction industry can read the technical content of the drawings. It is important for everyone to use the same drawing conventions, to ensure clear communication. Construction industry drawing conventions are covered by BS EN ISO 7519 which takes over from the withdrawn BS 1192 and BS 308. A drawing can be put to its best use if the projections/views are carefully chosen to show the most information with the maximum clarity. Most views in construction drawings are drawn orthographically (drawings in two dimensions), but isometric (30 ) and axonometric (45 ) projections should not be forgotten when dealing with complicated details. Typically drawings are split into: location, assembly and component. These might be contained on only one drawing for a small job. Drawing issue sheets should log issue dates, drawing revisions and reasons for the issue. Appropriate scales need to be picked for the different type of drawings: Location/site plans ± Used to show site plans, site levels, roads layouts, etc. Typical scales: 1:200, 1:500 and up to 1:2500 if the project demands. General arrangement (GA) ± Typically plans, sections and elevations set out as orthographic projections (i.e. views on a plane surface). The practical minimum for tender or construction drawings is usually 1:50, but 1:20 can also be used for more complicated plans and sections. Details ± Used to show the construction details referenced on the plans to show how individual elements or assemblies fit together. Typical scales: 1:20, 1:10, 1:5, 1:2 or 1:1 Structural drawings should contain enough dimensional and level information to allow detailing and construction of the structure. For small jobs or early in the design process, `wobbly line' hand drawings can be used to illustrate designs to the design team and the contractor. The illustrations in this book show the type of freehand scale drawings which can be done using different line thicknesses and without using a ruler. These sorts of sketches can be quicker to produce and easier to understand than computer drawn information, especially in the preliminary stages of design.

General Information

9

Line thicknesses cut section/slab edge/element to be highlighted elevations/infill details demolished structure under/hidden gridline/centre line outline of boundaries/adjacent parts limit of partially viewed element/cut-backline not at intersection breakline straight + tube

Hatching

Existing brickwork

New brickwork

New blockwork

Stonework

Concrete

Sawn softwood

Hardwood

Insulation

Subsoil

Hardcore

Mortar/ screed/ plaster

Plywood

Glass

Steel

Damp proof course or membrane

Steps, ramps and slopes

Stairs

Ramp

Landscape slope

Slope/pitch

Arrow indicates ‘up’

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Structural Engineer's Pocket Book

Common arrangement of work sections
The Common Arrangement of Work Sections for Building Work (CAWS) is intended to provide a standard for the production of specifications and bills of quantities for building projects, so that the work can be divided up more easily for costing and for distribution to subcontractors. The full document is very extensive, with sections to cover all aspects of the building work including: the contract, structure, fittings, finishes, landscaping and mechanical and electrical services. The following sections are extracts from CAWS to summarize the sections most commonly used by structural engineers:

A Preliminaries/ general conditions C Existing site/ buildings/ services

A1 A3 C1 C3 C5

The project generally Employer's requirements Demolition Alteration ± support Repairing/renovating metal/timber Investigation/ stabilization/ dewatering Piling Underpinning In situ concrete Reinforcement Precast concrete large units Brick/block walling Masonry accessories Structural/carcassing metal Metal/timber decking Drainage

A2 A4 C2 C4

The contract Contractor's general costs Alteration ± composite items Repairing/renovating concrete/masonry

D Groundwork

D1

D2

Excavation/filling

D3 D5 E In situ concrete/large precast concrete F Masonry G Structural/ carcassing in metal or timber R Disposal systems E1 E3 E5 F1 F3 G1 G3 R1

D4 E2 E4 E6 F2 G2

Diaphragm walling Formwork In situ concrete sundries Composite construction Stone walling Structural/carcassing timber

R2

Sewerage

There is a very long list of further subheadings which can be used to cover sections in more detail (e.g. F10 is specifically for Brick/block walling). However, the list is too extensive to be included here. Source: CPIC (1998).

General Information

11

Summary of ACE conditions of engagement
The Association of Consulting Engineers (ACE) represents the consulting sector of the engineering profession in the UK. The ACE Conditions of Engagement, Agreement B(1), 3rd Edition (2002) is used where the engineer is appointed directly to the client and works with an architect who is the lead consultant or contract administrator. A summary of the Normal Services from Agreement B(1) is given below with references to the lettered work stages (A±L) defined by the Royal Institute of British Architects (RIBA).

Feasibility Work Stage A Appraisal Identification of client requirements and development constraints by the Lead Consultant, with an initial appraisal to allow the client to decide whether to proceed and to select the probable procurement method. Confirmation of key requirements and constraints for or by the client, including any topographical, historical or contamination constraints on the proposals. Consider the effect of public utilities and transport links for construction and post construction periods on the project. Prepare a site investigation desk study and if necessary bring the full site investigation forward from Stage C. Identify the Project Brief, establish design team working relationships and lines of communication and discuss with the client any requirements for site staff or resident engineer. Collaborate on the design with the design team and prepare a stage report if requested by the client or lead consultant.

Stage B

Strategic briefing

Pre-construction phase Stage C Outline proposals Visit the site and study any reports available regarding the site. Advise the client on the need and extent of site investigations, arrange quotes and proceed when quotes are approved by the client. Advise the client of any topographical or dimensional surveys that are required. Consult with any local or other authorities about matters of principle and consider alternative outline solutions for the proposed scheme. Provide advice, sketches, reports or outline specifications to enable the Lead Consultant to prepare his outline proposals and assist the preparation of a Cost Plan. Prepare a report and, if required, present to the client. Develop the design of the detailed proposals with the design team for submission of the Planning Application by the Lead Consultant. Prepare drawings, specifications, calculations and descriptions in order to assist the preparation of a Cost Plan. Prepare a report and, if required, present to the client.

Stage D

Detailed proposals

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Structural Engineer's Pocket Book

Summary of ACE conditions of engagement ± continued
Pre-construction phase ± continued Stage E Final proposals Develop and co-ordinate all elements of the project in the overall scheme with the design team, and prepare calculations, drawings, schedules and specifications as required for presentation to the client. Agree a programme for the design and construction of the Works with the client and the design team. Develop the design with the design team and prepare drawings, calculations, schedules and specifications for the Tender Documentation and for Building Regulations Approval. Prepare any further drawings and schedules necessary to enable Contractors to carry out the Works, excluding drawings and designs for temporary works, formwork, and shop fabrication details (reinforcement details are not always included as part of the normal services). Produce a Designer's Risk Assessment in line with Health & Safety CDM Regulations. Advise the Lead Consultant on any special tender or contract conditions. Assist the Lead Consultant in identifying and evaluating potential contractors and/or specialists for the construction of the project. Assist the selection of contractors for the tender lists, assemble Tender Documentation and issue it to the selected tenderers. On return of tenders, advise on the relative merits of the contractors proposals, programmes and tenders.

Stage F

Production information

Stage G

Tender documents Tender action

Stage H

Construction phase Stage J Mobilization Assist the Client and Lead Consultant in letting the building contract, appointing the contractor and arranging site hand over to the contractor. Issue construction information to the contractor and provide further information to the contractor as and when reasonably required. Comment on detailed designs, fabrication drawings, bar bending schedules and specifications submitted by the Contractors, for general dimensions, structural adequacy and conformity with the design. Advise on the need for inspections or tests arising during the construction phase and the appointment and duties of Site Staff. Assist the Lead Consultant in examining proposals, but not including alternative designs for the Works, submitted by the Contractor. Attend relevant site meetings and make other periodic visits to the site as appropriate to the stage of construction. Advise the Lead Consultant on certificates for payment to Contractors. Check that work is being executed generally to the control documents and with good engineering practice. Inspect the construction on completion and, in conjunction with any Site Staff, record any defects. On completion, deliver one copy of each of the final structural drawings to the planning supervisor or client. Perform work or advise the Client in connection with any claim in connection with the structural works. Assist the Lead Consultant with any administration of the building contract after practical completion. Make any final inspections in order to help the Lead Consultant settle the final account.

Stage K

Construction to practical completion

Stage L

After practical completion

Source: ACE (1998).

2
Statutory Authorities and Permissions
Planning
Planning regulations control individuals' freedom to alter their property in an attempt to protect the environment in UK towns, cities and countryside, in the public interest. Different regulations and systems of control apply in the different UK regions. Planning permission is not always required, and in such cases the planning department will issue a Lawful Development Certificate on request and for a fee.

England and Wales
The main legislation which sets out the planning framework in England and Wales is the Town and Country Planning Act 1990. The government's statements of planning policy may be found in White Papers, Planning Policy Guidance Notes (PPGs), Mineral Policy Guidance Notes (MPGs), Regional Policy Guidance Notes (RPGs), departmental circulars and ministerial statements published by the Office of the Deputy Prime Minister (ODPM).

Scotland
The First Minister for Scotland is responsible for the planning framework. The main planning legislation in Scotland is the Town and Country Planning Act (Scotland) 1997 and the Planning (Listed Buildings and Conservation Areas) (Scotland) Act 1997. The legislation is supplemented by the Scottish Executive who publish National Planning Policy Guidelines (NPPGs) which set out the Scottish policy on land use and other issues. In addition, a series of Planning Advice Notes (PANs) give guidance on how best to deal with matters such as local planning, rural housing design and improving small towns and town centres.

Northern Ireland
The Planning (NI) Order 1991 could be said to be the most significant of the many different Acts which make up the primary and subordinate planning legislation in Northern Ireland. As in the other UK regions, the Northern Ireland Executive publishes policy guidelines called Planning Policy Statements (PPSs) which set out the regional policies to be implemented by the local authority.

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Structural Engineer's Pocket Book

Building regulations and standards
Building regulations have been around since Roman times and are now used to ensure reasonable standards of construction, health and safety, energy efficiency and access for the disabled. Building control requirements, and their systems of control, are different for the different UK regions. The legislation is typically set out under a Statutory Instrument, empowered by an Act of Parliament. In addition, the legislation is further explained by the different regions in explanatory booklets, which also describe the minimum standards `deemed to satisfy' the regulations. The `deemed to satisfy' solutions do not preclude designers from producing alternative solutions provided that they can be supported by calculations and details to satisfy the local authority who implement the regulations. Building control fees vary around the country but are generally calculated on a scale in relation to the cost of the work.

England and Wales
England and Wales has had building regulations since about 1189 when the first version of a London Building Act was issued. Today the relevant legislation is the Building Act 1984 and the Statutory Instrument Building Regulations 2000. The Approved Documents published by the ODPM are the guide to the minimum requirements of the regulations. Applications may be made as `full plans' submissions well before work starts, or for small elements of work as a `building notice' 48 hours before work starts. Completion certificates demonstrating Building Regulations Approval can be obtained on request. Third parties can become approved inspectors and provide building control services.

Approved documents (as amended)
A Structure A1 Loading A2 Ground Movement A3 and A4 Disproportionate Collapse B Fire Safety Site Preparation and Resistance to Moisture C D Toxic Substances E Resistance to the Passage of Sound F Ventilation G Hygiene Drainage and Waste Disposal H J Heat Producing Appliances K Stairs, Ramps and Guards L Conservation of Fuel and Power M Access and Facilities for Disabled People Glazing ± Materials and Protection N Regulation 7 Materials and Workmanship

Statutory Authorities and Permissions

15

Scotland
Building standards have been in existence in Scotland since around 1119 with the establishment of the system of Royal Burghs. The three principal documents which currently govern building control are the Building (Scotland) Act 1959 (as amended), the Building Standards (Scotland) Regulations 1990 (as amended) and the Technical Standards 1990 ± the explanatory guide to the regulations published by the Scottish Executive. Applications for all building and demolition work must be made to the local authority, who assess the proposals for compliance with the technical standards, before issuing a building warrant, which is valid for five years. Unlike the other regions in the UK, work may only start on site once a warrant has been obtained. Buildings may only be occupied at the end of the construction period once the local authority have issued a completion certificate. Building control departments typically will only assess very simple structural proposals and for more complicated work, qualified engineers must `self-certify' their proposals.

Technical standards
A B C D E F G H J K M N P Q R S General and Definitions Fitness of Materials Structure Structural Fire Precautions Means of Escape from Fire Heat Producing Installations and Storage of Liquid and Gaseous Fuels Preparation of Sites and Resistance to Moisture Resistance to Transmission of Sound Conservation of Fuel and Power Ventilation of Buildings Drainage and Sanitary Facilities Electrical Installations Miscellaneous Hazards Facilities for Dwellings Solid Waste Storage, Dungsteads and Farm Effluent Tanks Stairs, Ramps and Protective Barriers

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Structural Engineer's Pocket Book

Northern Ireland
The main legislation, policy and guidelines in Northern Ireland are the Building Regulations (Northern Ireland) Order 1979 as amended by the Planning and Building Regulations (Northern Ireland) (Amendment) Order 1990; the Building Regulations (NI) 2000 and the technical booklets ± which describe the minimum requirements of the regulations published by the Northern Ireland Executive. Building regulations in Northern Ireland are the responsibility of the Department of Finance and Personnel and are implemented by the district councils. Until recently the regulations operated on strict prescriptive laws, but the system is now very similar to the system in England and Wales. Applicants must demonstrate compliance with the `deemed to satisfy' requirements. Applications may be made as a `full plans' submission well before work starts, or as a `building notice' for domestic houses just before work starts. Builders must issue stage notices for local authority site inspections. Copies of the stage notices should be kept with the certificate of completion by the building owner.

Technical booklets
A B C D E F G H J K L N P R V Interpretation and General Materials and Workmanship Preparation of Sites and Resistance to Moisture Structure Fire Safety Conservation of Fuel and Power Sound Insulation Stairs, Ramps and Guarding Solid Waste in Buildings Ventilation Heat Producing Appliances and LPG Systems Drainage Sanitary Appliances and Unvented Hot Water Storage Systems Access and Facilities for Disabled People Glazing

Statutory Authorities and Permissions

17

Listed buildings
In the UK, buildings of `special architectural or historic interest' can be listed to ensure that their features are considered before any alterations are agreed to the exterior or interior. Buildings may be listed because of their association with an important architect, person or event or because they are a good example of design, building type, construction or use of material. Listed building consent must be obtained from the local authority before any work is carried out on a listed building. In addition, there may be special conditions attached to ecclesiastical, or old ecclesiastical, buildings or land by the local diocese or the Home Office.

England and Wales
English Heritage (EH) in England and CADW in Wales work for the government to identify buildings of `special architectural or historic interest'. All buildings built before 1700 (and most buildings between 1700 and 1840) with a significant number of original features will be listed. A building normally must be over 30 years old to be eligible for listing. There are three grades: I, II* and II, and there are approximately 500 000 buildings listed in England, with about 13 000 in Wales. Grades I and II* are eligible for grants from EH for urgent major repairs and residential listed buildings may be VAT zero rated for approved alterations.

Scotland
Historic Scotland maintains the lists and schedules for the Scottish Executive. All buildings before 1840 of substantially unimpaired character can be listed. There are over 40 000 listed buildings divided into three grades: A, B and C. Grade A is used for buildings of national or international importance or little altered examples of a particular period, style or building type, while a Grade C building would be of local importance or be a significantly altered example of a particular period, style or building type.

Northern Ireland
The Environment and Heritage Service (EHS) within the Northern Ireland Executive has carried out a survey of all the building stock in the region and keeps the Northern Ireland Buildings Database. Buildings must be at least 30 years old to be listed and there are currently about 8500 listed buildings. There are three grades of listing: A, B+ and B (with two further classifications B1 and B2) which have similar qualifications to the other UK regions.

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Structural Engineer's Pocket Book

Conservation areas
Local authorities have a duty to designate conservation areas in any area of `special architectural or historic interest' where the character or appearance of the area is worth preserving or enhancing. There are around 8500 conservation areas in England and Wales, 600 in Scotland and 30 in Northern Ireland. The character of an area does not just come from buildings and so the road and path layouts, greens and trees, paving and building materials and public and private spaces are protected. Conservation area consent is required from the local authority before work starts to ensure any alterations do not detract from the area's appearance.

Tree preservation orders
Local authorities have specific powers to protect trees by making Tree Protection Orders (TPOs). Special provisions also apply to trees in conservation areas. A TPO makes it an offence to cut down, lop, top, uproot, wilfully damage or destroy the protected tree without the local planning authority's permission. All of the UK regions operate similar guidelines with slightly different notice periods and penalties. The owner remains responsible for the tree(s), their condition and any damage they may cause, but only the planning authority can give permission to work on them. Arboriculturalists (who can give advice on work which needs to be carried out on trees) and contractors (who are qualified to work on trees) should be registered with the Arboricultural Association. In some cases (including if the tree is dangerous) no permission is required, but notice (about 5 days (or 6 weeks in a conservation area) depending on the UK region) must be given to the planning authority. When it is agreed that a tree can be removed, this is normally on the condition that a similar tree is planted as a replacement. Permission is generally not required to cut down or work on trees with a trunk less than 75 mm diameter (measured at 1.5 m above ground level) or 100 mm diameter if thinning to help the growth of other trees. Fines of up to £20 000 can be levied if work is carried out without permission.

Statutory Authorities and Permissions

19

Archaeology and ancient monuments
Archaeology in Scotland, England and Wales is protected by the Ancient Monuments and Archaeology Areas Act 1979, while the Historic Monuments and Archaeology Objects (NI) Order 1995 applies in Northern Ireland. Archaeology in the UK can represent every period from the camps of hunter gatherers 10 000 years ago to the remains of twentieth century industrial and military activities. Sites include places of worship, settlements, defences, burial grounds, farms, fields and sites of industry. Archaeology in rural areas tends to be very close to the ground surface, but in urban areas, deep layers of deposits were built up as buildings were demolished and new buildings were put directly on the debris. These deposits, often called `medieval fill', are an average of 5 m deep in places like the City of London and York. Historic or ancient monuments are structures which are of national importance. Typically monuments are in private ownership but are not occupied buildings. Scheduled monument consent is required for alterations and investigations from the regional heritage bodies: English Heritage, Historic Scotland, CADW in Wales and EHS in Northern Ireland. Each of the UK regions operates very similar guidelines in relation to archaeology, but through different frameworks and legislation. The regional heritage bodies develop the policies which are implemented by the local authorities. These policies are set out in PPG 16 for England and Wales, NPPG 18 for Scotland and PPS 6 for Northern Ireland. These guidance notes are intended to ensure that: 1. Archaeology is a material consideration for a developer seeking planning permission. 2. Archaeology strategy is included in the urban development plan by the local planning authority. 3. Archaeology is preserved, where possible, in situ. 4. The developer pays for the archaeological investigations, excavations and reporting. 5. The process of assessment, evaluation and mitigation is a requirement of planning permission. 6. The roles of the different types of archaeologists in the processes of assessment, evaluation and mitigation are clearly defined.

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Structural Engineer's Pocket Book

Where `areas of archaeological interest' have been identified by the local authorities, the regional heritage bodies act as curators (English Heritage, Historic Scotland, CADW in Wales and EHS in Northern Ireland). Any developments within an area of archaeological interest will have archaeological conditions attached to the planning permission to ensure that the following process is put into action: 1. Early consultation between the developers and curators so that the impact of the development on the archaeology (or vice versa) can be discussed and the developer can get an idea of the restrictions which might be applied to the site, the construction process and the development itself. 2. Desk study of the site by an archaeologist. 3. Field evaluation by archaeologists using field walking, trial pits, boreholes and/or geophysical prospecting to support the desk study. 4. Negotiation between the site curators and the developer's design team to agree the extent of archaeological mitigation. The developer must submit plans for approval by the curators. 5. Mitigation ± either preservation of archaeology in situ or excavation of areas to be disturbed by development. The archaeologists may have either a watching brief over the excavations carried out by the developer (where they monitor construction work for finds) or on significant sites, carry out their own excavations. 6. Post-excavation work to catalogue and report on the archaeology, either store or display the findings. Generally the preliminary and field studies are carried out by private consultants and contractors employed by the developers to advise the local authority planning department. In some areas advice can also be obtained from a regional archaeologist. In Northern Ireland, special licences are required for every excavation which must be undertaken by a qualified archaeologist. In Scotland, England and Wales, the archaeological contractors or consultants have a `watching brief'. Field evaluations can often be carried out using geotechnical trial pits with the excavations being done by the contractor or the archaeologist depending on the importance of the site. If an interesting find is made in a geotechnical trial pit and the archaeologists would like to keep the pit open for inspection by, say, the curators, the developer does not have to comply if there would be inconvenience to the developer or building users, or for health and safety reasons. Engineers should ensure for the field excavation and mitigation stages that the archaeologists record all the features in the excavations up to this century's interventions as these records can be very useful to the design team. Positions of old concrete footings could have as much of an impact on proposed foundation positions as archaeological features!

Statutory Authorities and Permissions

21

Party Wall etc. Act
The Party Wall etc. Act 1996 came into force in 1997 throughout England and Wales. In 2002 there is no equivalent legislation in Northern Ireland and in Scotland, the Law of the Tenement is only in draft form. Different sections of the Party Wall Act apply, depending on whether you propose to carry out work to an existing wall or structure shared with another property; build a freestanding wall or the wall of a building astride a boundary with a neighbouring property, and/or excavate within 3 m of a neighbouring building or structure. Work can fall within several sections of the Act at one time. A building Owner must notify his neighbours and agree the terms of a Party Wall Award before starting any work. The Act refers to two different types of Party Structure: `Party Wall' and `Party Fence Wall'. Party Walls are loosely defined as a wall on, astride or adjacent to a boundary enclosed by building on one or both sides. Party Fence Walls are walls astride a boundary but not part of a building; it does not include things like timber fences. A Party Structure is a wide term which can sometimes include floors or partitions. The Notice periods and sections 1, 2 and 6 of the Act are most commonly used, and are described below.

Notice periods and conditions
In order to exercise rights over the Party Structures, the Act says that the Owner must give Notice to Adjoining Owners; the building Owner must not cause unnecessary inconvenience, must provide compensation for any damage and must provide temporary protection for buildings and property where necessary. The Owner and the Adjoining Owner in the Act are defined as anyone with an interest greater than a tenancy from year to year. Therefore this can include shorthold tenants, long leaseholders and freeholders for any one property. A building Owner, or surveyor acting on his behalf, must send a Notice in advance of the start of the work. Different Notice periods apply to different sections of the Act, but work can start within the Notice period with the written agreement of the Adjoining Owner. A Notice is only valid for one year from the date that it is served and must include the Owner's name and address, the building's address (if different); a clear statement that the Notice is under the provisions of the Act (stating the relevant sections); full details of the proposed work (including plans where appropriate) and the proposed start date for the work. The Notice can be served by post, in person or fixed to the adjoining property in a `conspicuous part of the premises'. Once the Notice has been served, the Adjoining Owner can consent in writing to the work or issue a counter Notice setting out any additional work he would like to carry out. The Owner must respond to a counter Notice within 14 days. If the Owner has approached the Adjoining Owners and discussed the work with them, the terms of a Party Wall Award may have already been agreed in writing before a Notice is served.

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Structural Engineer's Pocket Book

If a Notice is served and the Adjoining Owner does not respond within 14 days, a dispute is said to have arisen. If the Adjoining Owner refuses to discuss terms or appoint a surveyor to act on his behalf, the Owner can appoint a surveyor to act on behalf of the Adjoining Owner. If the Owners discuss, but cannot agree terms they can jointly appoint a surveyor (or they can each appoint one) to draw up the Party Wall Award. If two surveyors cannot agree, a nominated Third Surveyor can be called to act impartially. In complex cases, this can often take over a year to resolve and in such cases the Notice period can run out, meaning that the process must begin again by serving another Notice. In all cases, the surveyors are appointed to consider the rights of the Owner over the wall and not to act as advocates in the negotiation of compensation! The building Owner covers the costs associated with all of the surveyors and experts asked about the work. When the terms have been agreed, the Party Wall Award should include a description (in drawings and/or writing) of what, when and how work is to be carried out; a record of the condition of the adjoining Owner's property before work starts; arrangements to allow access for surveyors to inspect while the works are going on and say who will pay for the cost of the works (if repairs are to be carried out as a shared cost or if the adjoining Owner has served a counter Notice and is to pay for those works). Either Owner has 14 days to appeal to the County Court against an Award if an Owner believes that the person who has drafted the Award has acted beyond their powers. An Adjoining Owner can ask the owner for a `bond'. The bond money becomes the property of the Adjoining Owner (until the work has been completed in accordance with the Award) to ensure that funds are available to pay for the completion of the works in case the Owner does not complete the works. The Owner must give 14 days' Notice if his representatives are to access the Adjoining Owner's property to carry out or inspect the works. It is an offence to refuse entry or obstruct someone who is entitled to enter the premises under the Act if the offender knows that the person is entitled to be there. If the adjoining property is empty, the Owner's workmen and own surveyor or architect may enter the premises if they are accompanied by a police officer.

Statutory Authorities and Permissions

23

Section 1 ± new building on a boundary line
Notice must be served to build on or astride a boundary line, but there is no right to build astride if your neighbour objects. You can build foundations on the neighbouring land if the wall line is immediately adjacent to the boundary, subject to supervision. The Notice is required at least 1 month before the proposed start date.

Section 2 ± work on existing party walls
The most commonly used rights over existing Party Walls include cutting into the wall to insert a DPC or support a new beam bearing; raising, underpinning, demolishing and/or rebuilding the Party Wall and/or providing protection by putting a flashing from the higher over the lower wall. Minor works such as fixing shelving, fitting electrical sockets or replastering are considered to be too trivial to be covered in the Act. A building Owner, or Party Wall Surveyor acting on the Owner's behalf must send a Notice at least 2 months in advance of the start of the work.

Section 6 ± excavation near neighbouring buildings
Notice must be served at least 1 month before an Owner intends to excavate or construct a foundation for a new building or structure within 3 m of an adjoining Owner's building where that work will go deeper than the adjacent Owner's foundations, or within 6 m of an adjoining Owner's building where that work will cut a line projecting out at 45 from the bottom of that building's foundations. This can affect neighbours who are not immediately adjacent. The Notice must state whether the Owner plans to strengthen or safeguard the foundations of the Adjoining Owner. Adjoining Owners must agree specifically in writing to the use of `special foundations' ± these include reinforced concrete foundations. After work has been completed, the Adjoining Owner may request particulars of the work, including plans and sections. Source: DETR (1997).

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Structural Engineer's Pocket Book

CDM
The Construction Design & Management (CDM) Regulations 1994 were developed to assign responsibilities for health and safety to the client, design team and principal contractor. The Approved Code of Practice is published by the Health and Safety Executive for guidance to the Regulations. The client is required to appoint a planning supervisor (PS) who has overall responsibility for co-ordinating health and safety aspects of the design and planning stages of a project. The duties of the PS can theoretically be carried out by any of the traditional design team professionals. The PS must ensure that the designers avoid, minimize or control health and safety risks for the construction and maintenance of the project, as well as ensuring that the contractor is competent to carry out the work. The PS prepares the pre-contract health and safety plan for inclusion in the tender documents which should include project-relevant health and safety information gathered from the client and designers. This should highlight any unusual aspects of the project (also highlighted on the drawings) that a competent contractor would not be expected to know. This document is taken on by the successful principal contractor and developed into the construction phase health and safety plan by the addition of the contractor's health and safety policy, risk assessments and method statements as requested by the designers. The health and safety plan is intended to provide a focus for the management and prevention of health and safety risks as the construction proceeds. The health and safety file is generally compiled at the end of the project by the contractor and the PS who collect the design information relevant to the life of the building. The PS must ensure that the file is compiled and passed to the client or the people who will use, operate, maintain and/or demolish the project. A good health and safety file will be a relatively compact maintenance manual including information to alert those who will be owners, or operators of the new structure, to the risks which must be managed when the structure and associated plant is maintained, repaired, renovated or demolished. After handover the client is responsible for keeping the file up to date.

3
Design Data
Design data checklist
The following design data checklist is a useful reminder of all of the limiting criteria which should be considered when selecting an appropriate structural form:

. . . . . . . . . . . . .

Description/building use Client brief and requirements Site constraints Loadings Structural form: load transfer, stability and robustness Materials Movement joints Durability Fire resistance Performance criteria: deflection, vibration, etc. Temporary works and construction issues Soil conditions, foundations and ground slab Miscellaneous issues

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Structural Engineer’s Pocket Book

Structural form, stability and robustness
Structural form
It is worth trying to remember the different structural forms when developing a scheme design. A particular structural form might fit the vision for the form of the building. Force or moment diagrams might suggest a building shape. The following diagrams of structural form are intended as useful reminders:

TRUSSES

Couple

Tied rafter

King post

Queen post

Howe (>10 m steel/ timber)

Double howe (8–15 m steel/ timber)

Fink (>10 m steel/ timber)

Double fink (5–14 m timber) (8–13 m steel)

Bowshing

Thrust Scissor (6–10 m steel/ timber) Double scissor (10–13 m steel/ timber)

Northlight (>5 m steel)

Northlight (5–15 m steel)

Fan (8–15 m steel)

French truss (12–20 m steel)

Bowshing (20–40 m steel) GIRDERS Pratt

Umbrella (~13 m steel)

Saw tooth (~5 m steel)

Warren

Modified warren

Howe

Fink

Modified fink

Double lattice

Vierendeel

Design Data

27

PORTAL FRAMES

All fixed ARCHES

2 pin

2 pin mansard

3 pin

Thrust SUSPENSION

Tied

3 pin

Cable stay WALLS Solid TIMBER

Suspension

Closed suspension

Piers

Chevron

Diaphragm

Ply/ply stressed skin RETAINING WALLS

Ply web

Ply/timber stressed skin

Flitched

Embedded

Cantilever

Gravity or reinforced earth

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Structural Engineer’s Pocket Book

Stability
Stability of a structure must be achieved in two orthogonal directions. Circular structures should also be checked for rotational failure. The positions of movement and/or acoustic joints should be considered and each part of the structure should be designed to be independently stable and robust. Lateral loads can be transferred across the structure and/or down to the foundations by using any of the following methods:

. Cross bracing which carries the lateral forces as axial load in diagonal members. . Diaphragm action of floors or walls which carry the forces by panel/plate/shear action. . Frame action with ‘fixed’ connections between members and ‘pinned’ connections at . Vertical cantilever columns with ‘fixed’ connections at the foundations. . Buttressing with diaphragm, chevron or fin walls.
Stability members must be located on the plan so that their shear centre is aligned with the resultant of the overturning forces. If an eccentricity cannot be avoided, the stability members should be designed to resist the resulting torsion across the plan. the supports.

Robustness and disproportionate collapse
All structural elements should be effectively tied together in each of the two orthogonal directions, both horizontally and vertically. This is generally achieved by specifying connections in steel buildings as being of certain minimum size, by ensuring that reinforced concrete junctions contain a minimum area of steel bars and by using steel straps to connect walls and floors in masonry structures. It is important to consider robustness requirements early in the design process. The Building Regulations require buildings of five or more storeys (excluding the roof) to be designed for disproportionate collapse. This is intended to ensure that accidental damage to elements of the building structure cannot cause the collapse of a disproportionately large area of a building. The disproportionate collapse requirement for public buildings with a roof span of more than 9 m appears to have been removed from the regulations. Typically the Building Regulations require that any collapse caused by the failure of a single structural element should be limited to an area of 70 m2 or 15% of any storey area (whichever is the lesser). Alternatively the designer can strengthen the structure to withstand the ‘failure’ of certain structural supports in order to prevent disproportionate collapse. In some circumstances the structure cannot be arranged to avoid the occurrence of ‘key elements’, which support disproportionately large areas of the building. These ‘key elements’ must be designed as protected members (to the code of practice for the relevant structural material) to provide extra robustness and damage resistance.

Design Data

29

Structural movement joints
Joints should be provided to control temperature, moisture, acoustic and ground movements. Movement joints can be difficult to waterproof and detail and therefore should be kept to a minimum. The positions of movement joints should be considered for their effect on the overall stability of the structure.

Primary movement joints
Primary movement joints are required to prevent cracking where buildings (or parts of buildings) are large, where a building spans different ground conditions, changes height considerably or where the shape suggests a point of natural weakness. Without detailed calculation, joints should be detailed to permit 15–25 mm movement. Advice on joint spacing for different building types can be variable and conflicting. The following figures are some approximate guidelines based on the building type:

Concrete Steel industrial buildings Steel commercial buildings Masonry

25 m (e.g. for roofs with large thermal differentials)– 50 m c /c. 100 m typical–150 m maximum c /c. 50 m typical–100 m maximum c /c. 40 m–50 m c /c.

Secondary movement joints
Secondary movement joints are used to divide structural elements into smaller elements to deal with the local effects of temperature and moisture content. Typical joint spacings are: Clay bricks Up to 12 m c/c on plan (6 m from corners) and 9 m vertically or every three storeys if the building is greater than 12 m or four storeys tall. 3 m–7 m c/c. 70 m c/c. 20 m c/c down the slope, no limit along the slope.

Concrete blocks Hardstanding Steel roof sheeting

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Structural Engineer’s Pocket Book

Fire resistance periods for structural elements
Fire resistance of structure is required to maintain structural integrity to allow time for the building to be evacuated. Generally, roofs do not require protection. Architects typically specify fire protection in consultation with the engineer.
Building types Minimum period of fire resistance minutes Basement storey including floor over Depth of a lowest basement Ground or upper storey

Height of top floor above ground, in a building or separated part of a building 6.1

* A load may project over one or both sides by up to 0.305 m, but the overall width is still limited as above.

Loads with a width of over 2.9 m or with loads projecting more than 0.305 m on either side of the vehicle must be marked to comply with the requirements of the Road Vehicles Lighting Regulations 1989.

Length of load
Total loaded length, L m L < 18.75 18.75 L 27.4

(all other trailers) L > 25.9

* The length of the front of an articulated motor vehicle is excluded if the load does not project over the front of the motor vehicle.

Projection of overhanging loads
Overhang position Rear Overhang length, L m L < 1.0 1.0 < L < 2.0 2.0 < L < 3.05 L > 3.05 Notification requirements

No special requirement Load must be made clearly visible Standard end marker boards are required Standard end marker boards are required plus police notification and an attendant is required No special requirement Standard end marker boards are required plus the driver is required to be accompanied by an attendant Standard end marker boards are required plus police notification and the driver is required to be accompanied by an attendant

Front

L < 1.83 2.0 < L < 3.05

L > 3.05

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Typical vehicle sizes and weights
Vehicle type 3.5 tonne van Weight, W kg 3500 Length, L m 5.5 Width, B m 2.1 Height, H m 2.6 Turning circle m 13.0

7.5 tonne van

7500

6.7

2.5

3.2

14.5

Single decker bus

16 260

11.6

2.5

3.0

20.0

Refuse truck

16 260

8.0

2.4

3.4

17.0

2 axle tipper

16 260

6.4

2.5

2.6

15.0

Van (up to 16.3 tonnes)

16 260

8.1

2.5

3.6

17.5

Skiploader

16 260

6.5

2.5

3.7

14.0

Fire engine

16 260

7.0

2.4

3.4

15.0

Bendy bus

17 500

18.0

2.6

3.1

23.0

51

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Structural Engineer’s Pocket Book

Temporary works toolkit
Steel trench prop load capacities
Better known as ‘Acrow’ props, these adjustable props should conform to BS 4704 or BS EN 1065. Verticality of the loads greatly affects the prop capacity and fork heads can be used to eliminate eccentricities. Props exhibiting any of the following defects should not be used:

. . . . .

A tube with a bend, crease or noticeable lack of straightness. A tube with more than superficial corrosion. A bent head or base plate. An incorrect or damaged pin. A pin not properly attached to the prop by the correct chain or wire.

Steel trench ‘acrow’ prop sizes and reference numbers to BS 4074
Prop size/reference* Height range Minimum m 1.07 1.75 1.98 2.59 3.20

Maximum m 1.82 3.12 3.35 3.96 4.87

0 1 2 3 4

*The props are normally identified by their length.

Steel trench prop load capacities
A prop will carry its maximum safe load when it is plumb and concentrically loaded as shown in the charts in BS 4074. A reduced safe working load should be used for concentric loading with an eccentricity, e 1.5 out of plumb as follows:

Capacity of props with e Height m Prop size 0, 1, 2 and 3 Prop size 4

1.5 (KN) £2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4.75 17 – 16 – 13 17 11 14 10 11 – 10 – 9 – 8 – 7

Design Data

53

Soldiers
Slim soldiers, also known as slimshors, can be used horizontally and vertically and have more load capacity than steel trench props. Lengths of 0.36 m, 0.54 m, 0.72 m, 0.9 m, 1.8 m, 2.7 m or 3.6 m are available. Longer units can be made by joining smaller sections together. A connection between units with four M12 bolts will have a working moment capacity of about 12 kNm, which can be increased to 20 kNm if stiffeners are used.

Slimshor section properties
Area cm2 19.64 Ixx cm4 1916 Iyy cm4 658 Zxx cm3 161 Zyy cm3 61 rx cm 9.69 ry cm 5.70 Mmax kNm 38 x Mmax kNm 7.5

y

Slimshor compression capacity yyAllowable bending moment (kNm) 50 40 30 20 10 0 20 40 60 80 100 120 140 160 Allowable axial load (kN) Factor of safety = 1.8 Use hi-load waler plate

x-

x-

Allowable load (kN)

m 5m =2 m , e 8m xis = 3 ya ,e xis ya

m 5m =2 m , e 8m xis = 3 xa ,e xis xa

150 140 120 100 80 60 40 20 0 2

4

6

8

10

Effective length (m) e = eccentricity of load Factor of safety = 2.0

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Structural Engineer’s Pocket Book

Slimshor moment capacity
Source: RMD Kwikform (2002).

Ladder beams
Used to span horizontally in scaffolding or platforms, ladder beams are made in 48.3f 3.2 CHS, 305 mm deep, with rungs at 305 mm centres. All junctions are saddle welded. Ladder beams can be fully integrated with scaffold fittings. Bracing of both the top and bottom chords is required to prevent buckling. Standard lengths are 3.353 m (110 ), 4.877 m (160 ) and 6.400 m (210 ). Manufacturers should be contacted for loading information. However, if the tension chord is tied at 1.5 m centres and the compression chord is braced at 1.8 m centres the moment capacity for working loads is about 8.5 kNm. If the compression chord bracing is reduced to 1.5 m centres, the moment capacity will be increased to about 12.5 kNm. The maximum allowable shear is about 12 kN.

Unit beams
Unit beams are normally about 615 mm deep, are about 2Z.5 times stronger than ladder beams and are arranged in a similar way to a warren girder. Loads should only be applied at the node points. May be used to span between scaffolding towers or as a framework for temporary buildings. As with ladder beams, bracing of both the top and bottom chords is required to prevent buckling, but diagonal plan bracing should be provided to the compression flange. Units can be joined together with M24 bolts to make longer length beams. Standard lengths are 1.8 m (60 ), 2.7 m (90 ) and 3.6 m (120 ) Manufacturers should be contacted for loading information. However, if the tension chord is tied at 3.6 m centres and the compression chord is braced at 2.4 m centres the moment capacity for working loads is about 13.5 kNm. If the compression bracing is reduced to 1.2 m centres, the moment capacity will be increased to about 27.5 kNm. The maximum allowable shear is about 14 kN.

4
Basic and Shortcut Tools for Structural Analysis
Load factors and limit states
There are two design considerations: strength and stiffness. The structure must be strong enough to resist the worst loading conditions without collapse and be stiff enough to resist normal working conditions without excessive d\eflection or deformation. Typically the requirements for strength and stiffness are split between the following `limit states': Ultimate limit state (ULS) ± Strength (including yielding, rupture, buckling and forming a mechanism), stability against overturning and swaying, fracture due to fatigue and brittle fracture. Serviceability limit state (SLS) ± Deflection, vibration, wind induced oscillation and durability. A factor of safety against structural failure of 2.0 to 10.0 will be chosen depending on the materials and workmanship. There are three main methods of applying the factor of safety to structural design: Allowable or permissible stress design ± Where the ultimate strengths of the materials are divided by a factor of safety to provide design stresses for comparison with unfactored loads. Normally the design stresses stay within the elastic range. This method is not strictly applicable to plastic (e.g. steel) or semi-plastic (e.g. concrete or masonry) materials and there is one factor of safety to apply to all conditions of materials, loading and workmanship. This method has also been found to be unsafe in some conditions when considering the stability of structures in relation to overturning. Load factor design ± Where working loads are multiplied by a factor of safety for comparison with the ultimate strength of the materials. This method does not consider variability of the materials and as it deals with ultimate loads, it cannot be used to consider deflection and serviceability under working loads. Ultimate loads or limit state design ± The applied loads are multiplied by partial factors of safety and the ultimate strengths of the materials are divided by further partial factors of safety to cover variation in the materials and workmanship. This method allows a global factor of safety to be built up using the partial factors at the designer's discretion, by varying the amount of quality control which will be available for the materials and workmanship. The designer can therefore choose whether to analyse the structure with working loads in the elastic range, or in the plastic condition with ultimate loads. Serviceability checks are generally made with unfactored, working loads.

Geometric section properties
56
Section A mm2 b2 b cx cy b

Cx mm b 2

Cy mm b 2

Ix cm3 b4 12

Iy cm3 b4 12

J (approx.) cm4 5b4 36

bd d cx cy b

d 2

b 2

bd3 12

db3 12

  d3 d4 16b À d 1À 4 3 12b for a > b

d2 4

d 2

d 2

d4 64

d4 64

d4 32

d

Cx

Cy

bd 2 d cx cy b

d 3

b 2

bd3 36

db3 48

b3 d 3 …15b2 ‡ 20d2 †

b cx cy b

t

b2 À …b À 2t†2

b 2

b 2

b4 À …b À 2t†4 12

b4 À …b À 2t†4 12

…b À t†3 t

Elastic modulus I/y, plastic modulus, S ˆ sum of first moments of area about central axis, the shape factor ˆ S/Z

57

Geometric section properties ± continued
Section A mm2 t d cx cy t1 d cx cy b t1 t2 d cx cy b t2

58

Cx mm d 2

Cy mm d 2

Ix cm3 …d4 À …d À 2t†4 † 64

Iy cm3 …d4 À …d À 2t†4 † 64

J (approx.) cm4 …d À t†3 t 4

   d2 À …d À 2t†2 4

2bt1 ‡ t2 …d À 2t1 †

d 2

b 2

bd3 À …b À t2 †…d À 2t1 †3 12

2t1 b3 À …d À 2t1 †t23 12

2t3 b À t23 d 1 3

bd À …d À 2t1 † …b À t2 †

d 2

2b2 t1 ‡ …d À 2t1 †t22 2…2bt1 ‡ …d À 2t1 †t2 †

bd3 À …b À t2 †…d À 2t1 †3 12

2t1 b3 À …d À 2t1 †t23  12 2 ‡2bt1 b À Cy 2  2 2 ‡t2 …d À 2t1 † Cy À t2

t3 …d ‡ 2b† 3

t1 cx d cy b t2

bt1 ‡ …d À t1 †t2

  1 bt1 d À t2 ‡ 1 …d À t1 †2 t2 2 A

b 2

bt13 ‡ t2 …d À t1 †3  12  t1 2 ‡ bt1 d À Cx À 2   d À t1 2 ‡ t2 …d À t1 † Cx À 2

t1 b3 À …d À t1 †t3 2 12

t13 b À t23 d 3

t2 d cx t1 b cy

dt2 ‡ …b À t2 †t1

d2 t2 …b À t2 †t12 2A

b2 t1 …d À t1 †t22 2A

t2 d3 À …b À t2 †t1 3 …d À 2t1 †3 12  2 d À Cx ‡ dt2 2   t1 2 ‡ …b À t2 †t1 Cx À 2

t1 b3 ‡ …d À t1 †t23 12  2 ‡bt1 b À Cy 2   t2 2 ‡…d À t1 †t2 Cy À 2

t3 b À t23 d 1 3

Elastic modulus I/y, plastic modulus, S ˆ sum of first moments of area about central axis, the shape factor ˆ S/Z

59

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Structural Engineer's Pocket Book

Parallel axis theorem
€ Ai yi yˆ € A ˆ Ai … y À yi †2 ‡ ˆ

Ixx ˆ

IC

Where: yi neutral axis depth of element from datum y depth of the whole section neutral axis from the datum Ai area of element A area of whole section Ixx moment of inertia of the whole section about the x-x axis IC moment of inertia of element

Composite sections
A composite section made of two materials will have a strength and stiffness related to the properties of these materials. An equivalent stiffness must be calculated for a composite section. This can be done by using the ratios of the Young's moduli to `transform' the area of the weaker material into an equivalent area of the stronger material. aE ˆ E1 E2 For concrete to steel, aE  15 For timber to steel, aE  35 Typically the depth of the material (about the axis of bending) should be kept constant and the breadth should be varied: b1 ˆ aE b2 . The section properties and stresses can then be calculated based on the transformed section in the stronger material.

Basic and Shortcut Tools for Structural Analysis

61

Material properties
Homogeneous: same elastic properties throughout. Isotropic: same elastic properties in all directions. Anisotropic: varying elastic properties in two different directions. Orthotropic: varying elastic properties in three different directions. All properties are given for a temperature of 20 C.

Properties of selected metals
Material Specific weight g kN/m3 Modulus of elasticity E kN/mm2 Shear modulus of elasticity G kN/mm2 25.5 26.6 46 Poisson's ratio n Proof or yield stress fy N/mm2 0 b

i3

2b br ˆ p p m ˆ 1 ‡ i1 ‡ 1 ‡ i3 a br

war br   a br 3 ‡ 12 ‡ 2i2 1 ‡ a br Top steel also required.

a i1

m i2 > 0

b

b war br   br ˆ p m ˆ 3 a br 1 ‡ i1 ‡ 3 ‡ i2 1 ‡ 2 a 2 br For opposite case, exchange a and b, I1 and I2. …a ‡ c†i1 ‡ …b ‡ d†i2 a‡b‡c‡d 3wab  m0 ˆ  a b 8 2‡ ‡ b a a4b42a

c a i1 i2 m

F ˆ 0:6

b

d



m0 À 0:15wcd 1‡F

d c m′

mH ˆ

w 2 …c ‡ d2 † 6 Bottom steel required for main span.

m

m′

if c ˆ 0:35a; m ˆ mH ˆ

wa2 16

c

a

c

Basic and Shortcut Tools for Structural Analysis

87

m



m′ c h

wh2  c wc2 0:39 À ; mH ˆ h 3 6 wa2 55

If c ˆ 0:33h; m ˆ mH ˆ

wh2 mˆ 2 m 2

 c 2 3 0:33 À 2h

3

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Structural Engineer's Pocket Book

Torsion
Elastic torsion of circular sections: T t Gf ˆ ˆ J r L Where, T is the applied torque, J is the polar moment of inertia, t is the torsional shear stress, r is the radius, f is the angle of twist, G is the shear modulus of elasticity and L is the length of member. The shear strain, g, is constant over the length of the member and rf gives the displacement of any point along the member. Materials yield under torsion in a similar way to bending. The material has a stress/strain curve with gradient G up to a limiting shear stress, beyond which the gradient is zero. The torsional stiffness of a member relies on the ability of the shear stresses to flow in a loop within the section shape which will greatly affect the polar moment of area, which is ‚ calculated from the relationship J ˆ r 2 dA. This can be simplified in some closed loop cases to J ˆ Izz ˆ Ixx ‡ Iyy .

Thin walled sections of arbitrary and open cross sections have less torsional stiffness than solid sections or tubular thin walled sections which allow shear to flow around the section. In thin walled sections the shear flow is only able to develop within the thickness of the walls and so the torsional stiffness comes from the sum of the stiffness of its parts: ‚ €À 3 Á J ˆ 1 section t3 ds. This can be simplified to J  bt =3 , where t ˆ Tt=J: 3 J for thick open sections are beyond the scope of this book, and must be calculated empirically for the particular dimensions of a section. For non-square and circular shapes, the effect of the warping of cross sections must be considered in addition to the elastic effects set out above.

Therefore for a solid circular section, J ˆ pd4 =32 for a solid square bar, J ˆ 5d4 =36 and 4 4 for thin walled circular tubes, J ˆ p…douter À rinner †=32 or J ˆ 2pr 3 t and the shear stress, t ˆ T=At where t is the wall thickness and A is the area contained within the tube.

Basic and Shortcut Tools for Structural Analysis

89

Taut wires, cables and chains
The cables are assumed to have significant self-weight. Without any externally applied loads, the horizontal component of the tension in the cable is constant and the maximum tension will occur where the vertical component of the tension reaches a maximum. The following equations are relevant where there are small deflections relative to the cable length. L h A ÁLs C E s ˆ h=L w V y D H Tmax x Span length Cable sag Area of cable Cable elongation due to axial stress Length of cable curve Modulus of elasticity of the cable Sag ratio Applied load per unit length Vertical reaction Equation for the deflected shape Height of elevation Horizontal reaction Maximum tension in cable Distance along cable

Uniformly loaded cables with horizontal chords
Tmax H y V x L yˆ 4h…Lx À x3 † L2 wL 2 Hˆ wL2 8h

w H h V



p Tmax ˆ H 1 ‡ 16s2 ÁLs  À Á HL 1 ‡ 16s 3 AE

  8 32 4 s ‡ ÁÁÁ C  L 1 ‡ s2 À 3 5

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Structural Engineer's Pocket Book

Uniformly loaded cables with inclined chords
Tmax H h V D

H V x

θ

y

L yˆ Vˆ 4h…Lx À x2 † L2 HD wL ‡ L 2  8s2 4y 3 sec Hˆ wL2 8h q À Á2 1 ‡ D ‡ 4s L

Tmax ˆ H

 C  L sec y 1 ‡

ÁLs 

À Á 16s HL 1 ‡ 3 sec4 y AE

Basic and Shortcut Tools for Structural Analysis

91

Vibration
When using long spans and lightweight construction, vibration can become an important issue. Human sensitivity to vibration has been shown to depend on frequency, amplitude and damping. Vibrations can detract from the use of the structure or can compromise the structural strength and stability. Vibrations can be caused by wind, plant, people, adjacent building works, traffic, earthquakes or wave action. Structures will respond differently depending on their mass and stiffness. Damping is the name given to the ability of the structure to dissipate the energy of the vibrations ± usually by friction in structural and non-structural components. While there are many sources of advice on vibrations in structures, assessment is not straightforward. In simple cases, structures should be designed so that their natural frequency is greater than 4.5 Hz to help prevent the structure from being dynamically excitable. Special cases may require tighter limits. A simplified method of calculating the natural frequency of a structure (f in Hz) is related to the static dead load deflection of the structure, where g is the acceleration due to gravity, d is the static dead load deflection estimated by normal elastic theory, k is the stiffness …k ˆ L=EI†, m is the total mass of the system, E is the modulus of elasticity, I is the moment of inertia and L is the length of the member. This method can be used to check the results of more complex analysis.
Member Estimate of natural frequency, af

General rule for structures with concentrated mass General rule for most structures with distributed loads Simplified rule for most structures

r g d r 1 k fˆ 2p m fˆ 1 2p 18 f ˆ p d r 1 48EI fˆ 2p mL3 r 1 EI fˆ 4 mL4 r EI fˆ mL4 r 1 3EI 2p r mL3 0:56 EI fˆ 2p rmL4  3:57 EI fˆ 2p mL4 fˆ

Simply supported, mass concentrated in the centre Simply supported, sagging, mass and stiffness distributed Simply supported, contraflexure, mass and stiffness distributed Cantilever, mass concentrated at the end Cantilever, mass and stiffness distributed Fixed ends, mass and stiffness distributed

For normal floors with span/depth ratios of 25 or less, there are unlikely to be any vibration problems. Typically problems are encountered with steel and lightweight floors with spans over about 8 m. Source: Bolton, A (1978).

5
Geotechnics
Geotechnics is the engineering theory of soils, foundations and retaining walls. This chapter is intended as a guide which can be used alongside information obtained from local building control officers, for feasibility purposes and for the assessment of site investigation results. Scheme design should be carried out on the basis of a full site investigation designed specifically for the site and structure under consideration. The relevant codes of practice are:

. BS 5930 for Site Investigation. . BS 8004 for Foundation Design and BS 8002 for Retaining Wall Design. . Eurocode 7 for Geotechnical Design.
The following issues should be considered for all geotechnical problems:

. UK (and most international codes) use unfactored loads, while Eurocodes use factored . All values in this chapter are based on unfactored loads. . Engineers not familiar with site investigation tests and their implications, soil theory and . The foundation information included in this chapter allows for simplified or idealized soil . All foundations must have an adequate factor of safety (normally g = 2 to 3) applied to f the ultimate bearing capacity to provide the allowable bearing pressure for design purposes. conditions. In practice, soil layers and variability should be allowed for in the foundation design. bearing capacity equations should not use the information in this chapter without using the sources listed in `Further Reading' for information on theory and definitions. loads.

. Settlement normally controls the design and allowable bearing pressures typically limit settlement to 25 mm. Differential settlements should be considered. Cyclic or dynamic loading can cause higher settlements to occur and therefore require higher factors of safety. Foundations in fine grained soils (such as clay, silt and chalk) need to be taken down to a depth below which they will not be affected by seasonal changes in the moisture content of the soil, frost action and the action of tree roots. Frost action is normally assumed to be negligible from 450 mm below ground level. Guidelines on trees and shallow foundations in fine grained soils are covered later in the chapter. Ground water control is key to the success of ground and foundation works and its effects must be considered, both during and after construction. Dealing with water within a site may reduce the water table of surrounding areas and affect adjoining structures. It is nearly always cheaper to design wide shallow foundations to a uniform and predetermined depth, than to excavate narrow foundations to a depth which might be variable on site.

.

.

.

Geotechnics

93

Selection of foundations and retaining walls
The likely foundation arrangement for a structure needs to be considered so that an appropriate site investigation can be specified, but the final foundation arrangement will normally only be decided after the site investigation results have been returned.

Foundations for idealized structure and soil conditions
Foundations must always follow the building type ± i.e. a large-scale building needs largescale/deep foundations. Pad and strip foundations cannot practically be taken beyond 3 m depth and these are grouped with rafts in the classification `shallow foundations', while piles are called deep foundations. They can have diameters from 75 mm to 2000 mm and be 5 m to 100 m in length. The smaller diameters and lengths tend to be bored cast in-situ piles, while larger diameters and lengths are driven steel piles.
Idealized extremes of structure type Idealized soil conditions Firm, uniform soil in an infinitely thick stratum Firm stratum of soil overlying an infinitely thick stratum of soft soil Pad or strip footings Soft, uniform soil in an infinitely thick stratum High water table and/or made ground Soft stratum of soil overlying an infinitely thick stratum of firm soil or rock Bearing piles or piers

Light, flexible structure Heavy rigid structure

Pad or strip footings Pad or strip footings

Friction piles or surface raft Buoyant raft or friction piles

Piles or surface raft Buoyant raft or piles

Buoyant raft or friction piles

Bearing piles or piers

Retaining walls for idealized site and soil conditions
Idealized site conditions Idealized soil types Dry sand and gravel Working space* available Saturated sand and gravel Clay and silt

.

Gravity or cantilever retaining wall

.

.

Dewatering during construction of gravity or cantilever retaining wall

.

Gravity or cantilever retaining wall

Reinforced soil, gabion or crib wall King post or sheet pile as temporary support Contiguous piled wall Diaphragm wall Soil nailing Contiguous piled wall Diaphragm wall

Limited working space

. . . .

. . .

Sheet pile and dewatering Secant bored piled wall Diaphragm wall

. . . . . .

King post or sheet pile as temporary support Contiguous piled wall Soil nailing Diaphragm wall Contiguous piled wall Diaphragm wall

Limited working space and special controls on ground movements

. .

. .

Secant bored piled wall Diaphragm wall

* Working space available to allow the ground to be battered back during wall construction.

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Structural Engineer's Pocket Book

Site investigation
In order to decide on the appropriate form of site investigation, the engineer must have established the position of the structure on the site, the size and form of the structure, and the likely foundation loads. BS 5390: Part 2 suggests that the investigation is taken to a depth of 1.5 times the width of the loaded area for shallow foundations. A loaded area can be defined as the width of an individual footing area, the width of a raft foundation, or the width of the building (if the foundation spacing is less than three times the foundation breadth). An investigation must be conducted to prove bedrock must be taken down 3 m beyond the top of the bedrock to ensure that rock layer is sufficiently thick.

Summary of typical site investigation requirements for idealized soil types
Soil type Type of geotechnical work Excavations Sand Shallow footings and rafts Deep foundations and piles

. .

Permeability for dewatering and stability of excavation bottom Shear strength for loads on retaining structures and stability of excavation bottom

. .

Shear strength for bearing capacity calculations Site loading tests for assessment of settlements

. .

Test pile for assessment of allowable bearing capacity and settlements Deep boreholes to probe zone of influence of piles Long-term test pile for assessment of allowable bearing capacity and settlements Shear strength and sensitivity testing to assess bearing capacity and settlements Deep boreholes to probe zone of influence of piles

Clay

.

Shear strength for loads on retaining structure and stability of excavation bottom Sensitivity testing to assess strength and stability and the possibility of reusing material as backfill

.

Shear strength for bearing capacity calculations

.

.

.

Consolidation tests for assessment of settlements

.

.

Moisture content and plasticity tests to predict heave potential and effects of trees

.

Geotechnics

95

Soil classification
Soil classification is based on the sizes of particles in the soil as divided by the British Standard sieves.
Sieve size mm 0.002 0.006 0.020 0.06 0.2 0.6 2.0 6.0 20 60 200

Description

Silt

Sand

Gravel

Cobbles

Boulders

Soil description by particle size
As soils are not normally uniform, standard descriptions for mixed soils have been defined by BS 5930. The basic components are boulders, cobbles, gravel, sand, silt and clay and these are written in capital letters where they are the main component of the soil. Typically soil descriptions are as follows:
Slightly sandy GRAVEL Very sandy GRAVEL Very gravelly SAND Slightly silty SAND (or GRAVEL) Very silty SAND (or GRAVEL) Clayey SAND (or GRAVEL) Sandy SILT (or CLAY) Very coarse up to 5% sand 20%±50% sand 20%±50% gravel up to 5% silt 15%±35% silt 5%±15% clay 35%±65% sand over 50% cobbles and boulders Sandy GRAVEL GRAVEL /SAND Slightly gravelly SAND Silty SAND (or GRAVEL) Slightly clayey SAND (or GRAVEL) Very clayey SAND (or GRAVEL) Gravelly SILT (or CLAY) 5%±20% sand equal proportions up to 5% gravel 5%±15% silt up to 5% clay 15%±35% clay 35%±65% gravel

Soil description by consistency
Homogeneous Heterogeneous Interstratified Weathered A deposit consisting of one soil type. A deposit containing a mixture of soil types. A deposit containing alternating layers, bands or lenses of different soil types. Coarse soils may contain weakened particles and/or particles sorted according to their size. Fine soils may crumble or crack into a `column' type structure. Breaks into multifaceted fragments along fissures. Uniform texture with no fissures. Recognizable plant remains present, which retains some strength. Uniform texture, with no recognizable plant remains.

Fissured clay Intact clay Fibrous peat Amorphous peat

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Structural Engineer's Pocket Book

Typical soil properties
The presence of water is critical to the behaviour of soil and the choice of shear strength parameters (internal angle of shearing resistance, f and cohesion, c) are required for geotechnical design. If water is present in soil, applied loads are carried in the short term by pore water pressures. For granular soils above the water table, pore water pressures dissipate almost immediately as the water drains away and the loads are effectively carried by the soil structure. However, for fine grained soils, which are not as free draining, pore water pressures take much longer to dissipate. Water and pore water pressures affect the strength and settlement characteristics of soil. The engineer must distinguish between undrained conditions (short-term loading, where pore water pressures are present and design is carried out for total stresses on the basis of fu and Cu) and drained conditions (long-term loading, where pore water pressures have dissipated and design is carried out for effective stresses on the basis of f' and c').

Drained conditions, f' > 0

Shear stress τ φ′

Failure envelope

Over consolidated clays c′ > 0

Effective stress circle

c′ = 0 generally

Direct stress σ

Geotechnics

97

Approximate correlation of properties for drained granular soils
Description SPT* N blows 0±4 4±10 10±30 30±50 >50 Effective internal angle of shearing resistance f' 26±28 28±30 30±36 36±42 42±46 Bulk unit weight g kN/m3 35 mm special detailing is required to reduce the risk of spalling.

Source: BS 8110: Part 1: 1997.

Preliminary sizing of concrete elements
Typical span/depth ratios
Element Typical spans m 5±6 6±11 4±8 6±14 8±14 9±10 3±10 5±15 2.5±8 2±4 2±8 Overall depth or thickness Simply supported L/22±30 L/24±35 L/27 L/23 L/15±20 L/35±40 L/12 L/10 H/10±20 H/30±35 ± Continuous L/28±36 L/34±40 L/36 L/31 L/18±24 L/38±45 L/15 L/12 H/10±20 H/45 ± Cantilever L/7±10 ± L/7±10 L/9 L/7 L/10±12 L/6 L/6 H/10 H/15±18 H/10±14

One way spanning slabs Two way spanning slabs Flat slabs Close centre ribbed slabs (ribs at 600 mm c/c) Coffered slabs (ribs at 900±1500 mm c/c) Post tensioned flat slabs Rectangular beams (width >250 mm) Flanged beams Columns Walls Retaining walls

NOTE: 125 mm is normally the minimum concrete floor thickness for fire resistance.

Reinforced Concrete

181

Preliminary sizing
Beams Although the span/depth ratios are a good indication, beams tend to need more depth to fit sufficient reinforcement into the section in order to satisfy deflection requirements. Check the detailing early ± especially for clashes with steel at column/beam junctions. The shear stress should be limited to 2 N/mm2 for preliminary design. Solid slabs Two way spanning slabs are normally about 90% of the thickness of one way spanning slabs. Profiled slabs Obtain copies of proprietary mould profiles to minimize shuttering costs. The shear stress in ribs should be limited to 0.6 N/mm2 for preliminary design. Columns A plain concrete section with no reinforcement can take an axial stress of about 0.45Fcu. The minimum column dimensions for a stocky braced column ˆ clear column height/17.5. A simple allowance for moment transfer in the continuous junction between slab and column can be made by factoring up the load from the floor immediately above the column being considered (by 1.25 for interior, 1.50 for edge and 2.00 for corner columns). The column design load is this factored load plus any other column loads. For stocky columns, the column area (Ac) can be estimated by: Ac ˆ N/15, N/18 or N/21 for columns in RC35 concrete containing 1%, 2% or 3% high yield steel respectively.

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Structural Engineer's Pocket Book

Reinforcement
The ultimate design strength is fy ˆ 250 N/mm2 for mild steel and fy ˆ 460 N/mm2 high yield reinforcement.

Weight of reinforcement bars by diameter (kg/m)
6 mm 0.222 8 mm 0.395 10 mm 0.616 12 mm 0.888 16 mm 1.579 20 mm 2.466 25 mm 3.854 32 mm 6.313 40 mm 9.864

Reinforcement area (mm2) for groups of bars
Number of bars Bar diameter mm 6 1 2 3 4 5 6 7 8 9 28 57 85 113 141 170 198 226 254 8 50 101 151 201 251 302 352 402 452 10 79 157 236 314 393 471 550 628 707 12 113 226 339 452 565 679 792 905 1018 16 201 402 603 804 1005 1206 1407 1608 1810 20 314 628 942 1257 1571 1885 2199 2513 2827 25 491 982 1473 1963 2454 2945 3436 3927 4418 32 804 1608 2413 3217 4021 4825 5630 6434 7238 40 1257 2513 3770 5027 6283 7540 8796 10 053 11 310

Reinforcement area (mm2/m) for different bar spacing
Spacing mm Bar diameter mm 6 50 75 100 125 150 175 200 225 250 565 377 283 226 188 162 141 126 113 8 1005 670 503 402 335 287 251 223 201 10 1571 1047 785 628 524 449 393 349 314 12 2262 1508 1131 905 754 646 565 503 452 16 4021 2681 2011 1608 1340 1149 1005 894 804 20 6283 4189 3142 2513 2094 1795 1571 1396 1257 25 9817 6545 4909 3927 3272 2805 2454 2182 1963 32 ± 10 723 8042 6434 5362 4596 4021 3574 3217 40 ± ± 12 566 10 053 8378 7181 6283 5585 5027

Reinforced Concrete

183

Reinforcement mesh to BS 4483
Fabric reference Longitudinal wires Diameter mm Pitch mm Area mm2/m 393 252 193 142 98 1131 785 503 385 283 196 785 636 503 385 283 98 49 Cross wires Diameter mm 10 8 7 6 5 8 8 8 7 7 7 6 6 5 5 5 5 2.5 Cross wires Width 2.4 m Pitch mm 200 200 200 200 200 200 200 200 200 200 200 400 400 400 400 400 200 100 Area mm2/m 393 252 193 142 98 252 252 252 193 193 193 70.8 70.8 49 49 49 98 49 Mass kg/m2

Square mesh ± High tensile steel A393 10 200 A252 8 200 A193 7 200 A142 6 200 A98 5 200 Structural mesh ± High tensile steel B131 12 100 B785 10 100 B503 8 100 B385 7 100 B283 6 100 B196 5 100 Long mesh ± High tensile steel C785 10 100 C636 9 100 C503 8 100 C385 7 100 C283 6 100 Wrapping mesh ± Mild steel D98 5 D49 2.5 Stock sheet size 200 100

6.16 3.95 3.02 2.22 1.54 10.9 8.14 5.93 4.53 3.73 3.05 6.72 5.55 4.34 3.41 2.61 1.54 0.77 Sheet area 11.52 m2

Longitudinal wires Length 4.8 m

Source: BS 4486: 1985.

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Shear link reinforcement areas
Shear link area, Asv mm2 No. of link legs Link diameter mm 6 8 10 2 56 100 158 226 3 84 150 237 339 4 112 200 316 452 6 168 300 474 678 Shear link area/link bar spacing, Asv /Sv mm2/mm Link spacing, Sv mm 12 100 0.560 1.000 1.580 2.260 0.840 1.500 2.370 3.390 1.120 2.000 3.160 4.520 1.680 3.000 4.740 6.780 125 0.448 0.800 1.264 1.808 0.672 1.200 1.896 2.712 0.896 1.600 2.528 3.616 1.344 2.400 3.792 5.424 150 0.373 0.667 1.053 1.507 0.560 1.000 1.580 2.260 0.747 1.333 2.107 3.013 1.120 2.000 3.160 4.520 175 0.320 0.571 0.903 1.291 0.480 0.857 1.354 1.937 0.640 1.143 1.806 2.583 0.960 1.714 2.709 3.874 200 0.280 0.500 0.790 1.130 0.420 0.750 1.185 1.695 0.560 1.000 1.580 2.260 0.840 1.500 2.370 3.390 225 0.249 0.444 0.702 1.004 0.373 0.667 1.053 1.507 0.498 0.889 1.404 2.009 0.747 1.333 2.107 3.013 250 0.224 0.400 0.632 0.904 0.336 0.600 0.948 1.356 0.448 0.800 1.264 1.808 0.672 1.200 1.896 2.712 275 0.204 0.364 0.575 0.822 0.305 0.545 0.862 1.233 0.407 0.727 1.149 1.644 0.611 1.091 1.724 2.465 300 0.187 0.333 0.527 0.753 0.280 0.500 0.790 1.130 0.373 0.667 1.053 1.507 0.560 1.000 1.580 2.260

Reinforced Concrete

185

Concrete design to BS 8110
Partial safety factors for ultimate limit state
Load combination Load type Dead Adverse Dead and imposed (and earth and water pressure) Dead and wind (and earth and water pressure) Dead and wind and imposed (and earth and water pressure) 1.4 Beneficial 1.0 Live Adverse 1.6 Beneficial 0.0 Earth and water pressures 1.4 ± Wind

1.4

1.0

±

±

1.4

1.4

1.2

1.2

1.2

1.2

1.2

1.2

Effective depth
Effective depth, d, is the depth from compression face of section to the centre of area of the main reinforcement group allowing for layering, links and concrete cover.

Design of beams
Design moments and shears in beams with more than three spans
At outer support Near middle of end span WL 11 ± At first interior support ÀWL 9 W 16 At middle of interior span At interior supports

Moment Shear

0 W 2

WL 14 ±

ÀWL 12:5 5W 9

Source: BS 8110: Part 1: 1997.

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Structural Engineer's Pocket Book

Ultimate moment capacity of beam section

Mu ˆ 0.156Fcubd2 where there is less than 10% moment redistribution.

Factors for lever arm (z/d) and neutral axis (x/d) depth kˆ z/d x/d M Fcu bd2 0.043 0.050 0.070 0.090 0.110 0.130 0.145 0.156

0.950 0.13

0.941 0.15

0.915 0.19

0.887 0.25

0.857 0.32

0.825 0.39

0.798 0.45

0.777 0.50

Where z ˆ lever arm and x ˆ neutral axis depth.

Area of tension reinforcement for rectangular beams
If the applied moment is less than Mu, then the area of tension reinforcement, z ASrequired ˆ M/‰0:95…d†fy dŠ. If the applied moment is greater than Mu, then the area of compression steel is AHSrequired ˆ …K À 0:156† Fcu bd2 /‰0:95fy …d À dH †Š and the area of tension reinforcement is,
H ASrequired ˆ 0:156Fcu bd2 /‰0:95fy zŠ ‡ As if redistribution is less than 10%.

Equivalent breadth and depth of neutral axis for flanged beams
Flanged beams T beam L beam Simply supported bw ‡ L /10 bw ‡ L /5 Continuous bw ‡ L /7 bw ‡ L /13 Cantilever bw bw

Where bw ˆ breadth of web, L ˆ actual flange width or beam spacing, hf is the depth of the flange.

Calculate k using bw. From k, calculate 0.9x from the tabulated values of the neutral axis depth, x/d. If 0.9x hf, the neutral axis is in the beam flange and steel areas can be calculated as rectangular beams. If 0.9x > hf, the neutral axis is in the beam web and steel areas can be calculated as BS 8110: clause 3.4.4.5. Source: BS 8110: Part 1: 1997.

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Shear stresses in beams
The applied shear stress is  ˆ V/bn d.

Shear capacity of concrete
The shear capacity of concrete, vc, relates to the section size, effective depth and percentage reinforcement.

1.3 1.2 Shear capacity,Vc (N/mm2) 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0 0.5 1.0 1.5 2.0 2.5 100As % bd 3.0 3.5 4.0 d = 125 (mm) d = 150 d = 175 d = 200 d = 225 d = 250 d = 300 d = 400

Form and area of shear reinforcement in beams
Value of applied shear stress v (N/mm2) v < 0.5vc throughout beam Form of shear reinforcement to be provided Minimum links should normally be provided other than in elements of minor importance such as lintels, etc. Minimum links for whole length of beam to provide a shear resistance of 0.4 N/mm2 Links provided for whole length of beam at no more than 0.75d spacing along the span. No tension bar should be more than 150 mm from a vertical shear link leg* Area of shear reinforcement to be provided (mm2) Suggested minimum in fyv ˆ 250 N/mm2 ASn > 0:2bn d 100

0.5vc < v < (0.4 ‡ vc) …0:4 0:8 p ‡ vc † < v < 2 F cu or 5 N=mm

Asn > Asn >

0:4bn d 0:95fyn bn Sn …n À nc † 0:95fyn

*Bent-up bars can be used in combination with links as long as no more than 50% of the shear resistance comes from the bent-up bars as set out in BS 8110: clause 3.4.5.6.

Source: BS 8110: Part 1: 1997.

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Design of solid slabs
Solid slabs are supported on walls or beams. With simple supports the applied moment is about M ˆ wlx ly =24 allowing for bending in two directions, where lx and ly can be different span lengths.

Design moments and shear forces for a one way spanning continuous solid slab
End support/slab connection Simple support At outer support Moment 0 W 2:5 Near middle of end span WL 11:5 ± Continuous At outer support ÀWL 25 6W 13 Near middle of end span WL 13 ± ÀWL 11:5 3W 5 WL 15:5 ± ÀWL 15:5 W 2 At first interior support At middle of interior span At interior supports

Shear

Where W is the load on one span and L is the length of one span.

Design moments for a two way spanning continuous solid slab
Where ly =lx 1:5 the following formulae and coefficients can be used to calculate moments in orthogonal directions Mx ˆ bxWlx and My ˆ byWly for the given edge conditions:
Type of panel Moments considered* Coefficient bx for short span lx Coefficient by for long span ly

ly ˆ 1:0 lx
Interior panel Continuous edge Midspan À1 32 1 41 À1 26 1 34 À1 26 1 33 À1 21 1 27

ly ˆ 1:2 lx
À1 23 1 29 À1 20 1 26 À1 17 1 22 À1 15 1 20

ly ˆ 1:5 lx
À1 19 1 25 À1 17 1 23 À1 13 1 18 À1 12 1 17 À1 31 1 41 À1 27 1 35 À1 27 1 35 À1 22 1 29

One short edge discontinuous

Continuous edge Midspan

One long edge discontinuous

Continuous edge Midspan

Two adjacent edges discontinuous

Continuous edge Midspan

* These moments apply to the full width of the slab in each direction. The area of reinforcement to be provided top and bottom, both ways, at corners where the slab is not continuous ˆ 75% of the reinforcement for the short span, across a width lx =5 both ways.

Form and area of shear reinforcement in solid slabs
The allowable shear stress, vc, is the same as that calculated for beams, but the slab section should be sized to avoid shear reinforcement. If required, Table 3.16 in BS 8110 sets out the reinforcement requirements. Source: BS 8110: Part 1: 1997.

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189

Design of flat slabs
Flat slabs are solid slabs on concrete which sit on points or columns instead of linear wall or beam supports. Slab depth should be selected to satisfy deflection requirements and to resist shear around the column supports. Any recognized method of elastic analysis can be used, but BS 8110 suggests that the slabs be split into bay-wide subframes with columns or sections of columns projecting above and below the slab.

Simplified bending moment analysis in flat slabs
A simplified approach is permitted by BS 8110 which allows moments to be calculated on the basis of the values for onep way spanning solid slabs on continuous supports less the value of 0.15Whc where hc ˆ 4Acol =p and Acol ˆ column area. Alternatively, the following preliminary moments for regular grid with a minimum of three bays can be used for feasibility or preliminary design purposes only:

Preliminary target moments and forces for flat slab design
End support/slab connection Simple support Continuous At first At middle At interior interior of interior supports support span

At outer Near At outer Near support middle support middle of end of end span span Column strip moments Middle strip moments 0 WL2 11 WL2 11 ÀWL2 20 ÀWL2 20 WL2 10 WL2 10 À2WL2 13 ÀWL2 20 WL2 11 WL2 11 À2WL2 15 ÀWL2 20

0

W is a UDL in kN/m2, L is the length of one span and M is in kNm/m width of slab.

Moment transfer between the slab and exterior columns is limited to Mt max. ˆ 0.15Fcubed2 where be depends on the slab to column connection as given in Figure 3.13 in BS 8110. Subframe moments may need to be adjusted to keep the assumed moment transfer within the value of Mt max.

Distribution of bending moments in flat slabs
The subframes used in the analysis are further split into middle and column strips. Loads are more concentrated on the column strips. Typically, for hogging (negative) moments, 75% of the total subframe design moment will be distributed to the column strip. For sagging (positive) moments, 55% of the total subframe design moment will be distributed to the column strip. Special provision must be made for holes in panels and panels with marginal beams or supporting walls. BS 8110 suggests that where ly =lx 2:0, column strips are normally lx =2 wide centred on the grid. The slab should be detailed so that 66 per cent of the support reinforcement is located in the width lx =4 centred over the column.

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Punching shear forces in flat slabs
The critical shear case for flat slabs is punching shear around the column heads. The basic shear, V, is equal to the full design load over the area supported by the column which must be converted to effective shear forces to account for moment transfer between the slab and columns. For slabs with equal spans, the effective shears are: Veff ˆ 1.15V for internal columns, Veff ˆ 1.25V for corner columns and Veff ˆ 1.25V for edge columns for moments parallel to the slab edge or Veff ˆ 1.4V edge columns for moments perpendicular to the slab edge.

Punching shear checks in flat slabs
The shear stress at the column face should be checked: o ˆ Veff =Uo d (where Uo is the p column perimeter in contact with the slab). This should be less than the lesser of 0.8 Fcu or 5 N/mm2. Perimeters radiating out from the column should then be checked: i ˆ Veff =Ui d where Ui is the perimeter of solid slab spaced off the column. The first perimeter checked (i ˆ 1) is spaced 1.5d from the column face with subsequent shear perimeters spaced at 0.75d intervals. Successive perimeters are checked until the applied shear stress is less than the allowable stress, vc. BS 8110: clause 3.7.76 sets out the detailing procedure and gives rules for the sharing of shear reinforcement between perimeters. The position of the column relative to holes and free edges must be taken into account when calculating the perimeter of the slab/column junction available to resist the shear force.

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191

Stiffness and deflection
BS 8110 gives basic span/depth ratios which limit the total deflection to span/250 and live load and creep deflections to the lesser of span/500 or 20 mm, for spans up to 10 m.

Basic span/depth ratios for beams
Support conditions Rectangular sections bw b = 1.0 7 20 26 Flanged section bw b £ 0.3 5.6 16.0 20.8

Cantilever Simply supported Continuous

For values of bw =b > 0:3 linear interpolation between the flanged and rectangular values is permitted.

Allowable span/depth ratio
Allowable span/depth ˆ F1  F2  F3  F4  Basic span/depth ratio Where: F1 F2 F3 F4 modification factor to reduce deflections in beams with spans over 10 m. F1 ˆ 10=span, where F1 < 1.0 modification for tension reinforcement modification for compression reinforcement

modification for stair waists where the staircase occupies at least 60% of the span and there is no stringer beam, F4 ˆ 1.15 2fy As required The service stress in the bars, fs ˆ 3As provided Source: BS 8110: Part 1: 1997.

Modification factor for tension reinforcement
2.0 1.9

192

Modification factor for tension reinforcement F2

1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 100 120 140 160 180 200 220 240 260 280 3.0 4.0 5.0 6.0 0.75 1.0

M = 0.5 bd 2

1.5 2.0

Service stress, fs =

2fyAs req 3As prov

(N/mm2)

Modification factor for compression reinforcement
1.6 Modification factor for compression reinforcement, F3

1.5

1.4

1.3

1.2

1.1

1.0 0 0.5 1.0 1.5 2.0 ′ 100As bd 2.5 3.0 3.5 4.0

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Columns
Vertical elements (of clear height, l, and dimensions, b  h) are considered as columns if h > 4b, otherwise they should be considered as walls. Generally the clear column height between restraints should be less than 60b. It must be established early in the design whether the columns will be in a braced frame where stability is to be provided by shear walls or cores, or whether the columns will be unbraced, meaning that they will maintain the overall stability for the structure. This has a huge effect on the effective length of columns, le ˆ bl, as the design method for columns depends on their slenderness, lex =b or ley =h. A column is considered `stocky' if the slenderness is less than 15 for braced columns or 10 for unbraced columns. Columns exceeding these limits are considered to be `slender'.

Effective length coefficient (b) for columns
End condition at top of column End condition at base of column Condition 1 Condition 2 Condition 3

Braced Unbraced Braced Unbraced Braced Unbraced Condition 1 'Moment' connection to a beam or foundation which is at least as deep as the column dimension* Condition 2 'Moment' connection to a beam or foundation which is shallower than the column dimension* Condition 3 'Pinned' connection Condition 4 'Free' end 0.75 1.20 0.80 1.30 0.90 1.60

0.80

1.30

0.85

1.50

0.95

1.80

0.90 n/a

1.60 2.20

0.95 n/a

1.80 n/a

1.00 n/a

n/a n/a

*Column dimensions measured in the direction under consideration.

Source: BS 8110: Part 1: 1997.

Framing moments transferred to columns

M FU = Me KU

KU KL + KU + 0.5KB Unfactored dead load WD kN/m KB1 KL KL + KU + 0.5KB KL KU

KU M FU = Mes KL + KU + 0.5K1 + 0.5K2 Total factored load WT kN/m KB2 KL M FL = Mes KL + KU + 0.5K1 + 0.5K2

W kN/m

KB KL M FL = Me

Stiffness, k = I L Me = Fixed end beam moment Mes =Total out of balance fixed end moment

195

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Column design methods
Column design charts must be used where the column has to resist axial and bending stresses. Stocky columns need only normally be designed for the maximum design moment about one axis. The minimum design moment is the axial load multiplied by the greater of the eccentricity or h/20 in the plane being considered. If a full frame analysis has not been carried out, the effect of moment transfer can be approximated by using column subframes or by using increasing axial loads by 10% for symmetrical simply supported loads. Where only a nominal eccentricity moment applies, stocky columns carrying axial load can be designed for: N ˆ 0.4fcu Ac ‡ 0.75Asfy. Slender columns can be designed in the same way as short columns, but must resist an additional moment due to eccentricity caused by the deflection of the column as set out in clause 3.8.3 of BS 8110.

Biaxial bending in columns
When it is necessary to consider biaxial moments, the design moment about one axis is enhanced to allow for the biaxial bending effects and the column is designed about the enhanced axis. Where M is the applied moment, dx is the effective depth across the x±x axis and dy is the effective depth across the y±y axis: If If
Mx My x ! dy the increased moment about the x±x axis is Mxenhanced ˆ Mx ‡ d

bdx My dy .

My dy M x 40e Slender if D/t > 80e2 t ¼ plate thickness, d ¼ web depth, py ¼ design strength,

For simplicity only design methods for Class 1 and 2 sections are covered in this book. Source: BS 5950: Part 1: 2000.

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Tension members
Bolted connections: Pt ¼ (Ae À 0.5a2) py Welded connections: Pt ¼ (Ae À 0.3a2) py If a2 ¼ Ag À a1 where Ag is the gross section area, Ae is the effective area (which is the net area multiplied by 1.2 for S275 steel, 1.1 for S355 or 1.0 for S460) and a1 is the area of the connected part (web or flange, etc.).

Flexural members
Shear capacity, Pv
Pv ¼ 0.6py Av Where Av is the shear area, which should be taken as: tD AD=ðD þ BÞ t (D À T ) 0.6A 0.9A for rolled I sections (loaded parallel to the web) and rolled T sections for rectangular hollow sections for welded T sections for circular hollow sections solid bars and plates

t ¼ web thickness, A ¼ cross sectional area, D ¼ overall depth, B ¼ overall breadth, T ¼ flange thickness.

If d=t > 70 for a rolled section, or >62 for a welded section, shear buckling must be allowed for (see BS 5950: clause 4.4.5). Source: BS 5950: Part 1: 2000.

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Moment capacity MC
The basic moment capacity (Mc) depends on the provision of full lateral restraint and the interaction of shear and bending stresses. Mc is limited to 1.2py Z to avoid irreversible deformation under serviceability loads. Full lateral restraint can be assumed if the construction fixed to the compression flange is capable of resisting not less than 2.5% of the maximum compression force in the flange, distributed uniformly along the length of the flange. Moment capacity (Mc) is generally the controlling capacity for class 1 and 2 sections in the following cases:
. . . .

Bending about the minor axis. CHS, SHS or small solid circular or square bars. RHS in some cases given in clause 4.3.6.1 of BS 5950. UB, UC, RSJ, PFC, SHS or RHS if  < 34 for S275 steel and  < 30 for S355 steel in Class 1 and 2 sections, where  ¼ LE =rÁ Mc ¼ pyS

Low shear (Fv < 0.6Pv)
F Where  ¼ 2 Pv À 1 v Pv. 2

High shear (Fv > 0.6Pv) Mc ¼ py (S À rSn)  

and Sv ¼ the plastic modulus of the shear area used to calculate

Lateral torsional buckling capacity Mb
Lateral torsional buckling (LTB) occurs in tall sections or long beams in bending if not enough restraint is provided to the compression flange. Instability of the compression flange results in buckling of the beam, preventing the section from developing its full plastic capacity, Mc. The reduced bending moment capacity, Mb, depends on the slenderness of the section, LT. For Class 1 and 2 sections, LT ¼ .

A simplified and conservative method of calculating Mb for rolled sections uses D=T and LT to determine an ultimate bending stress pb (from the following graph) where Mb ¼ pbSx for Class 1 and 2 sections. Source: BS 5950: Part 1: 2000.

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Ultimate bending strengths for rolled sections, pb

270 260 250 240 230 220 210 Ultimate bending stress, pb (N/mm2) 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 25 50 75 100 125 150 175 200 225
20 25 30 35 40 45 50 15 10 D = 5 T

250

Slenderness (Le/ry)

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Compression members
The compression capacity of Class 1 and 2 sections can be calculated as Pc ¼ Agpc, where Ag is the gross area of the section and pc can be estimated depending on the expected buckling axis and the section type for steel of 40 mm thickness.

Type of section

Strut curve for value of pc Axis of buckling x–x y–y a b c b a a b b Any axis: c

Hot finished structural hollow section Rolled I section Rolled H section Round, square or flat bar Rolled angle, channel or T section/paired rolled sections/compound rolled sections Ultimate compression stresses for rolled sections, pc

Ultimate compression stresses for rolled sections, pc

280 260 240 220 Ultimate compressive stress, pc (N/mm2) 200 180 160 140 120 100 80 60 40 20 0 50 100 150 200 250 Slenderness (Le/ry) 300 c b a Strut curve

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Combined bending and compression
Although each section should have its classification checked for combined bending and axial compression, the capacities from the previous tables can be checked against the following simplified relationship for section Classes 1 and 2:

My F Mx þ < 1:0 þ P Mcx or Mb Mcy

Section 4.8 in BS 5950 should be referred to in detail for all the relevant checks.

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Connections
Welded connections

W

W

The of  resultant2 combined longitudinal and transverse forces should be checked:  FL 2 FT þ < 1:0 : PL PT

Ultimate fillet weld capacities for S275 elements joined at 90°
Leg length s mm 4 6 8 12 Throat thickness a ¼ 0.7s mm 2.8 4.2 5.6 8.4 Longitudinal capacity* PL ¼ pw kN/mm 0.616 0.924 1.232 1.848 Transverse capacity* L ¼ pwaK kN/mm 0.770 1.155 1.540 2.310

* Based on values for S275, pw ¼ 220 N/mm2 and K ¼ 1.25.

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Bolted connections
Limiting bolt spacings
1.25D
2.5D 1.25D 2.5D 1.25D Rolled, machine cut or flame cut, sawn or planed edge.

Direct shear
W Single shear W

W Double shear

W

Simple moment connection bolt groups e P
F1 F2 F3

X4 X3

X2 X1

F4

X3 X4

X2

X1

  P Mcap ¼ no. rows1 of bolts Pt x2 i x V ¼ Pt n xn Fn ¼ Pt xnÀ1

Where x1 ¼ max xi and xi ¼ depth from point of rotation to centre of bolt being considered, Pt is the tension capacity of the bolts, n is the number of bolts, V is the shear on each bolt and F is the tension in each bolt. This is a simplified analysis which assumes that the bolt furthest from the point of rotation carries the most load. As the connection elements are likely to be flexible, this is unlikely to be the case; however, more complicated analysis requires a computer or standard tables. For bolts in shear or tension see the following tabulated values. For bolts in shear and tension check: ðFv =Pv Þ þ ðFt =Pt Þ 1:4 where F indicates the factored design load and P indicates the ultimate bolt capacity.

Bolt capacity checks

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Selected ultimate bolt capacities for non-pre-loaded ordinary bolts in S275 steel
Diameter of bolt, f mm Tensile Tension Shear stress capacity capacity area kN Single Double mm2 kN kN 3.9 7.0 11.1 16.2 30.1 47.0 67.8 107.7 9.0 16.4 26.0 37.8 70.3 109.8 158.1 251.3 3.2 5.9 9.3 13.5 25.1 39.2 56.5 89.8 7.5 13.7 21.8 31.6 58.9 91.9 132.4 210.4 6.4 11.7 18.6 27.0 50.2 78.4 113.0 179.5 15.1 27.5 43.5 63.2 117.8 183.8 264.8 420.8 Bearing capacity for end distance ¼ 2f kN Thickness of steel passed through mm 5 13.8 18.4 23.0 27.6 36.8 46.0 55.2 69.0 13.8 18.4 23.0 27.6 36.8 46.0 55.2 69.0 6 8 10 12 15 41.4 55.2 69.0 82.8 110.4 138.0 165.6 207.0 41.4 55.2 69.0 82.8 110.4 138.0 165.6 207.0 20 55.2 73.6 92.0 110.4 147.2 184.0 220.8 276.0 55.2 73.6 92.0 110.4 147.2 184.0 220.8 276.0

Grade 4.6 6 20.1 8 36.6 10 58 12 84.3 16 157 20 245 24 353 30 561 Grade 8.8 6 20.1 8 36.6 10 58 12 84.3 16 157 20 245 24 353 30 561

16.6 22.1 27.6 33.1 22.1 29.4 36.8 44.2 27.6 36.8 46.0 55.2 33.1 44.2 55.2 66.2 44.2 58.9 73.6 88.3 55.2 73.6 92.0 110.4 66.2 88.3 110.4 132.5 82.8 110.4 138.0 165.6 6.6 22.1 27.6 33.1 22.1 29.4 36.8 44.2 27.6 36.8 46.0 55.2 33.1 44.2 55.2 66.2 44.2 58.9 73.6 88.3 55.2 73.6 92.0 110.4 66.2 88.3 110.4 132.5 82.8 110.4 138.0 165.6

NOTES: 2 mm clearance holes for f < 24 or 3 mm clearance holes for f < 24. . Tabulated tension capacities are nominal tension capacity ¼ 0.8A p which accounts for prying forces. t t . Bearing values shown in bold are less than the single shear capacity of the bolt. . Bearing values shown in italic are less than the double shear capacity of the bolt. . Multiply tabulated bearing values by 0.7 if oversized or short slotted holes are used.
. . .

Multiply tabulated bearing values by 0.5 if kidney shaped or long slotted holes are used. Shear capacity should be reduced for large packing, grip lengths or long joints.

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Selected ultimate bolt capacities for non-pre-loaded countersunk bolts in S275 steel
Diameter of bolt, f mm Tensile Tension Shear stress capacity capacity area kN Single Double mm2 kN kN 3.9 7.0 11.1 16.2 30.1 47.0 67.8 9.0 16.4 26.0 37.8 70.3 109.8 158.1 3.2 5.9 9.3 13.5 25.1 39.2 56.5 7.5 13.7 21.8 31.6 58.9 91.9 132.4 6.4 11.7 18.6 27.0 50.2 78.4 113.0 15.1 27.5 43.5 63.2 117.8 183.8 264.8 Bearing capacity for end distance ¼ 2f kN Thickness of steel passed through (mm) 5 8.6 – – – – – – 8.6 – – – – – – 6 11.3 12.9 – – – – – 11.3 12.9 – – – – – 8 16.8 20.2 21.9 – – – – 16.8 20.2 21.9 – – – – 10 22.4 27.6 31.1 34.5 – – – 22.4 27.6 31.1 34.5 – – – 12 27.9 35.0 40.3 45.5 55.2 62.1 – 27.9 35.0 40.3 45.5 55.2 62.1 – 15 36.2 46.0 54.1 62.1 77.3 89.7 85.6 36.2 46.0 54.1 62.1 77.3 89.7 85.6 20 50.0 64.4 77.1 89.7 114.1 135.7 140.8 50.0 64.4 77.1 89.7 114.1 135.7 140.8

Grade 4.6 6 20.1 8 36.6 10 58 12 84.3 16 157 20 245 24 353 Grade 8.8 6 20.1 8 36.6 10 58 12 84.3 16 157 20 245 24 353

NOTES: . Values are omitted from the table where the bolt head is too deep to be countersunk into the thickness of the plate. . 2 mm clearance holes for f 63 mm for Grade 50 (S355) and t > 25 mm for Grade 55 (S460). The allowable axial stress, Pc, reduces as the slenderness of the element increases as shown in the following chart:

180

160

140

120 Allowable compressive stress, Pc (N/mm2)

100

80

60

40

20

0 50 100 150 200 250 300 350

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Allowable average shear stress Pv in unstiffened webs
Form Sections, bars, plates, wide flats and hollow sections Steel grade 43 (S275) 50 (S355) 55 (S460) Thickness mm d 40 40 < d d 63 63 < d d 25 100 100 Pv* N/mm2 110 100 140 130 170

* See Table 12 in BS 449: Part 2 for allowable average shear stress in stiffened webs.

Section capacity checks
Combined bending and axial load
Compression: fbc fc fbcx þ y þ Pc Pbcx Pbcy 1:0 and 1:0 fbc fbcx þ y Pbcx Pbcy

Tension:

ft fbt þ Pt Pbt

1:0

Combined bending and shear
 2 p 2 p 2 2 2 0 fe ¼ ðfbt þ 3fq Þ or fe ¼ ðfbc þ 3fq Þ and fe < Pe and ðfbc =Po Þ2 þ fq =P0q 1:25

0 Where fe is the equivalent stress, fq is the average shear stress in the web, Po is defined in 0 BS 449 subclause 20 item 2b iii and Pq is defined in clause 23. From BS 449: Table 1, the allowable equivalent stress Pe ¼ 250 N/mm2 for Grade 43 (S275) steel < 40 mm thick.

Combined bending, shear and bearing p 2 p 2 2 2 2 2 fe ¼ ðfbt þ fb þ fbt fb þ 3fq Þ or fe ¼ ðfbc þ fb þ fbc fb þ 3fq Þ and À Á2 À 0 0 Á2 À Á 1:25 fbc =Po þ fq =Pq þ fcw =Pcw fe < Pe and

Source: BS 449: Part 2: 1969.

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Connections
Selected fillet weld capacities for Grade 43 (S275) steel
Leg length s mm 4 6 8 12 Throat thickness a = 0.7s mm 2.8 4.2 5.6 8.4 Weld capacity* kN/mm 0.32 0.48 0.64 0.97

* When a weld is subject to a combination of stresses, the combined effect should be checked using the same checks as used for combined loads on sections to BS 449.

Selected full penetration butt weld capacities for Grade 43 (S275) steel
Thickness mm 6 15 20 30 Shear capacity kN/mm 0.60 1.50 2.00 3.00 Tension or compression capacity* kN/mm 0.93 2.33 3.10 4.65

* When a weld is subject to a combination of stresses, the combined effect should be checked using the same checks as used for combined loads on sections to BS 449. Source: BS 449: Part 2: 1969.

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Allowable stresses in non-pre-loaded bolts
Description Close tolerance and turned bolts Bolts in clearance holes Bolt grade 4.6 8.8 4.6 8.8 Axial tension N/mm2 120 280 120 280 Shear N/mm2 100 230 80 187 Bearing N/mm2 300 350 250 350

Allowable stresses on connected parts of bolted connections (N/mm2)
Description Allowable stresses on connected parts for different steel grades N/mm2 43 (S275) Close tolerance and turned bolts Bolts in clearance holes Source: BS 449: Part 2: 1969. 300 250 50 (S355) 420 350 55 (S460) 480 400

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Selected working load bolt capacities for non-pre-loaded ordinary bolts in grade 43 (S275) steel
Diameter of bolt, f mm Tensile Tension Shear stress capacity capacity area kN Single Double mm2 kN kN Bearing capacity for end distance ¼ 2f kN Thickness of steel passed through 5 6 8 10 12 15 20

Grade 4.6 6 8 10 12 16 20 24 30 Grade 8.8 6 8 10 12 16 20 24 30 20.1 36.6 58 84.3 157 245 353 561 4.5 8.2 13.0 18.9 35.2 54.9 79.1 125.7 3.8 6.8 10.8 15.8 29.4 45.8 66.0 104.9 7.5 13.7 21.7 31.5 58.7 91.6 132.0 209.8 7.5 10.0 12.5 15.0 20.0 25.0 9.0 12.0 15.0 18.0 24.0 30.0 12.0 16.0 20.0 24.0 32.0 40.0 15.0 20.0 25.0 30.0 40.0 50.0 18.0 24.0 30.0 36.0 48.0 60.0 22.5 30.0 30.0 40.0 37.5 50.0 45.0 60.0 60.0 80.0 75.0 100.0 20.1 36.6 58 84.3 157 245 353 561 1.9 3.5 5.6 8.1 15.1 23.5 33.9 53.9 1.6 2.9 4.6 6.7 12.6 19.6 28.2 44.9 3.2 5.9 9.3 13.5 25.1 39.2 56.5 89.8 7.5 10.0 12.5 15.0 20.0 25.0 30.0 9.0 12.0 15.0 18.0 24.0 30.0 36.0 12.0 16.0 20.0 24.0 32.0 40.0 48.0 15.0 20.0 25.0 30.0 40.0 50.0 60.0 18.0 24.0 30.0 36.0 48.0 60.0 72.0 22.5 30.0 30.0 40.0 37.5 50.0 45.0 60.0 60.0 80.0 75.0 100.0 90.0 120.0

37.5 45.0 60.0 75.0 90.0 112.5 150.0

30.0 36.0 48.0 60.0 72.0 90.0 120.0 37.5 45.0 60.0 75.0 90.0 112.5 150.0

NOTES: . 2 mm clearance holes for f < 24 or 3 mm clearance holes for f < 24. . Bearing values shown in bold are less than the single shear capacity of the bolt. . Bearing values shown in italic are less than the double shear capacity of the bolt. . Multiply tabulated bearing values by 0.7 if oversized or short slotted holes are used. . Multiply tabulated bearing values by 0.5 if kidney shaped or long slotted holes are used.
.

Shear capacity should be reduced for large packing, grip lengths or long joints.

Bolted connection capacity check for combined tension and shear f t fs þ Pt Ps 1:4

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Stainless steel to BS 5950
Stainless steels are a family of corrosion and heat resistant steels containing a minimum of 10.5% chromium which results in the formation of a very thin self-healing transparent skin of chromium oxide – which is described as a passive layer. Alloy proportions can be varied to produce different grades of material with differing strength and corrosion properties. The stability of the passive layer depends on the alloy composition. There are five basic groups: austenitic, ferritic, duplex, martensitic and precipitation hardened. Of these, only austenitic and Duplex are really suitable for structural use.

Austenitic
Austenitic is the most widely used for structural applications and contains 17–18% chromium, 8–11% nickel and sometimes molybdenum. Austenitic stainless steel has good corrosion resistance, high ductility and can be readily cold formed or welded. Commonly used alloys are 304L (European grade 1.4301) and 316L (European grade 1.4401).

Duplex
Duplex stainless steels are so named because they share the strength and corrosion resistance properties of both the austenitic and ferritic grades. They typically contain 21–26% chromium, 4–8% nickel and 0.1–4.5% molybdenum. These steels are readily weldable but are not so easily cold rolled. Duplex stainless steel is normally used where an element is under high stress in a severely corrosive environment. A commonly used alloy is Duplex 2205 (European grade 1.44062).

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Material properties
The material properties vary between cast, hot rolled and cold rolled elements. Density Tensile strength Poisson’s ratio Modulus of elasticity 78–80 kN/m3 200–450 N/mm2 0.2% proof stress depending on grade. 0.3 E varies with the stress in the section and the direction of the stresses. As the stress increases, the stiffness decreases and therefore deflection calculations must be done on the basis of the secant modulus. 76.9 kN/mm2 17 Â 10À6/ C for 304L (1.4301) 16.5 Â 10À6/ C for 316L (1.4401) 13 Â 10À6/ C for Duplex 2205 (1.4462)

Shear modulus Linear coefficient of thermal expansion Ductility

Stainless steel is much tougher than mild steel and so BS 5950 does not apply any limit on the thickness of stainless steel sections as it does for mild steel.

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Elastic properties of stainless steel alloys for design
The secant modulus, Es ¼ Esi ¼   E m 
1þk
f1 or 2 Py

ðEs1 þ Es2 Þ , where 2

where i = 1 or 2, k ¼ 0:002E=Py and m is a constant. Values of the secant modulus are calculated below for different stress ratios ðfi =Py Þ

Values of secant modulus for selected stainless steel alloys for structural design
Stress Secant modulus ratio* fi Py kN/mm2 304L 316L Duplex 2205

Longitudinal Transverse Longitudinal Transverse Longitudinal Transverse 0.0 0.2 0.3 0.4 0.5 0.6 0.7 200 200 199 197 191 176 152 200 200 200 200 198 191 173 190 190 190 188 184 174 154 195 195 195 195 193 189 174 200 200 199 196 189 179 165 205 205 204 200 194 183 168

* Where i ¼ 1 or 2 for the applied stress in the tension and compression flanges respectively.

Typical stock stainless steel sections
There is no UK-based manufacturer of stainless steel and so all stainless steel sections are imported. Two importers who will send out information on the sections they produce are Valbruna and IMS Group. The sections available are limited. IMS has a larger range including hot rolled equal angles (from 20 Â 20 Â 3 up to 100 Â 100 Â 10), unequal angles (20 Â 10 Â 3 up to 200 Â 100 Â 13), I beams (80 Â 46 up to 400 Â 180), H beams (50 Â 50 up to 300 Â 300), channels (20 Â 10 up to 400 Â 110) and tees (20 Â 20 Â 3 up to 120 Â 120 Â 13) in 1.4301 and 1.4571. Valbruna has a smaller selection of plate, bars and angles in 1.4301 and 1.4404. Source: Nickel Development Institute (1994).

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Durability and fire resistance
Suggested grades of stainless steel for different atmospheric conditions
Stainless steel grade 304L (1.4301) 316L (1.4401) Duplex 2205 (1.4462) O O O O O O O O 3 O O 3 O O O O 3 3 3 3 (3) 3 3 (3) Location Rural Urban Industrial Marine

Low Med High Low Med High Low Med High Low Med High 3 3 3 3 3 (3) (3) (3) X 3 (3) X

Where: 3 ¼ optimum specification, (3) ¼ may require additional protection, X ¼ unsuitable, O ¼ overspecified.

Note that this table does not apply to chlorinated environments which are very corrosive to stainless steel. Grade 304L (1.4301) can tarnish and is generally only used where aesthetics are not important; however, marine Grade 316L (1.4401) will maintain a shiny surface finish.

Corrosion mechanisms
Durability can be reduced by heat treatment and welding. The surface of the steel forms a self-healing invisible oxide layer which prevents ongoing corrosion and so the surface must be kept clean and exposed to provide the oxygen required to maintain the corrosion resistance. Pitting Mostly results in the staining of architectural components and is not normally a structural problem. However, chloride attack can cause pitting which can cause cracking and eventual failure. Alloys rich in molybdenum should be used to resist chloride attack. Crevice corrosion nuts and washers. Chloride attack and lack of oxygen in small crevices, e.g. between

Bi-metallic effects The larger the cathode, the greater the rate of attack. Mild steel bolts in a stainless steel assembly would be subject to very aggressive attack. Austenitic grades typically only react with copper to produce an unsightly white powder, with little structural effect. Prevent bi-metallic contact by using paint or tape to exclude water as well as using isolation gaskets, nylon/Teflon bushes and washers.

Fire resistance
Stainless steels retain more of their strength and stiffness than mild steels in fire conditions, but typically as stainless steel structure is normally exposed, its fire resistance generally needs to be calculated as part of a fire engineered scheme. Source: Nickel Development Institute (1994).

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Preliminary sizing
Assume a reduced Young’s modulus depending on how heavily stressed the section will be and assume an approximate value of maximum bending stress for working loads of 130 N/mm2. A section size can then be selected for checking to BS 5950.

Stainless steel design to BS 5950: Part 1
The design is based on ultimate loads calculated on the same partial safety factors as for mild steel.

Ultimate mechanical properties for stainless steel design to BS 5950
Alloy type Steel designation European grade (UK grade) Minimum 0.2% proof stress N/mm2 Basic austenitic Molybdenum austenitic2 Duplex
1

Ultimate tensile strength N/mm2

Minimum elongation after fracture %

X5CrNi 18-9 X2CrNiMo 17-12-2 X2CrNi MoN 22-5-3

304L (1.4301) 316L (1.4401) Duplex 2205 (1.4462)

210 220 460

520–720 520–670 640–840

45 40 20

NOTES: 1. Most commonly used for structural purposes. 2. Widely used in more corrosive situations. The alloys listed in the table above are low carbon alloys which provide good corrosion resistance after welding and fabrication. As for mild steel, the element cross section must be classified to BS 5950: Part 1 in order to establish the appropriate design method. Generally this method is as given for mild steels; however, as there are few standard section shapes, the classification and design methods can be laborious. Source: Nickel Development Institute (1994).

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Connections
Bolted and welded connections can be used. Design data for fillet and butt welds requires detailed information about which particular welding method is to be used. The information about bolted connections is more general.

Bolted connections
Requirements for stainless steel fasteners are set out in BS EN ISO 3506 which split fixings into three groups: A = Austenitic, F = Ferritic and C = Martensitic. Grade A fasteners are normally used for structural applications. Grade A2 is equivalent to Grade 304L (1.4301) with a 0.2% proof stress of 210 N/mm2 and Grade A4 is equivalent to Grade 316L (1.4401) with a 0.2% proof stress of 450 N/mm2. There are three further property classes within Grade A: 50, 70 and 80 to BS EN ISO 3506. An approximate ultimate bearing strength for connected parts can be taken as 460 N/mm2 for preliminary sizing.

Ultimate stress values for bolted connection design
Grade A property class Shear strength* Bearing strength* Tensile strength* N/mm2 N/mm2 N/mm2 50 70 (most common) 80 140 310 380 510 820 1000 210 450 560

* These values are appropriate with bolt diameters less than M24 and bolts less than 8 diameters long. Sources: Nickel Development Institute (1994).

10
Composite Steel and Concrete
Composite steel and concrete flooring, as used today, was developed in the 1960s to economically increase the spans of steel framed floors while minimizing the required structural depths.

Composite flooring elements
Concrete slab There are various types of slab: solid in situ, in situ on profiled metal deck and in situ on precast concrete units. Solid slabs are typically 125±150 mm thick and require formwork. The precast and metal deck systems both act as permanent formwork, which may need propping to control deflections. The profiled metal deck sheets have a 50±60 mm depth to create a 115±175 mm slab, which can span 2.5 to 3.6 m. Precast concrete units 75±100 mm thick with 50±200 mm topping can span 3±8 m. Steelwork Generally the steel section is sized to support the wet concrete and construction loads with limited deflection, followed by the full design loading on the composite member. Secondary beams carry the deck and are in turn supported on primary beams which are supported on the columns. The steel beams can be designed as simply supported or continuous. Long span beams can be adversely affected by vibration, and should be used with caution in dynamic loading situations. Shear studs Typically 19 mm diameter and 95 mm or 120 mm tall. Other heights for deep profiled decks are available with longer lead times. Larger diameter studs are available but not many subcontractors have the automatic welding guns to fix them. Welded studs will carry about twice the load of proprietary `shot fired' studs. Economic arrangement Secondary beam spacing is limited to about arrangement 2.5±3 m in order to keep the slab thickness down and its fire resistance up. The most economic geometrical arrangement is for the primary span to be about 3/4 of the secondary span.

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Summary of material properties
The basic properties of steel and concrete are as set out in their separate sections. Concrete grade Normal weight concrete RC 30±50 and lightweight concrete RC 25±40. Density Modular ratio …aE ˆ ES =EC † Normal weight concrete 24 kN/m3 and lightweight concrete 17 kN/m3. Normal concrete Lightweight concrete Steel grade aE ˆ 6 short term and aE ˆ 18 long term. aE ˆ 10 short term aE ˆ 25 long term.

S275 is used where it is required in small quantities (less than 40 tonnes) or where deflection, not strength, limits the design. Otherwise S355 is more economical, but will increase the minimum number of shear studs which are required by the code.

Durability and fire resistance
. The basic durability requirements of steel and concrete are as set out in their separate . Concrete slabs have an inherent fire resistance. The slab thickness may be controlled by . . the minimum thickness required for fire separation between floors, rather than by deflection or strength. Reinforcing mesh is generally added to the top face of the slab to control surface cracking. The minimum required is 0.1 per cent of the concrete area, but more may be required for continuous spans or in some fire conditions. Additional bars are often suspended in the troughs of profiled metal decks to ensure adequate stability under fire conditions. Deck manufacturers provide guidance on bar areas and spacing for different slab spans, loading and thickness for different periods of fire resistance. Precast concrete composite planks have a maximum fire resistance of about 2 hours. The steel frame has to be fire and corrosion protected as set out in the section on structural steelwork. sections.

. .

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Preliminary sizing of composite elements
Typical span/depth ratios
Element Typical spans m Primary Universal beam sections Universal column sections Fabricated sections Fixed end/haunched beams (Haunch length L/10 with maximum depth 2D) Castellated beams (Circular holes f ˆ 2D/3 at about 1.5f c/c, D is the beam depth) Proprietary composite trusses n/a 6±16 L/17 L/20 6±10 6±10 >12 >12 Secondary 8±18 8±18 >12 >12 Total structural depth (including slab and beams) for simply supported beams Primary L/19 L/22 L/15 L/25 (midspan) Secondary L/23 L/29 L/25 L/32 (midspan)

>12

>12

L/12

L/16

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Preliminary sizing
Estimate the unfactored moment which will be applied to the beam in its final (rather than construction) condition. Use an allowable working stress of 160 N/mm2 for S275, or 210 N/mm2 for S355, to estimate the required section modulus ( Z ) for a non-composite beam. A preliminary estimate of a composite beam size can be made by selecting a steel beam with 60±70% of the non-composite Z. Commercial office buildings normally have about 1.8 to 2.2 shear studs (19 mm diameter) per square metre of floor area. Deflections, response to vibration and service holes should be checked for each case.

Approximate limits on holes in rolled steel beams
Reduced section capacity due to holes through the webs of steel beams must be considered for both initial and detailed calculations. Where D is the depth of the steel beam, limit the size of openings to 0.6D depth and 1.5D length in unstiffened webs, and to 0.7D and 2D respectively where stiffeners are provided above and below the opening. Holes should be a minimum of 1.5D apart and be positioned centrally in the depth of the web, in the middle third of the span for uniformly loaded beams. Holes should be a minimum of D from any concentrated loads and 2D from a support position. Should the position of the holes be moved off centre of depth of the beam, the remaining portions of web above and below the hole should not differ by a factor of 1.5 to 2.

Preliminary composite beam sizing tables for S275 and normal weight concrete
4 kN/m2 live loading + 1 kN/m2 for partitions
Primary span m Secondary span m No. of secondary beams per grid 2 2 2 3 3 3 3 3 3 4 4 4 4 4 4 Secondary beam spacing m 3.00 3.00 3.00 2.67 2.67 2.67 3.00 3.00 3.00 2.50 2.50 2.50 3.00 3.00 3.00 Beam sizes for minimum steel weight Primary beam 457 Â 152 UB 67 533 Â 210 UB 92 610 Â 229 UB 101 533 Â 210 UB 92 610 Â 229 UB 125 762 Â 267 UB 147 610 Â 229 UB 101 686 Â 254 UB 140 762 Â 267 UB 173 686 Â 254 UB 140 838 Â 292 UB 176 914 Â 305 UB 201 762 Â 267 UB 173 914 Â 305 UB 224 914 Â 305 UB 289 Secondary beam 406 Â 178 UB 54 610 Â 229 UB 113 762 Â 267 UB 173 356 Â 171 UB 57 610 Â 229 UB 101 762 Â 267 UB 173 406 Â 178 UB 54 610 Â 229 UB 113 762 Â 267 UB 173 356 Â 171 UB 57 610 Â 229 UB 101 762 Â 267 UB 173 406 Â 178 UB 54 610 Â 229 UB 113 762 Â 267 UB 173 Steel weight kN/m3 0.26 0.45 0.64 0.33 0.48 0.75 0.31 0.49 0.69 0.40 0.55 0.83 0.40 0.56 0.77 Beam sizes for minimum floor depth Primary beam Secondary beam Steel weight kN/m3 0.61 1.09 1.97 0.65 1.30 1.94 0.74 1.20 2.10 0.79 1.46 2.18 0.93 1.49 2.01

6

8 12 15 8 12 15 8 12 15 8 12 15 8 12 15

254 Â 254 UC 132 305 Â 305 UC 158 305 Â 305 UC 198 305 Â 305 UC 198 305 Â 305 UC 283 356 Â 406 UC 287 305 Â 305 UC 240 356 Â 406 UC 287 356 Â 406 UC 393 356 Â 406 UC 287 356 Â 406 UC 393 356 Â 406 UC 467 356 Â 406 UC 467 356 Â 406 UC 634 914 Â 419 UB 289

254 Â 254 UC 132 356 Â 406 UC 287 356 Â 406 UC 551 254 Â 254 UC 107 305 Â 305 UC 283 356 Â 406 UC 467 254 Â 254 UC 132 356 Â 406 UC 287 356 Â 406 UC 551 254 Â 254 UC 107 305 Â 305 UC 283 356 Â 406 UC 467 254 Â 254 UC 132 356 Â 406 UC 287 356 Â 406 UC 551

8

9

10

12

Check floor natural frequency 1.5 m as it will not contain impact loads as Class A to BS 6206.

Laminated glass floors and stair treads
UDL kN/m2 1.5 5.0 4.0 4.0 Point load kN 1.4 3.6 4.5 4.0 Glass thickness (top ‡ bottom annealed)* mm ‡ mm 19 ‡ 10 25 ‡ 15 25 ‡ 25 25 ‡ 10 Typical use

Domestic floor or stair Dance floor Corridors Stair tread

* Based on a floor sheet size of 1 m2 or a stair tread of 0.3 Â 1.5 m supported on four edges with a minimum bearing length equal to the thickness of the glass unit. The 1 m2 is normally considered to be the maximum size/weight which can be practically handled on site.

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Glass mullions or fins in toughened safety glass
Mullion height m 65%) Form of construction Comments

Type B ± RC to BS 8110 (with crack widths limited to 0.3 mm)

Provides integral protection and needs waterstops at construction joints. Medium risk. Consider ground chemicals for durability and effect on finishes. The BS 8102 description of a workshop is not as good as the workshop environment described in the Building Regulations Requires drainage to external basement perimeter below the level of the wall/floor membrane lap. Medium risk with multiple membrane layers and strict site control Provides integral protection and needs waterstops at construction joints. Medium risk. Consider ground chemicals for durability and effect on finishes. Additional tanking is likely to be needed to meet retail storage requirements Not recommended unless drainage is provided above the wall/floor membrane lap position and the site is relatively free draining. High risk Provides integral protection and needs waterstops at construction joints. Medium risk. Consider ground chemicals for durability and effect on finishes. Additional tanking is recommended A drained cavity allows the wall to leak and it is therefore foolproof. Sumps may need back-up pumps. High safety factor Unikely to be able to provide the controlled conditions required. Very high risk High risk. Medium risk with addition of a drainage cavity to reduce water penetration Medium risk. Addition of a water resistant concrete wall would provide the maximum possible safety for sensitive environments

2

No water penetration but moisture tolerable (typical relative humidity ˆ 35±50%)

Type A

Type B ± RC to BS8007

3

Ventilated residential and working areas, offices, restaurants and leisure centres

Dry environment, but no specific control on moisture vapour (typical relative humidity 40±60%)

Type A

Type B ± RC to BS 8007

Type C ± wall and floor cavity system 4 Archives and computer stores Totally dry environment with strict control of moisture vapour (typical relative humidity ˆ 35% for books ± 50% for art storage) Type A Type B ± RC to BS 8007 plus vapour barrier Type C ± wall and floor cavity system with vapour barrier to inner skin and floor cavity with DPM

NOTES: 1. Type A ˆ tanked construction, Type B ˆ integral structural waterproofing and Type C ˆ drained protection. 2. Relative humidity indicates the amount of water vapour in the air as a percentage of the maximum amount of water vapour which would be possible for air at a given temperature and pressure. Typical values of relative humidity for the UK are about 40±50% for heated indoor conditions and 85% for unheated external conditions.

Source: BS 8102: 1990.

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Remedial work
Failed basement systems require remedial work. Application of internal tanking in this situation is not normally successful. The junction of the wall and floor is normally the position where water leaks are most noticeable. An economical remedial method is to turn the existing floor construction into a drained floor by chasing channels in the existing floor finishes around the perimeter. Additional channels may cross the floor where there are large areas of open space. Proprietary plastic trays with perforated sides and bases can be set into the chases, connected up and drained to a sump and pump. New floor finishes can then be applied over the original floor and its new drainage channels, to provide ground water protection with only a small thickness of additional floor construction.

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Screeds
Screeds are generally specified by an architect as a finish to structural floors in order to provide a level surface, to conceal service routes and/or as a preparation for application of floor finishes. Historically screeds fail due to inadequate soundness, cracking and curling and therefore, like waterproofing, it is useful for the engineer to have some background knowledge. Structural toppings generally act as part of a precast structural floor to resist vertical load or to enhance diaphragm action. The structural issues affecting the choice of screed are: type of floor construction, deflection, thermal or moisture movements, surface accuracy and moisture condition.

Deflection
Directly bonded screeds can be successfully applied to solid reinforced concrete slabs as they are generally sufficiently rigid, while floating screeds are more suitable for flexible floors (such as precast planks or composite metal decking) to avoid reflective cracking of the screed. Floating screeds must be thicker than bonded screeds to withstand the applied floor loadings and are laid on a slip membrane to ensure free movement and avoid reflective cracking.

Thermal/moisture effects
Drying shrinkage and temperature changes will result in movement in the structure, which could lead to the cracking of an overlying bonded screed. It is general practice to leave concrete slabs to cure for 6 weeks before laying screed or applying rigid finishes such as tiles, stone or terrazzo. For other finishes the required floor slab drying times vary. If movement is likely to be problematic, joints should be made in the screed at predetermined points to allow expansion/ contraction/stress relief. Sand:cement screeds must be cured by close covering with polythene sheet for 7 days while foot traffic is prevented and the screed is protected from frost. After this the remaining free moisture in the screed needs time to escape before application of finishes. This is especially true if the substructure and finish are both vapour proof as this can result in moisture being trapped in the screed. Accurate prediction of screed drying times is difficult, but a rough rule is 4 weeks per 25 mm of screed thickness (to reach about 75% relative humidity). Accelerated heating to speed the drying process can cause the screed to crack or curl, but dehumidifiers can be useful.

Surface accuracy
The accuracy of surface level and flatness of a laying surface is related to the type of base, accuracy of the setting out and the quality of workmanship. These issues should be considered when selecting the overall thickness of the floor finishes to avoid problems with the finish and/or costly remedial measures.

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Precast concrete hollowcore slabs
The values for the hollowcore slabs set out below are for precast prestressed concrete slabs by Tarmac Topfloor. The prestressing wires are stretched across long shutter beds before the concrete is extruded or slip formed along beds up to 130 m long. The prestress in the units induces a precamber. The overall camber of associated units should not normally exceed L /300. Some planks may need a concrete topping (not screed) to develop their full bending capacity or to contribute to diaphragm action. Minimum bearing lengths of 100 mm are required for masonry supports, while 75 mm is acceptable for supports on steelwork or concrete. Planks are normally 1200 mm wide at their underside and are butted up tight together on site. The units are only 1180 to 1190 mm wide at the top surface and the joints between the planks are grouted up on site. Narrower planks are normally available on special order in a few specific widths. Special details, notches, holes and fixings should be discussed with the plank manufacturer early in the design.

Typical spiroll hollowcore working load capacities
Nominal hollowcore plank depth mm 150 200 260 320 400 up to 2 up to 2 2 2 2
2

Fire resistance hours

Typical selfweight kN/m2 2.33 2.94 3.97 3.97 4.83

Clear span for imposed loads* m 1.5 kN/m2 5.5±7.5 7.5±10.0 10.0±12.0 12.0±14.5 14.5±17.5 3 kN/m2 5.0±7.0 7.0±8.5 8.5±11.0 11.0±13.0 13.0±15.5 5 kN/m2 4.5±6.0 6.0±7.5 7.5±10.0 10.0±11.5 11.5±14.0 10 kN/m2 3.5±4.5 4.5±6.0 6.0±8.0 8.0±9.5 9.5±11.5

*An allowance of 1.5 kN/m for screeds and finishes has been included in addition to the plank self-weight. Source: Tarmac Topfloor (2002). Note that this information is subject to change at any time. Consult the latest Tarmac literature for up to date information.

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Bi-metallic corrosion
When two dissimilar metals are put together with an `electrolyte' (normally water) an electrical current passes between them. The further apart the metals are on the galvanic series, the more pronounced this effect becomes. The current consists of a flow of electrons from the anode (the metal higher in the galvanic series) to the cathode, resulting in the `wearing away' of the anode. This effect is used to advantage in galvanizing where the zinc coating slowly erodes, sacrificially protecting the steelwork. Alloys of combined metals can produce mixed effects and should be chosen with care for wet or corrosive situations in combination with other metals. The amount of corrosion is dictated by the relative contact surface (or areas) and the nature of the electrolyte. The effect is more pronounced in immersed and buried objects. The larger the cathode, the more aggressive the attack on the anode. Where the presence of electrolyte is limited, the effect on mild steel sections is minimal and for most practical building applications where moisture is controlled, no special precautions are needed. For greater risk areas where moisture will be present, gaskets, bushes, sleeves or paint systems can be used to separate the metal surfaces.

The galvanic series
Anode Magnesium Zinc Aluminium Carbon and low alloy steels (structural steel) Cast iron Lead Tin Copper, brass, bronze Nickel (passive) Titanium Stainless steels (passive) Cathode

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Structural adhesives
There is little definite guidance on the use of adhesives in structural applications which can be considered if factory controlled conditions are available. Construction sites rarely have the quality control which is required. Adhesive manufacturers should be consulted to ensure that a suitable adhesive is selected and that it will have appropriate strength, durability, fire resistance, effect on speed of fabrication, creep, surface preparation, maintenance requirements, design life and cost. Data for specific products should be obtained from manufacturers.

Adhesive families
Epoxy resins Good gap filling properties for wide joints, with good strength and durability; low cure shrinkage and creep tendency and good operating temperature range. The resins can be cold or hot cure, in liquid or in paste form but generally available as two part formulations. Relatively high cost limits their use to special applications. Very versatile, but slightly weaker than epoxies. Good durability properties (resistance to water, oils and chemicals but generally not alkalis) with operating temperatures of up to 60 C. Moisture is generally required as a catalyst to curing, but moisture in the parent material can adversely affect the adhesive. Applications include timber and stone, but concrete should generally be avoided due to its alkalinity. Toughened acrylics are typically used for structural applications which generally need little surface preparation of the parent material to enhance bond. They can exhibit significant creep, especially at higher operating temperatures and are best suited to tight fitting (thin) joints for metals and plastics. Polyesters exhibit rapid strength gain (even in extremely low temperatures) and are often used for resin anchor fixings etc. However they can exhibit high cure shrinkage and creep, and have poor resistance to moisture. Intended for use primarily with timber. Curing can be achieved at room temperature and above. These adhesives are expensive but strong, durable, water and boil proof and will withstand exposure to salt water. They can be used for internal and external applications, and are generally used in thin layers, e.g. finger joints in glulam beams. Typically used in factory `hot press' fabrication of structural plywood. Cold curing types use strong acids as catalysts which can cause staining of the wood. The adhesives have similar properties to RF and PRF adhesives.

Polyurethanes

Acrylics

Polyesters

Resorcinolformaldehydes (RF) and phenol-resorcinolformaldehydes (PRF) Phenol-formaldehydes (PF)

Melamine-ureaAnother adhesive typically used for timber, but these need protection from formaldehydes (MUF) moisture. These are best used in thin joints (of less than 0.1 mm) and cure and urea-formaldehydes above 10 C. (UF) Caesins Polyvinyl acetates and elastomerics Adhesive tapes Derived from milk proteins, these adhesives are less water resistant than MUF and UF adhesives and are susceptible to fungal attack. Limited to non-loadbearing applications indoors as they have limited moisture resistance. Double sided adhesive tapes are typically contact adhesives and are suitable for bonding smooth surfaces where rapid assembly is required. The tapes have a good operating temperature range and can accommodate a significant amount of strain. Adhesive tapes are typically used for metals and/or glass in structural applications.

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Surface preparation of selected materials in glued joints
Surface preparation is essential for the long-term performance of a glued joint and the following table describes the typical steps for different materials. Specific requirements should normally be obtained from the manufacturer of the adhesive.

Material Concrete

Surface preparation 1. Test parent material for integrity 2. Grit blast or water jet to remove the cement rich surface, curing agents and shutter oil, etc. 3. Vacuum dust and clean surface with solvent approved by the glue manufacturer 4. Apply a levelling layer to the roughened concrete surface before priming for the adhesive 1. Degrease the surface 2. Mechanically wire brush, grit blast or water jet to remove millscale and surface coatings 3. Vacuum dust then prime surface before application of the adhesive 1. Test the steel /zinc interface for integrity 2. Degrease the surface 3. Lightly abrade the surface and avoid rupturing the zinc surface 4. Vacuum dust and then apply an etch primer 5. Thoroughly clean off the etch primer and prime the surface for the adhesive Factory method: 1. Acid etch the surface and clean thoroughly 2. Apply primer Site method: 1. Degrease the surface with solvent 2. Grit blast 3. Apply chemical bonding agent, e.g. silane

Typical adhesive Epoxies are commonly used with concrete, while polyesters are used in resin fixings and anchors. Polyurethanes are not suitable for general use due to the alkalinity of the concrete

Steel and cast iron

Epoxies are the most common for use with structural iron/steel. Where high strength is not required acrylic or polyurethane may be appropriate, but only where humidity can be controlled or creep effects will not be problematic Epoxies are suitable for structural applications. Acrylics are not generally compatible with the zinc surface

Zinc coated steel

Stainless steel

Toughened epoxies are normally used for structural applications

Aluminium

Factory method: 1. Degrease with solvent 2. Use alkaline cleaning solution 3. Acid etch, then neutralize 4. Prime surface before application of the adhesive Site method (as 1 and 2): 3. Grit blast 4. Apply a silane primer/bonding agent

Epoxies and acrylics are most commonly used. Anodized components are very difficult to bond

Timber

1. Remove damaged parent material 2. Dry off contact surfaces and ensure both surfaces have a similar moisture content (which is also less than 20) 3. Plane to create a clean flat surface or lightly abrade for sheet materials 4. Vacuum dust then apply adhesive promptly

Epoxies are normally limited to special repairs. RF and PRF adhesives have long been used with timber. Durability of the adhesive must be carefully considered. They are classified: WBP±Weather Proof and Boil Proof BR±Boil Resistant MR±Moisture Resistant INT±Interior Epoxies usual for normal applications. In dry conditions polyurethanes can be used, and acrylics if creep effects are not critical Structural bonding tape or modified epoxies. The use of silicon sealant adhesives if curing times are not critical

Plastic and fibre composites

1. Dust and degrease surface 2. Abrade surface to remove loose fibres and resin rich outer layers 3. Remove traces of solvent and dust 1. Degreasing should be the only surface treatment. Abrading or etching the surface will weaken the parent material 2. Silane primer is occasionally used

Glass

304

Structural Engineer's Pocket Book

Fixings and fastenings
Although there are a great number of fixings available, the engineer will generally specify nails, screws or bolts. Within these categories there are variations depending on the materials to be fixed. The fixings included here are standard gauges generally available in the UK.

Selected round wire nails to BS 1202

Length mm

Diameter (standard wire gauge (swg) and mm) 11 swg 3.0 mm
.

10 3.35
. .

9 3.65
.

8 4.0
. .

7 4.5

6 5.0

5 5.6

50 75 100 125 150

.

. . .

Selected wood screws to BS 1210

Length mm

Diameter (standard gauge (sg) and mm) 6 sg 3.48 mm
. . .

7 3.50
. .

8 4.17
. . .

10 4.88
. . . . .

12 5.59
. . . . .

14 6.30
. . . .

16 6.94
. . . .

25 50 75 100 125

Selected self-tapping screws to BS 4174
Self-tapping screws can be used in metal or plastics, while thread cutting screws are generally used in plastics or timber.

AB

B

D Metals

U

BF Plastics

Metals + plastics

Building Elements, Materials, Fixings and Fastenings

305

Selected ISO metric black bolts to BS 4190 and BS 3692
Bolt head Shank Thread Nut Nut Dome head nut

Pitch – the distance between points of threads

Nominal diameter mm

Coarse pitch mm

Maximum width of head and nut mm Across flats Across corners 11.5 15.0 19.6 21.9 27.7 34.5 41.6 53.1

Maximum height of head mm

Maximum thickness of nut (black) mm

Minimum distance between centres mm

Tensile stress area mm2

Normal size* (Form E) round washers to BS 4320

Inside diameter mm 4.375 5.875 7.450 8.450 10.450 13.900 15.900 20.050 5.375 6.875 8.450 10.450 13.550 16.550 19.650 24.850 15 20 25 30 40 50 60 75 20.1 36.6 58.0 84.3 157.0 245.0 353.0 561.0 6.6 9.0 11.0 14.0 18.0 22.0 26.0 33.0

Outside diameter mm 12.5 17.0 21.0 24.0 30.0 37.0 44.0 56.0

Nominal thickness mm 1.6 1.6 2.0 2.5 3.0 3.0 4.0 4.0

M6 M8 M10 M12 M16 M20 M24 M30

1.00 1.25 1.50 1.75 2.00 2.50 3.00 3.50

10 13 17 19 24 30 36 46

* Larger diameter washers as Form F and Form G are also available to BS 4320.

Length* mm 30 50 70 100 120 140 150 180

Bolt size M6 M8 M10 M12 M16 M20 M24

. .

. . .

. . . .

. . . . . .

. . . . . .

. . . . .

. . . .

* Intermediate lengths are available.

M6, M8, M10 and M12 threaded bar (called studding) is also available in long lengths.

Spanner and podger dimensions
2(D

3.2 D

15°

) +1

15°

Podger

Spanner

306

Structural Engineer's Pocket Book

Selected metric machine screws to BS 4183
Available in M3 to M20, machine screws have the same dimensions as black bolts but they are threaded full length and do not have a plain shank. Machine screws are often used in place of bolts and have a variety of screw heads:

Pozidrive

Slotted

Cheese

Hexagonal

Cross slot

Round

Phillips

Pig nose

Counter sunk

Square

Allen key

Fillister

Selected coach screws to BS 1210

Typically used in timber construction. The square head allows the screw to be tightened by a spanner.
Length* mm 25 37.5 50 75 87.5 100 112 125 150 200 Diameter 6.25 7.93 9.52 12.5

. . . . . . . .

. . . . . . . .

. . . . . .

. . . . .

Selected welding symbols to BS 449

Fillet

Vee butt

Double vee butt

Spot

Seam

Spot flush one side Spot flush two sides

Building Elements, Materials, Fixings and Fastenings

307

Cold weather working
Cold weather and frosts can badly affect wet trades such as masonry and concrete; however, rain and snow may also have an effect on ground conditions, make access to the site and scaffolds difficult, and cause newly excavated trenches to collapse. Site staff should monitor weather forecasts to plan ahead for cold weather.

Concreting
Frost and rain can damage newly laid concrete which will not set or hydrate in temperatures below 1 C. At lower temperatures, the water in the mixture will freeze, expand and cause the concrete to break up. Heavy rain can dilute the top surface of a concrete slab and can also cause it to crumble and break up.
. Concrete should not be poured below an air temperature of 2 C or if the temperature is

due to fall in the next few hours. Local conditions, frost hollows or wind chill may reduce temperatures further. . If work cannot be delayed, concrete should be delivered at a minimum temperature of 5 C and preferably at least 10 C, so that the concrete can be kept above 5 C during the pour. . Concrete should not be poured in more than the lightest of rain or snow showers and poured concrete should be protected if rain or snow is forecast. Formwork should be left in place longer to allow for the slower gain in strength. Concrete which has achieved 5 N/mm2 is generally considered frost safe. . Mixers, handling plant, subgrade/shuttering, aggregates and materials should be free from frost and be heated if necessary. If materials and plant are to be heated, the mixing water should be heated to 60 C. The concrete should be poured quickly and in extreme cases, the shuttering and concrete can be insulated or heated.

Bricklaying
Frost can easily attack brickwork as it is usually exposed on both sides and has little bulk to retain heat. Mortar will not achieve the required strength in temperatures below 2 C. Work exposed to temperatures below 2 C should be taken down and rebuilt. If work must continue and a reduced mortar strength is acceptable, a mortar mix of 1 part cement to 5 to 6 parts sand with an air entraining agent can be used. Accelerators are not recommended and additives containing calcium chloride can hold moisture in the masonry resulting in corrosion of any metalwork in the construction.
. Bricks should not be laid at air temperatures below 2 C or if the temperature is due to

fall in the next few hours. Bricklaying should not be carried out in winds of force 6 or above, and walls without adequate returns to prevent instability in high winds should be propped. . Packs, working stacks and tops of working sections should be covered to avoid soaking, which might lead to efflorescence and/or frost attack. An airspace between any polythene and the brickwork will help to prevent condensation. Hessian and bubble wrap can be used to insulate. The protection should remain in place for about 7 days after the frost has passed. In heavy rain, scaffold boards nearest the brickwork can be turned back to avoid splashing, which is difficult to clean off. . If bricks have not been dipped, a little extra water in the mortar mix will allow the bricks to absorb excess moisture from the mortar and reduce the risk of expansion of the mortar due to freezing.

308

Structural Engineer's Pocket Book

Effect of fire on construction materials
This section is a brief summary of the effect of fire on structural materials to permit a quick assessment of how a fire may affect the overall strength and stability of a structure. It is necessary to get an accurate history of the fire and an indication of the temperatures achieved. If this is not available via the fire brigade, clues must be gathered from the site on the basis of the amount of damage to the structure and finishes. At 150 C paint will be burnt away, at 240 C wood will ignite, at 400±500 C PVC cable coverings will be charred, zinc will melt and run off and aluminium will soften. At 600±800 C aluminium will run off and glass will soften and melt. At 900±1000 C most metals will be melting and above this, temperatures will be near the point where a metal fire might start. The effect of heat on structure generally depends on the temperature, the rate and duration of heating, and the rate of cooling. Rapid cooling by dousing with water normally results in the cracking of most structural materials.

Reinforced concrete
Concrete is likely to blacken and spall, leaving the reinforcement exposed. The heat will reduce the compressive strength and elastic modulus of the section, resulting in cracking and creep/permanent deflections. For preliminary assessment, reinforced concrete heated to 100±300 C will have about 85% of its original strength, by 300±500 C it will have about 40% of its original strength and above 500 C it will have little strength left. As it is a poor conductor of heat only the outer 30±50 mm will have been exposed to the highest temperatures and therefore there will be temperature contours within the section which may indicate that any loss of strength reduces towards the centre of the section. At about 300 C concrete will tend to turn pink and at about 450±500 C it will tend to become a dirty yellow colour. Bond strengths can normally be assumed to be about 70% of pre-fire values.

Building Elements, Materials, Fixings and Fastenings

309

Prestressed concrete
The concrete will be affected by fire as listed for reinforced concrete. More critical is the behaviour of the steel tendons, as non-recoverable extension of the tendons will result in loss of prestressing forces. For fires with temperatures of 350±400 C the tendons may have about half of their original capacity.

Timber
Timber browns at 120±150 C, blackens at 200±250 C and will ignite and char at temperatures about 400 C. Charring may not affect the whole section and there may be sufficient section left intact which can be used in calculations of residual strength. Charring can be removed by sandblasting or planing. Large timber sections have often been found to perform better in fire than similarly sized steel or concrete sections.

Brickwork
Bricks are manufactured at temperatures above 1000 C, therefore they are only likely to be superficially or aesthetically damaged by fire. It is the mortar which can lose its strength as a result of high temperatures. Cementitious mortar will react very similarly to reinforced concrete, except without the reinforcement and section mass, it is more likely to be badly affected. Hollow blocks tend to suffer from internal cracking and separation of internal webs from the main block faces.

Steelwork
The yield strength of steel at 20 C is reduced by about 50% at 550 C and at 1000 C it is 10% or less of its original value. Being a good conductor of heat, the steel will reach the same temperature as the fire surrounding it and transfer the heat away from the area to affect other remote areas of the structure. Steelwork heated up to about 600 C can generally be reused if its hardness is checked. Cold worked steel members are more affected by increased temperature. Connections should be checked for thread stripping and general soundness. An approximate guide is that connections heated to 450 C will retain full strength, to 600 C will retain about 80% of their strength and to 800 C will retain only about 60% of their strength.

310

Structural Engineer's Pocket Book

Aluminium
Aluminium is extracted from ore and has little engineering use in its pure form. Aluminium is normally alloyed with copper, magnesium, silicon, manganese, zinc, nickel and chromium to dramatically improve its strength and work hardening properties. Aluminium has a stiffness of about one third of that for steel and therefore it is much more likely to buckle in compression than steel. The main advantage of aluminium is its high strength:weight ratio, particularly in long span roof structures. The strength of cold worked aluminium is reduced by the application of heat, and therefore jointing by bolts and rivets is preferable to welding. For structural purposes wrought aluminium alloy sections are commonly used. These are shaped by mechanical working such as rolling, forging, drawing and extrusion. Heat treatments are also used to improve the mechanical properties of the material. This involves the heating of the alloy followed by rapid cooling, which begins a process of ageing resulting in hardening of the material over a period of a few days following the treatment. The hardening results in increased strength without significant loss of ductility. Wrought alloys can be split into non-heat treatable and heat treatable according to the amount of heat treatment and working received. The temper condition is a further classification, which indicates the processes which the alloy has undergone to improve its properties. Castings are formed from a slightly different family of aluminium alloys.

Summary of material properties
Density Poisson's ratio Modulus of elasticity, E Modulus of ridigity, G Linear coefficient of thermal expansion 27.1 kN/m3 0.32 70 kN/mm2 23 kN/mm2 24 Â 10
À6 

/ C

Notation for the classification of structural alloys
Heat treatable alloys T4 T6 Heat treated ± naturally aged Heat treated ± artificially aged Fabricated Annealed Strain hardened

Non-heat treatable alloys F O H

Summary of main structural aluminium alloys to BS 8118
Values of limiting stresses depend on whether the products are extrusions, sheet, plate or drawn tubes.
Alloy Temper Types of product* Typical thicknesses mm Durability Approx. loss of strength due to welding (%) 0 50 Limiting stresses Py N/mm2 65 160 Pc / Pt N/mm2 85 175 Pv N/mm2 40 95

Heat treatable

6063

T4 T6

Thin walled extruded sections and tubes as used in curtain walling and window frames Solid and hollow extrusions

1±150 1±150

B

6082

T4 T6

1±150 1±20 20±150 0.2±80 3±25 0.2±6 ±

B

0 50 50 0 0 45 ±

115 255 270 105 130 235

145 275 290 150 170 270

70 155 160 65 75 140

Non-heat treatable

5083

O F H22 F

Sheet and plate. Readily welded. Often used for plating and tanks Mainly sand castings in simple shapes with high surface polish Good for complex shaped castings Sand castings Chill castings

A

LM 5

A

Strengths of castings determined in consultation with castings manufacturer. Approx. values:

LM 6

F

±

B

±

40±120

70±140

25±75

±

*British Aluminium Extrusions do a range of sections in heat treatable aluminium alloys.

311

Source: BS 8118: Part 1: 1997

312

Structural Engineer's Pocket Book

Durability
Corrosion protection guidelines are set out in BS 8118: Part 2. Each type of alloy is graded as A or B. Corrosion protection is only required for A rated alloys in severe industrial, urban or marine areas. Protection is required for B rated alloys for all applications where the material thickness is less than 3 mm, otherwise protection is only required in severe industrial, urban or marine areas and where the material is immersed in fresh or salt water. Substances corrosive to aluminium include: timber preservatives; copper naphthanate, copper-chrome-arsenic or borax-boric acid; oak, chestnut and western red cedar unless they are well seasoned; certain cleaning agents and building insulation. Barrier sealants (e.g. bituminous paint) are therefore often used.

Fire protection
Aluminium conducts heat four times as well as steel. Although this conductivity means that `hot spots' are avoided, aluminium has a maximum working temperature of about 200 to 250 C (400 C for steel) and a melting temperature of about 600 C (1200 C for steel). In theory fire protection could be achieved by using thicker coatings than those provided for steel, aluminium is generally used in situations where fire protection is not required. Possible fire protection systems might use ceramic fibre, intumescent paints or sacrificial aluminium coatings.

Selected sizes of extruded aluminium sections to BS 1161
Section type Range of sizes (mm) Minimum Equal angles Unequal angles Channels I sections Tee sections 30 Â 30 Â 2.5 50 Â 38 (web 3, flange 4) 60 Â 30 (web 5, flange 6) 60 Â 30 (web 4, flange 6) 50 Â 38 Â 3 Maximum 120 Â 120 Â 10 140 Â 105 (web 8.5, flange 11) 240 Â 100 (web 9, flange 13) 160 Â 80 (web 7, flange 11) 120 Â 90 Â 10

Rolled plates in thicknesses of 6.5±155 mm can be obtained in widths up to 3 m and lengths up to 15 m.

Building Elements, Materials, Fixings and Fastenings

313

Structural design to BS 8118: Part 1
Partial safety factors for applied loads
BS 8118 operates a two tier partial safety factor system. Each load is first factored according to the type of load and when loads are combined, their total is factored according to the load combination. Dynamic effects are considered as imposed loads and must be assessed to control vibration and fatigue. This is not covered in detail in BS 8118 which suggests `special' modelling.

Primary load factors
Load type Dead Imposed Wind Temperature effects gf1 1.20 or 0.80 1.33 1.20 1.00

Secondary load factors for load combinations
Load combinations Dead load Imposed or wind load giving the component Imposed or wind load giving action on the component Imposed or wind load giving action on the component Imposed or wind load giving action on the component the most severe loading action on the second most severe loading the third most severe loading the fourth most severe loading gf2 1.0 1.0 0.8 0.6 0.4

Partial safety factors for materials depending on method of jointing
Type of construction gm Members Riveted and bolted Welded Bonded/glued 1.2 1.2 1.2 Joints 1.2 1.3 or 1.6 3.0

Comment on aluminium design to BS 8118
As with BS 5950 for steel, the design of the structural elements depends on the classification of the cross section of the element. An initial estimate of bending strength would be Mb ˆ py S=gm but detailed reference must be given to the design method in the code. Strength is usually limited by local or overall buckling of the section and deflections often govern the design. Source: BS 8118: Part 1: 1997.

13
Useful Mathematics
Trigonometric relationships
Addition formulae sin…A Æ B† ˆ sin A cos B Æ cos A sin B cos…A Æ B† ˆ cos A cos B Ç sin A sin B tan…A Æ B† ˆ tan A Æ tan B 1 Ç tan A tan B

Sum and difference formulae sin A ‡ sin B ˆ 2 sin 1 …A ‡ B† cos 1 …A ‡ B† 2 2 sin A À sin B ˆ 2 cos 1 …A ‡ B† sin 1 …A À B† 2 2 cos A ‡ cos B ˆ 2 cos 1 …A ‡ B† cos 1 …A À B† 2 2 cos A À cos B ˆ À2 sin 1 …A ‡ B† sin 1 …A À B† 2 2 sin…A ‡ B† cos A cos B sin…A À B† tan A À tan B ˆ cos A cos B tan A ‡ tan B ˆ

Product formulae
2 sin A cos B ˆ sin…A À B† ‡ sin…A ‡ B† 2 sin A sin B ˆ cos…A À B† À cos…A À B† 2 cos A cos B ˆ cos…A À B† ‡ cos…A ‡ B†

Multiple angle and powers formulae sin 2A ˆ 2 sin A cos A cos 2A ˆ cos2 A À sin2 A cos 2A ˆ 2 cos2 A À 1 cos 2A ˆ 1 À 2 sin2 A tan 2A ˆ 2 tan A 1 À tan2 A

sin2 A ‡ cos2 A ˆ 1

sec2 A ˆ tan2 A ‡ 1

Useful Mathematics

315

Relationships for plane triangles
D b C a
Pythagoras for right angled triangles: Sin rule:

A B

c

a2 ‡ b2 ˆ c2 a b c ˆ ˆ sin A sin B sin C 2 p s…s À a†…s À b†…s À c†; bc where s ˆ …a ‡ b ‡ c†=2

sin A ˆ

Cosine rule:

a2 ˆ b2 ‡ c2 À 2bc cos A a2 ˆ b2 ‡ c2 ‡ 2bc cos D cos A ˆ b2 ‡ c2 À a2 2bc

316

Structural Engineer's Pocket Book

Special triangles

5 4

3

√2 1 45°

1

2

60° 1

30° √3

f

b a e c d a = c = e d f b

Useful Mathematics

317

Algebraic relationships
Quadratics
ax2 ‡ bx ‡ c ˆ 0 x2 ‡ 2xy ‡ y 2 ˆ …x ‡ y†2 x2 À y2 ˆ …x ‡ y†…x À y† x3 À y3 ˆ …x À y†…x2 ‡ xy ‡ y 2 † xˆ Àb Æ p b2 À 4ac 2a

Powers ax À ay ˆ ax‡y

ax ˆ axÀy ay

…ax †y ˆ axy

Logarithms x  eloge x  eln x x  log10 …10x †  log10 …antilog10 x†  10log10 x e ˆ 2:71828 ln x ˆ log10 x ˆ 2:30259 log10 x log10 e

Equations of curves
Circle x ‡y ˆa
2 2 2

Ellipse x2 y 2 ‡ ˆ1 a2 b2

a

b

a

318

Structural Engineer's Pocket Book
Parabola y 2 ˆ ax

Hyperbola x2 y 2 À ˆ1 a2 b2

a

Circular arc Rˆ  d2 ‡  L2 1 4 2d
L
Arc of circle

d

R
Centre of circle

Useful Mathematics

319

Rules for differentiation and integration d dv du …uv† ˆ u ‡ v dx dx dx   d u  1 du dv ˆ 2 v Àu dx v v dx dx d dw dv du …uvw† ˆ uv ‡ uw ‡ vw dx dx dx dx  …uv†dx ˆ u  …v†dx À  du dx  …v†dx

Standard differentials and integrals d n x ˆ nxnÀ1 dx d 1 ln x ˆ dx x d ax e ˆ aeax dx d x a ˆ ax ln a dx d x x ˆ xx …1 ‡ ln x† dx d sin x ˆ cos x dx d cos x ˆ Àsin x dx d tan x ˆ sec2 x dx d cot x ˆ Àcosec2 x dx d 1 sinÀ1 x ˆ p dx 1 À x2 d 1 tanÀ1 x ˆ dx 1 ‡ x2 d À1 cotÀ1 x ˆ dx 1 ‡ x2              xn dx ˆ xn‡1 n‡1 n Tˆ 1

1 dx ˆ ln x x eax dx ˆ ax dx ˆ eax a a Tˆ 0 a > 0; a Tˆ 0

ax ln a

ln x dx ˆ x…ln x À 1† sin x dx ˆ Àcos x cos x dx ˆ sin x tan x dx ˆ Àln…cos x† cot x dx ˆ ln…sin x† sec2 x dx ˆ tan x cosec2 x dx ˆ Àcot x 1 p dx ˆ sinÀ1 x jxj

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Analysis on the Decreasing Number of Graduating Civil Engineering Students in Southern Luzon State University

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Issues in African Development

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