Gateway Arch | U.S. National Register of Historic Places | U.S. National Historic Landmark | The Gateway Arch in April 2010. | Location: | Missouri | Coordinates: | 38°37′31″N 90°11′0″W / 38.62528°N 90.183333°W / 38.62528; -90.183333Coordinates: 38°37′31″N 90°11′0″W / 38.62528°N 90.183333°W / 38.62528; -90.183333 | Area: | 62.165 acres[1] | Built/Founded: | constructed 1963-1965 design created 1947 | Architect: | Eero Saarinen | Governing body: | National Park Service | Added to NRHP: | May 28, 1987[2] | Designated NHL: | May 28, 1987[3] |
The Gateway Arch, also known as the Gateway to the West, is an integral part of the Jefferson National Expansion Memorial and the iconic image of St. Louis, Missouri. It was constructed as a monument to the westward expansion of the United States. It was designed by Finnish-American architect Eero Saarinen and structural engineer Hannskarl Bandel in 1947. It is 630 feet (192 m) wide at its base and stands 630 feet (192 m) tall, making it the tallest monument in the United States.[3] Construction started on February 12, 1963, and ended on October 28, 1965.[4] The monument opened to the public on July 10, 1967.[5] * |
Physical description

Welds on the exterior skin of the arch seal gaps between large (4' x 8') sheets of stainless steel. Various graffiti, which covers the lower 5-7' of the monument, is also apparent.
The design of the Arch was chosen in a national architectural competition in 1947 from among 147 entries. The competition was coordinated by architect George Howe, and the seven jury members included Fiske Kimball, Richard Neutra, Roland Wank, and William Wurster. After narrowing the field down to a smaller number of finalists, Saarinen's design was chosen unanimously [6].
The cross-sections of its legs are equilateral triangles, narrowing from 54 feet (16 m) per side at the base to 17 feet (5.2 m) at the top. Each wall consists of a stainless steel skin covering a sandwich of two carbon steel walls with reinforced concrete in the middle from ground level to 300 feet (91 m), with carbon steel and rebar from 300 feet (91 m) to the peak.[7] The Arch is hollow and contains a unique tram system that brings visitors to an observation deck at the top. The interior also contains two stairwells of 1,076 steps each for use in emergencies.[8]

The base of each leg at ground level had an engineering tolerance of one-64th of an inch (0.40 mm) or the two legs would not meet at the top.[4] During construction, both legs were built simultaneously. When the time came to connect the legs at the apex, thermal expansion of the sunward-facing south leg prevented it from aligning precisely with the north leg. The St. Louis Fire Department sprayed the south leg with water from firehoses, cooling it until it aligned with the north leg.
The tram is an egg-shaped "elevator". It is operated by the quasi-governmental Bi-State Development Agency under an agreement with the NPS.
From the visitor center, one may move to either base (one on the north end and the other on the south end) of the Arch and enter the tramway much as one would enter an ordinary elevator, through narrow double doors. The north queue area includes displays that interpret the design and construction of the Gateway Arch; the south queue area includes displays about the St. Louis riverfront during the mid-19th century.
Passing through the doors, passengers in groups of five enter an egg-shaped compartment containing five seats and a flat floor. Because of the car shape, the compartments have sloped ceilings low enough to force taller riders to lean forward while seated (for this reason it's recommended that the tallest of the five passengers in the car sit in the center seat facing the door). Eight compartments are linked to form a train, meaning that both trains have a capacity of 40, and that 80 people can be transported at one time. These compartments rotate 5 degrees as they travel, keeping them upright while the entire train follows curved tracks up one leg of the arch. The trip to the top takes four minutes, and the trip down takes three minutes. The car doors have narrow windows, allowing passengers to see the interior stairways and structure of the Arch during the trip. Interior of the tram capsule in the Gateway Arch. | Entrance doors as viewed from inside the tram capsule. |
Observation area
Near the top of the arch, the rider exits the compartment and climbs a slight grade to enter the arched observation area. Thirty-two windows (16 per side) measuring 7 by 27 inches (180 × 690 mm) allow views across the Mississippi River and southern Illinois with its prominent Mississippian culture mounds to the east at Cahokia Mounds, and the City of Saint Louis and St. Louis County to the west beyond the city.[9] On a clear day, one can see up to 30 miles (48 km).[10] Observation area on top of the arch | Gateway Arch view from Observation Area | Southern entrance to the subterranean visitor center. |

Mathematics of the Arch

Curvature detail with the windows of the observation deck located around the apex of the arch.
The geometric form of the Arch was set by mathematical equations provided to Saarinen by Dr. Hannskarl Bandel. Bruce Detmers and other architects expressed the geometric form in blueprints with this equation:[11]
,
with the constants

where fc = 625.0925 ft (191 m) is the maximum height of centroid, Qb = 1,262.6651 sq ft (117 m2) is the maximum cross sectional area of arch at base, Qt= 125.1406 sq ft (12 m2) is the minimum cross sectional area of arch at top, and L = 299.2239 ft (91 m) is the half width of centroid at the base.
This hyperbolic cosine function describes the shape of a catenary. A chain that supports only its own weight forms a catenary; in this configuration, the chain is strictly in tension.[12][13] An inverted catenary arch that supports only its own weight is strictly in compression, with no shear. The gateway arch itself is not a catenary, but a more general curve called a flattened catenary of the form y=Acosh(Bx);[14] a catenary is the special case when AB=1. While a catenary is the ideal shape for an arch of constant thickness, the gateway arch does not have constant thickness as it is narrower near the top.

An arch is a structure that spans a space while supporting weight (e.g. a doorway in a stone wall). Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.
The semicircular arch was followed in Europe by the pointed Gothic arch or ogive whose centreline more closely followed the forces of compression and which was therefore stronger. The semicircular arch can be flattened to make an elliptical arch as in the Ponte Santa Trinita. The parabolic and catenary arches are now known to be the theoretically strongest forms. Parabolic arches were introduced in construction by the Spanish architect Antoni Gaudí, who admired the structural system of Gothic style, but for the buttresses, which he termed “architectural crutches”. The catenary and parabolic arches carry all horizontal thrust to the foundation and so do not need additional elements.
The horseshoe arch is based on the semicircular arch, but its lower ends are extended further round the circle until they start to converge. The first examples known are carved into rock in India in the first century AD, while the first known built horseshoe arches are known from Aksum (modern day Ethiopia and Eritrea) from around the 3rd–4th century, around the same time as the earliest contemporary examples in Syria, suggesting either an Aksumite or Syrian origin for the type of arch.[1]
The segmental arch was first built by the Chinese who realized that an arch in a bridge did not have to be a semicircle.[2]

Construction
An arch requires all of its elements to hold it together, raising the question of how an arch is constructed. One answer is to build a frame (historically, of wood) which exactly follows the form of the underside of the arch. This is known as a centre or centring. The voussoirs are laid on it until the arch is complete and self-supporting. For an arch higher than head height, scaffolding would in any case be required by the builders, so the scaffolding can be combined with the arch support. Occasionally arches would fall down when the frame was removed if construction or planning had been incorrect. (The A85 bridge at Dalmally, Scotland suffered this fate on its first attempt, in the 1940s). The interior and lower line or curve of an arch is known as the intrados.
Old arches sometimes need reinforcement due to decay of the keystones, forming what is known as bald arch.
The gallery shows arch forms displayed in roughly the order in which they were developed. Triangular arch | Round arch or Semi-circular arch | Segmental arch or arch that is less than a semicircle | Unequal round arch or Rampant round arch | Lancet arch | Equilateral pointed arch | Shouldered flat arch -see also jack arch | Three-foiled cusped arch | Horseshoe arch | Three-centered arch | Elliptical arch | Inflexed arch | Ogee arch | Reverse ogee arch | Tudor arch | Catenary or Parabolic arch | Technical aspects

Roman Pont-Saint-Martin
The arch is significant because, in theory at least, it provides a structure which eliminates tensile stresses in spanning an open space. All the forces are resolved into compressive stresses. This is useful because several of the available building materials such as stone, cast iron and concrete can strongly resist compression but are very weak when tension, shear or torsional stress is applied to them. By using the arch configuration, significant spans can be achieved. This is because all the compressive forces hold it together in a state of equilibrium. This even applies to frictionless surfaces. However, one downside is that an arch pushes outward at the base, and this needs to be restrained in some way, either with heavy sides and friction or angled cuts into bedrock or similar.
This same principle holds when the force acting on the arch is not vertical such as in spanning a doorway, but horizontal, such as in arched retaining walls or dams.
Even when using concrete, where the structure may be monolithic, the principle of the arch is used so as to benefit from the concrete's strength in resisting compressive stress. Where any other form of stress is raised, it has to be resisted by carefully placed reinforcement rods or fibres. (See Arch bridge.)

The dry stone bridge, so called Porta Rosa (4th century BC), in Elea | Arch of Constantine, Rome, Italy commemorating a victory by Constantine I in 312 AD | The Gateway Arch in Saint Louis, Missouri; a sculpture based on a catenary arch | Doubled round archivolts - Igreja de Nossa Senhora da Assunção, Linhares da Beira, Portugal. | Stonework arches seen in a ruined stonework building - Burg Lippspringe, Germany | Several arches at the Casa Simón Bolívar in Havana, Cuba | Arches in the Armenian monastery of Geghard. | Arches in the nave of the church in monastery of Alcobaça, Portugal | The Arc de Triomphe, Paris; a 19th century triumphal arch modeled on the classical Roman design | The Second Wembley Stadium, in London, built in 2007 | Catenary arches inside Casa Milà in Barcelona, Spain by Antoni Gaudí | Arches in one of the porticos of Mosque of Uqba also known as the Great Mosque of Kairouan, city of Kairouan, Tunisia |