This week in 1941: Galloping Gertie bridge collapses

Published 3 September 2008

The Tacoma Narrows bridge, known as Galloping Gertie, was a 5,000 ft-long, two-lane suspension bridge — the third longest of its kind in the world; it was the first suspension bridge to use plate girders, rather than open lattice beam trusses, to support the roadbed meaning that wind could not pass through the truss but was diverted above and below the structure; 67 years ago this week the wind was just too strong

Yoggi Bera said that “Nostalgia ain’t what it used to be,” but still: this week in 1941 the Tacoma Narrows bridge, known as Galloping Gertie, collapsed, in the process offering a lesson in how not to design a bridge. There is an interesting article in the Engineer analyzing the collapse and its causes.

The 5,000 ft-long, two-lane suspension bridge was the third longest of its kind in the world. It was also the first suspension bridge of its type to use plate girders, rather than open lattice beam trusses, to support the roadbed. This meant that with the new design, wind could not pass through the truss but was diverted above and below the structure, leading the bridge to sway and buckle in winds.

This motion was not thought to threaten the bridge’s structural integrity but, four months later, 40 mph winds caused it to twist and eventually collapse into the river. The Engineer writes that despite being “well designed and built to resist safely all static forces usually considered in the design of similar structures,” the bridge was simply unable to withstand the “excessive oscillations.”  These, writes the magazine, were caused by “the structure’s extraordinary degree of flexibility and its relatively small capacity to absorb dynamic forces.”

The article said it was not realized that the forces that had proved disastrous to lighter, shorter flexible suspension bridges would affect a structure of such magnitude as Tacoma. “While there have been a number of bridge failures attributed to wind, that of Tacoma brought the question of aerodynamic action on suspension bridges into greater prominence than ever before,” concluded the Engineer