The first half of the last century witnessed a burgeoning enthusiasm for the suspension bridge, especially for the clean lines and simple elegance of the so-called suspended deck design. The world’s most famous example, the Golden Gate Bridge straddling San Francisco Bay, opened in 1937. Three years later the quest for ever more slender and graceful structures culminated in the Tacoma Narrows Bridge across Puget Sound in Washington State. Connecting the under-developed Olympic peninsula with the Washington mainland, the bridge was not designed for heavy traffic volumes so, even though it had a central span of 853 m (2,800 ft), it had a width of just 11.9 m (39 ft) – enough for a single carriageway and walkway in each direction.
The Tacoma Narrows Bridge opened with much fanfare in July 1940. For all the high expectations its life proved spectacularly short. Just four months later the deck of the bridge began to twist; in the lateral forces of a stiff cross-wind blowing up the Narrows one autumn morning.
A certain amount of vibration along the length of the deck had in fact been observed in lighter winds as soon as the bridge was finished, and this had gained it the nickname of ‘Galloping Gertie’.
On this occasion the carriageways were not only rippling lengthwise but the torsional oscillations were also pushing one side of the deck as much as 8.5 m (28 ft) above the other. This extraordinary sight continued for over an hour until the bridge could take the strain no longer and its central span broke and fell into the water. This has become a particularly famous engineering disaster because the whole episode was captured on film and many a school physics lesson has been enlivened by this remarkable footage.
When: November 7 1940
Where: Puget Sound, Washington, USA
Death toll: The bridge’s protracted death-throes gave good notice of the collapse so there were fortunately no human casualties on this occasion. The only fatality was a dog too frightened to leave a car that had been abandoned on the bridge.
You should know: Because of world War ll it was ten years before a new bridge was built across the Narrows, this time with a wider deck and open trusses used as stiffeners instead of the previous solid-plate girders.
