Gertie - .PDF file.
Dramatic film footage showing the collapse of the Tacoma Narrows bridge, known as Galloping Gertie, provided an object lesson in how not to design a bridge. An article in The Engineer analysed the report into the collapse.
The 5,000ft-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, 40mph winds caused it to twist and eventually collapse into the river.
wrote 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, wrote 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 realised 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.