The Gateway Arch was built as two separate legs measured to meet precisely at the top. On October 28, 1965, the morning sun expanded the south leg. The last piece would not fit. Workers ran fire hoses to the top and soaked the steel until it shrank. Hydraulic jacks applied 450 tons of force, and the final section slid in.
The Gateway Arch Almost Didn't Close
The Gateway Arch in St. Louis stands 630 feet tall and is the tallest monument in the United States. What most visitors never learn is how close the project came to failing on its very last day.
Two Legs, One Shot at the Top
Construction began in February 1963. Workers built two separate legs simultaneously, rising toward each other from opposite sides of the Jefferson National Expansion Memorial grounds. Each triangular steel section was stacked and welded onto the one below. Measurements were taken at night to avoid the distortion caused by sunlight heating the steel - any error at the base would compound with every section added, so precision was everything.
The Morning Everything Went Wrong
On October 28, 1965, ironworkers arrived to fit the final piece - a 10-ton, eight-foot-long triangular keystone section at the very top. The problem: the south leg had absorbed the morning sun and expanded. The gap between the two legs was now too narrow for the keystone to fit. Thermal expansion had constricted the 8.5-foot gap by 5 inches - just enough to stop the section sliding home.
Fire Hoses at the Top
Workers ran fire hoses to the top of the south leg, with fire trucks on the ground pumping a steady stream of cold water up to them. They soaked the south leg until the steel contracted enough to open the gap again.
450 Tons of Force
Cooling the leg alone was not enough. Hydraulic jacks applied roughly 450 tons of force to push the two legs apart to the required 8.5 feet. Fifteen ironworkers guided the keystone into place. It took just 13 minutes. Vice President Hubert Humphrey watched the whole thing from a helicopter overhead.
A Monument Built on Precision and a Garden Hose
The Gateway Arch opened to visitors in 1967 and today attracts millions each year. The structure is designed to flex up to 18 inches in high winds. But on the day it all came together, a cold-water hose and 450 tons of hydraulic force was what stood between an engineering masterpiece and an unfinished arch.
Frequently Asked Questions
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When was the Gateway Arch completed?
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Verified Fact
Verified 2026-06-08. 6 sources checked. Primary: Wikipedia (Gateway_Arch, read in full). Secondary: UPI Archives Oct 29 1965 (contemporary report, 403 on direct fetch - content retrieved via search snippet), NPS Fact Sheet (nps.gov/jeff), NPS Architecture page, gatewayarch.com FAQ, designingbuildings.co.uk. Claims checked: - Core drama (Oct 28 1965 keystone, thermal expansion, fire hoses, hydraulic jacks): CONFIRMED - Wikipedia, UPI, NPS - "1/64 of an inch" tolerance: REMOVED - figure absent from Wikipedia, NPS fact sheet, gatewayarch.com FAQ, and UPI contemporary report; only found on unsourced engineering blogs (designingbuildings.co.uk, earthcontactproducts.com) with no cited primary source. Replaced with qualitative "extreme precision" + "measurements taken at night" (both sourced). - "550-foot level" for fire hoses: REMOVED - zero sources specify this height. Wikipedia/UPI say only "fire hoses to the top of the south leg". Replaced with "to the top of the south leg". - "almost a foot" expansion (engagement comment): CORRECTED to "5 inches" - Wikipedia explicitly states "thermal expansion had constricted the 8.5-foot gap at the top by 5 inches (13 cm)." - 450 tons of force: CONFIRMED - UPI contemporary report (1965). Note: gatewayarch.com FAQ says "over 500 tons"; this is a documented source conflict. 450 (contemporary UPI) is retained as the primary sourced figure, with article language hedged as "roughly 450 tons". - 8.5-foot gap: CONFIRMED - Wikipedia - 13 minutes: CONFIRMED - Wikipedia - 15 ironworkers: CONFIRMED - Wikipedia ("Fifteen iron workers eased the section into place") - 10-ton, eight-foot keystone: CONFIRMED - Wikipedia ("10-short-ton, eight-foot-long triangular section"). Fixed "eight-foot-wide" -> "eight-foot-long" in article. - VP Humphrey from helicopter: CONFIRMED - Wikipedia - Oct 28 1965: CONFIRMED - Wikipedia, UPI - 18-inch wind flex: CONFIRMED - NPS Fact Sheet ("Deflection of Arch - 18'' in 150 MPH wind") and NPS Architecture page - 6 inches to spare: CONFIRMED - Wikipedia ("only 6 inches remaining"); UPI says "3 inches to spare on either side" (same total 6 inches, different framing). Kept. - "perfect catenary curve" in FAQ Q1: CORRECTED to "catenary curve" (it is a weighted catenary, not a perfect catenary). - Construction start Feb 1963: CONFIRMED - Wikipedia - Visitor center opened 1967: CONFIRMED - Wikipedia - 630 feet height + width: CONFIRMED - NPS Fact Sheet - Saarinen design, 1947-48 competition, died 1961: CONFIRMED - Wikipedia - No image_social exists (pre-imaging gate - correct, no image to null). - No scheduled_posts found for this fact. Fields corrected: text, social_text, social_caption, social_engagement_comment, article, faqs. source_url (Wikipedia Gateway_Arch): backs all confirmed claims above. Retained.
Wikipedia - Gateway ArchRelated Topics
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