St. Louis Gateway Arch (Jefferson National Expansion Memorial), St. Louis, Mo.

A structural engineering landmark for more than 50 years, the Gateway Arch has a height and span of 630 feet. It is an equilateral triangle in cross-section, the sides of which taper from 54 ft. wide at the base to 17 ft. at the peak. Severud Associates designed the tubular, torsionally stable shape, which creates an interior space for the observation platform, tramway, and mechanical systems, as a stressed orthotropic skin composed of two layers of steel separated by a gap, which also varies in width.

The outer layer is made from stainless steel to protect the structure from weather and to provide the metallic finish desired by its famed architect, Eero Saarinen. The inner layer and the intermittent stiffeners connecting it to the outer layer are made from carbon steel. To resist lateral loads (and the unbalanced dead weight of each separate leg during construction), the gap between the inner and outer steel skins is filled with post-tensioned concrete to a height of 300 ft. This cutting-edge design limits the arch to a lateral deflection of only 18 inches at its apex in winds of up to 150 mph.