July 1940: The collapse of the Tacoma Narrows bridge

One of the best-known engineering disasters resulted from a lack of knowledge of materials behaviour and of how the size of the structure would affect how it interacted with the wind

This month marks the 75th anniversary of one of engineering’s most infamous failures. The Tacoma Narrows Bridge, which (briefly!) spanned the Puget Sound in the US State of Washington, opened in July of 1940. Just five months later, the bridge was to suffer a catastrophic collapse, the iconic video footage becoming synonymous with engineering disasters.

One of the main towers of the Tacoma Narrows bridge, as pictured in The Engineer

Christened the ‘Galloping Gertie’ by construction workers due to its vertical movement in high winds, the 1,800m long structure had a main span of 853m, the third longest in the world at the time behind the Golden Gate Bridge in San Francisco and George Washington Bridge in New York City. Though several measures were introduced to correct the flaws in the bridge’s design, on 7 November Gertie went for her final gallop. In winds of just 40mph – relatively mild for the Pacific North West – the bridge began to sway and buckle dangerously. Aerostatic fluttering caused the central span to twist and contort, with the amplitude ultimately causing the suspension cables to fail.

A rare colour image of the Tacoma bridge collapse

Although the dramatic pictures of the bridge collapse were yet to reach these shores by the following week, the incident still gained several column inches in The Engineer. The report contains the testimony of a “venturesome reporter” who “when it was first seen to be cracking had the temerity to drive out upon the bridge in his car,” and “only escaped by crawling 500 yards, while the road bed was tipping to an angle of 45 degrees.”

In the 1940 article, our predecessors at Engineer Towers quote from their American contemporary Civil Engineering, published by the American society of Civil Engineers. One part of the report gives some insight into one of the underlying causes of the collapse: “The span to width ratio of the Tacoma Bridge at 72 may be contrasted with the comparable figure of 42 for the Golden Gate Bridge at San Francisco, itself at the time of its design considerably higher than that of any earlier bridge.”