How Things Work: The Gateway Arch

Sarah Mogin Oct 30, 2006

The Gateway Arch, 630 feet tall and another 630 feet wide, is an elegant frame to the historic city of St. Louis. A gigantic endeavor in its time, the arch is almost 100 feet taller than the Washington Monument and more than twice the height of the Statue of Liberty. It contains over 5000 tons of steel and nearly 40,000 tons of concrete.

In 1935, the United States government sanctioned the riverfront location of the arch, officially known as the Jefferson National Expansion Memorial. The initiative accrued about 40 blocks worth of city space, but the project sank into a 12-year hiatus in the chaos of the Second World War.

Memorial plans resurfaced in 1947, when architect Eero Saarinen won a countrywide competition for its design. His proposal, the original concept of the Gateway Arch, described a monument in the shape of an inverse catenary curve.

The catenary, often confused with the parabola, is the curve formed by a hanging chain held only at its ends. Responding to the force of gravity, the slope of the chain is shallowest at its center, growing ever steeper towards its hanging points, where the most weight must be supported.
The catenary curve conforms to the shape of a hyperbolic cosine function.

The Gateway Arch is composed of many different pieces, or sections, in the shapes of equilateral triangles. The widest bottom sections are 54 feet in length (with a 40-foot hollow in the center) and 12 feet high, while the two keystone pieces are 17 feet long (15 1/2 inside) and eight feet tall.

Each section contains an inner layer made up of three-eighths of an inch of A-7 carbon steel. The smooth, radiant exterior consists of 900 tons of stainless steel panels, each as thick as 14 inches.
The initial sections of the arch were so large that they had to be welded on site. Each triangular section arrived as three individual legs, connected via welding on a 56-foot by 125-foot pad of concrete. Subsequent sections to the arch were small enough that this part of process could be completed at the factory.

Ground-operated creeper cranes lowered each section into place for the first 72 feet of the arch. Separated by jacks enforcing a four-inch gap, the sections were welded inside and out, and concrete was poured to fill the spaces between the layers of carbon and stainless steel.

Workers continued using concrete in this way until the 300th level of construction. After that, steel diaphragms served as the primary connections between the inner and outer walls.

Past the height of 72 feet, the use of cranes was no longer practicable. Workers switched to creeper derricks, two 100-ton pieces of equipment, which literally climbed the arch as construction progressed.

Each derrick had a pair of adjustable steel legs able to hug the arch. The legs adjusted to the varying curvature and height of the completed sections. At this point in the process, the additions weighed about 50 tons per section, but required only about 30 minutes to be properly positioned.
When construction reached a height of 530 feet, workers added a stabilizing truss to secure each separate leg. The truss was made of alloy steel and constructed on location.

About 60 tons in weight, the 225-foot truss was lifted into place through the use of both of the creeper derricks. Steel harnesses held the truss in place on either side, directing forces due to gravity and wind towards the arch’s strong foundations.

Workers added another 42 sections, leaving only the two keystones. While these were being positioned, a jacking frame put pressure on the arch. The creeper derricks were then used to remove the stabilizing truss and each vertical track.

Though construction of the Gateway Arch hadn’t begun until 1963, the arch was completed in 1965, costing only $15 million. By 1968, its one-of-a-kind tram system was open to the public.

Elevator businessman Dick Bowser’s proposal was one part elevator, one part Ferris wheel, in light of the unique circumstances of the arch’s structure. Like in a Ferris wheel, the passengers’ weight helps to stabilize their environment. There are two trams, one for each leg. Composed of eight five-passenger barrels, the trams can hold up to 40 visitors at a time. Each barrel is comparable in size to that of a cement mixer.

Throughout the trip, the barrels rotate 155 degrees. At the base of the arch, the barrels hang from the track, but by the time they reach the top the track is beneath them.

It only takes four minutes for each tram to make its ascent and three to return to the bottom. About 160 people can fit on the observation deck at the arch’s apex. Loading occurs in the Museum of Westward Expansion, located underground directly beneath the arch.