Monday, August 4, 2008

Gateway Arch

The 630-foot-high (192-meter) stainless-steel Gateway Arch rises from a wooded park within what became the Jefferson National Expansion Memorial Park on the bank of the Mississippi in St. Louis, Missouri. Taller than the Washington Monument in the national capital and twice as high as the Statue of Liberty, the sleek and seamless Gateway Arch (now known as the St. Louis Arch) is a major achievement of twentieth-century architecture and structural engineering.

A decision was taken in 1935 to establish a national monument in St. Louis, Missouri, to commemorate the nineteenth-century westward expansion that pursued Thomas Jefferson’s dream of a continental United States. A large tract of riverfront land in the older part of the city was acquired and cleared, but the project was interrupted by the country’s involvement in World War II. With the return to peace, in 1947–1948 the Jefferson National Expansion Memorial Association sponsored a design competition for an appropriate monument. The Finnish-American architect Eero Saarinen was awarded first prize.

Work on design development began in 1961, the year of the architect’s death, the project being managed by his firm, Eero Saarinen Associates. Fred Severud of the structural engineering practice Severud, Elstad, Krueger and Associates undertook a feasibility study about the buildability of the daring concept, and Dr. Hannskarl Bandel generated exacting calculations for the weighted catenary (an inverted version of the curve of a suspended chain) that forms the basis of the structure. Bruce Detmers and other architects converted the mathematics into working drawings.

The main contractor was MacDonald Construction, and the steel was fabricated and erected by Pittsburgh-Des Moines Steel. The first concrete pour for the massive 60-foot-deep (18-meter) foundations took place late in June 1962 and construction of the arch itself began eight months later. The span of the arch is the same as its height, and the composite structure consists of 142 welded, stainless-steel-faced sections of equilateral triangular cross sections. The length of their side at the base is 54 feet (16.5 meters), and the sections are 12 feet (3.6 meters) high; at the top, they have a side length of 17 feet (5.4 meters) and are 8 feet (2.4 meters) high. The legs have double walls with an inner skin of 0.375-inch-thick (about 10-millimeter) carbon steel and an outer skin of stainless steel, set 3 feet (90 centimeters) apart; at the 400-foot (120-meter) level, the gap between the skins reduces to less than 8 inches (20 centimeters). For the first 300 feet the space between the walls is filled with concrete; above that, to the crown of the arch, the structure is braced with steel stiffeners. It is clear that the engineering design is highly complicated, but all that can be seen from the outside is the sheer skin of polished stainless steel.

The wall components were fabricated and bolted together in Pennsylvania and transported to St. Louis by rail. On-site, the triangular sections were welded by highly skilled tradesmen. In July 1998 their specialized work was recognized by the American Welding Society’s Historical Welded Structure Award. The completed 50-ton (45.5-tonne) double-walled sections were transported to the site on a specially designed railroad car and lifted into place. For the first 72 feet (21.6 meters), conventional cranes on the ground were used; above that, purpose-made creeper cranes handled the sections. In effect, each leg of the arch was a vertical cantilever and therefore had no need of scaffolding. But when the 530-foot (162-meter) level was reached, a steel stabilizing truss was lifted into place and fixed to brace the two legs while the remaining twenty-one sections and the central “keystone” were located. The arch was completed on 28 October 1965. As the creeper cranes moved back to the ground, their tracks were dismantled and bolt holes in the stainless-steel surface were made good.

In 1967–1968 passenger trams were constructed in the hollow core of the arch, to carry visitors—there were 4 million in 1999—to a 140-person observation platform at the top, where tiny plate-glass windows give access to views up to 30 miles (50 kilometers) eastward and westward. The total cost of the arch, including $2 million for the internal transportation system, was $13 million. The building received the American Institute of Architects 25 Year Award in 1990.

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