The Gateway Arch in St. Louis is 630 ft (192 m) high and 630 ft wide at its base. Its section is an equilateral triangle, 54 ft a side at the ground and 17 ft at the top, built from double-walled segments with a stainless steel outer skin and a carbon steel inner skin. There is no internal skeleton: the two skins, filled with concrete to the 300 ft level and stiffened with steel above it, form the structure.

Architecturally the Arch is one continuous curve. In construction terms it was two cantilevers that had to stand on their own for most of the build, then meet in the air with almost no tolerance for error. The first section was set on 12 February 1963 and the last on 28 October 1965. The prime contractor was MacDonald Construction Company, with the Pittsburgh-Des Moines Steel Company fabricating and erecting the arch as subcontractor.

630 ft
Height and span (192 m)
142
Triangular sections
54 → 17 ft
Section side, base to top
5,199 t
Steel (short tons)
530 ft
Temporary truss level
Phase 1 — The base
Crawler cranes for the first sections
Each leg is founded on a massive concrete base, with 25,980 short tons of concrete below ground across the two foundations. The first sections, up to a height of 72 ft, were lifted by crawler cranes working from the ground. Section heights range from 12 ft at the base to 8 ft for the two sections at the crown, and each section was welded inside and out to the one below.
Phase 2 — The climb
Creeper derricks riding the legs
Above the reach of the ground cranes, purpose-built creeper derricks took over. Each derrick rode a pair of track beams bolted to its leg and was hauled upward by cables and pulleys as the leg grew, lifting each new section into position. The same track beams later guided the workmen's elevator. The derricks were still on the legs after the closure: a National Park Service photograph shows them backing down in January 1966.
Phase 3 — The approach
A temporary truss between two cantilevers
For most of the build the legs were independent, free-standing cantilevers leaning towards each other. As they rose above 530 ft, a temporary truss was installed between them to steady them, and a catch net was slung across the narrowing gap. The truss was a temporary works item only, and was later removed.
Phase 4 — The closure
Jacks, fire hoses and 3/8 of an inch
The final section could only go in if the gap between the legs was opened. Hydraulic jacks forced the legs apart; the National Park Service's J.E.N. Jensen described a force of about a million pounds, maintained on both legs through the operation. Sun on the south leg was heating it more than the north leg, threatening to lengthen and twist it at the critical moment, so fire hoses sprayed water up the leg to keep the two legs at about the same temperature. When the last section was placed on 28 October 1965, the legs were reported to be only 3/8 of an inch out.
Four-panel schematic of the Gateway Arch erection: crawler cranes setting the first sections, creeper derricks climbing rails on both legs, a temporary truss tying the legs above 530 ft, and the closure with hydraulic jacks spreading the legs and fire hoses cooling the sunlit south leg
Two free-standing cantilevers, one closure in the air. Conceptual schematic, not to scale.
Sources: National Park Service; J.E.N. Jensen as quoted in G. Hartzog, Battling for the National Parks; St. Louis Post-Dispatch archive. Diagram: EE&HL Network, 2026.

The arch itself is simple. Closing it was the engineering problem: two legs built apart had to arrive at the same point, at the same temperature, at the same moment.

The closure was not the only point of tension. According to the account in George Hartzog's memoir, Battling for the National Parks, the erection subcontractor requested a change during construction, contending that the structure as designed was unsafe. The dispute reached the Secretary of the Interior. Engineers from the Bureau of Reclamation sided with the subcontractor; the National Park Service's own team, including the structural engineer Fred Severud, stood by the design. Hartzog, by then Director of the National Park Service, told the Secretary to let the work continue and to fire him if the arch fell. It was built as designed.

The episode is a clear case of where erection risk sits when a structure is unprecedented. The party carrying the lifting and the temporary condition questioned the design, the owner's engineers defended it, and the decision was made above both. The permanent structure and the construction-stage structure were the same object, but the parties responsible for each saw it differently.

The closure also has a direct parallel on this site. Some 77 years earlier, the Eiffel Tower's four legs were set on sandboxes and hydraulic jacks so that their geometry could be corrected before the first platform closed them together. See the Eiffel Tower case study, and for the principles, Temporary Works & Erection Strategy and Erection Sequence Risk.

Related case studies: Eiffel Tower · London Eye · Chenab Bridge · Sheikh Zayed Bridge · Singapore Sports Hub

Core concepts: What is erection engineering? · What is heavy lift engineering?

The two unfinished legs of the Gateway Arch rising towards each other in June 1965, each topped by a creeper crane, with the red steel truss lying on the ground between them and the Mississippi River behind
The Gateway Arch in June 1965, before the truss used to support the two legs was lifted into place; the truss lies on the ground between them, with the MacArthur Bridge behind. Resized from the original for web display.
Photo: Kevin William Kelly, via Wikimedia Commons, CC BY-SA 4.0.

Sources: National Park Service — Gateway Arch fact sheet (dimensions, section count, steel and concrete quantities); National Park Service — J.E.N. Jensen, “The Construction of the Arch” (section geometry, double-wall construction, section heights); National Park Service — workmen's elevator, prime contractor and erection subcontractor; National Park Service — Arch architecture (crawler cranes to 72 ft, creeper derricks, the 255 ft stabilizing truss at 530 ft, the jacking frame used at closure); The Architect's Newspaper — start of arch construction on 12 February 1963; St. Louis Post-Dispatch — archive photographs and captions (derrick rails, truss above 530 ft, jacks, 3/8 in closure); National Park History — extracts from G. Hartzog, Battling for the National Parks, including J.E.N. Jensen's account of the closure and the change-order dispute. Secondary accounts give different figures for the width of the jacked gap and the size of the thermal closure error; those figures are not stated here.

Frequently Asked Questions

The two legs were built up from their foundations as independent cantilevers, one double-walled triangular section at a time, 142 sections in all. Crawler cranes lifted the first sections from the ground to a height of 72 ft; above that, two purpose-built creeper derricks rode track beams bolted to each leg and lifted the sections into place. Above 530 ft the legs were tied together by a temporary truss, and the final section was inserted between them on 28 October 1965.

Sunlight on the south leg was heating it more than the north leg, and the resulting expansion threatened to lengthen and twist it just as the final section was due to go in. According to J.E.N. Jensen of the National Park Service, fire hoses were used to spray water up the leg to keep both legs at approximately the same temperature, so that their geometry matched at the moment of closure.

The gap between the two legs had to be opened to receive the last section. Hydraulic jacks forced the legs apart, with Jensen describing a force of about a million pounds applied to the two legs and maintained through the closure. Contemporary reporting in the St. Louis Post-Dispatch recorded that the legs were only 3/8 of an inch out when the piece was placed.

The prime contractor was MacDonald Construction Company of St. Louis, and the fabrication and erection of the arch were carried out by the Pittsburgh-Des Moines Steel Company as its subcontractor. The structural engineering was by Fred Severud's firm, working from Eero Saarinen's design, with the National Park Service as owner.