What they were working with
- 100 t
- Each creeper derrick. One per leg, taking over from the crawler cranes at 72 ft.1
- 43 × 32 ft
- The derrick platform. It carried a tool shed, a toilet and the comms gear, and telescoping legs kept it level as the leg curved over.1
- 2 × 12WF
- The track. Two beams 24 ft apart, about 2 ft off the outside face, each bracket held with four 1¼ in bolts.1
- 4 pins
- 5¾ in pins held the sled to the track.1
- 48 ft
- One length of track. The derrick moved up after every fourth section.1
- 50 t
- A 12 ft section off the derrick. About half an hour per lift.1
- 3 jacks
- Each section landed about 4 in high on three 35-ton screw jacks, which left the joint open for the weld.1
- ¼ in
- Type 304 stainless outside, ⅜ in carbon steel inside, 1¾ in at the corners.1
- 300 ft
- Concrete and tension bars between the skins up to here. Steel stiffeners only above.1
- 142
- Sections in the Arch.2 This model sets 70 per leg and counts the last piece at the crown as number 71 for both.
- 255 ft
- The stabilizing truss. 40 ft wide, 15 ft deep, about 60 tons, 325 high-strength bolts, picked in one piece by both derricks.1
- 1 elevator
- The gang rode a workmen's elevator up the leg to the platform. Its run got longer as the Arch grew.1
What the derricks looked like
No written source gives a paint color. The one color photo the Park Service publishes shows the derrick framing in red-orange, the color of shop primer, so the model's derricks are now painted that way and not crane yellow. The black and white shot shows PDM lettered across the platform, a white shed on the deck and a lattice boom. In the truss photo the booms are banded light and dark, so the model's booms are banded red and white. The band count is by eye.1
MacDonald Construction was the general contractor. Pittsburgh-Des Moines Steel built the steel.4 Boilermakers of Local 659 in Warren, Pennsylvania fabricated about 80 percent of the sections, and ironworkers erected them on site.5
The raising gang lifted, landed, fitted and welded every section, by the account of a man who worked on it.4 They worked hundreds of feet up with no nets.3 Men on the job have described walking scaffold boards with very little protection, wind off the river running 35 to 70 miles an hour, and tying off to the Arch with a rope so the wind would not take them.4
Two things are left off this page on purpose: the ironworkers' local number and the size of the crew. Neither turned up in a source solid enough to print.
Topping day
October 28, 1965. The legs sat about 2.5 ft apart at the top and the last piece was 8 ft wide. Jacks forced the legs to 8.5 ft so it would go in.6
Sun on the south leg was the problem. Heat grows the steel on one side and the leg moves. A raising-gang radio man has said readings had been running about a foot off, and that the St. Louis Fire Department ran cold water down the back of the south leg that morning to hold it.4 It is also why the surveyors shot the legs at night.3
The last piece was set at 11:00 in the morning.3 The operator was on the ground and could not see the work, so the radio man talked it in.4
The jacks put thrust into the crown before they came off. Then the derricks lowered the truss and backed down the legs, taking their track with them, and every bolt hole in the stainless was plugged and ground smooth.1
Check the math
y = 693.8597 − 68.7672 · cosh(0.0100333 · x)
That is the published curve for the centers of the triangles, in feet, for x from −299.2239 to +299.2239.7 The model builds the legs straight from it.
area = 125.1406 · cosh(0.0100333 · x) sq ft
That is the published area of each triangle.8 Run both and you get 54 ft sides at the base, 17 ft at the crown, and top of steel at 630.00 ft.
What is not from a drawing
Section heights between the 12 ft base sections and the crown, the pick weights (scaled from the 50-ton figure by size), the shape of the derricks and cranes (the derrick color is matched by eye to the Park Service color photo), the rigging, the pick path beside the leg, where the pad and track sit in the yard, the scaffold at the joint, and where each man stands. The truss is drawn 255 ft long with a harness making up the difference to the legs. How far the tips lean in before jacking is drawn to match the 2.5 ft gap. Stories are figured at 10 ft each.
This is a learning study. It is not engineering, and it is not a rigging or erection plan.
Sources
- Gateway Arch architecture, National Park Service
- Gateway Arch fact sheet, National Park Service
- The builders of the Gateway Arch, TIME, October 28, 2015
- Builders of the Gateway Arch say it was a monumental task and an honor, St. Louis Public Radio, 2015
- Boilermakers who built the Arch sections, St. Louis/Southern Illinois Labor Tribune, 2012
- A lesson in Gateway Arch architecture, Gateway Arch Park Foundation
- The Gateway Arch, Differential Calculus: From Practice to Theory
- Gateway Arch model notes, with the section area equation, Thingiverse
A learning study by Andrew Windisch, Steel Erection Field Notes. Built with Claude.