On November 7, 1940, at about 11:00 am, the first Tacoma Narrows suspension bridge, the third longest of its kind in the world at the time, collapsed. Situated on the Tacoma Narrows in Puget Sound, near the city of Tacoma, Washington, the bridge had only been open for traffic for a few months. Even in that short time, the bridge had become known for its violent oscillations during high winds. On November 7, these oscillations became sufficiently large to snap a support cable in the centre of the bridge, and the unbalanced load that was created caused the bridge to begin twisting in the wind, which eventually led to the bridge’s collapse.
See the bridge twisting … a dramatic short mpeg movie (833k)
Bridge designers in the mid twentieth century had been using steel trusses for stiffening in bridges, and the immense weight of all that steel made most bridges so stable that the designers were unaware of the effects of wind. When heavy railroad traffic began to decline and automobile traffic increase, many more bridges were needed, and the lighter loads presented by road vehicles allowed designers in turn to build much lighter bridges. The Tacoma Narrows bridge was built using very light sheet steel beams, and the weight of these was considered sufficient to damp out any vibrations caused by wind.
They were wrong. The winds in Puget Sound were sufficient to make the bridge bounce up and down … a lot. All by itself, this kind of vibration wouldn’t have hurt the bridge. But a support bracket on the centre span slipped during one series of vibrations, causing the center cables to loosen. This immediately resulted in severe twisting motions as the wind continued to gust, which soon caused the bridge to break up.
The collapse of the Tacoma Narrows bridge was due to a combination of factors. Its light weight and narrow structure made it prone to resonance … a vibration in synch with the 42 mph wind gusts (much like pumping a playground swing to make it go higher). Severe turbulence on the leeward side of the bridge may also have been a factor.
Architects and engineers today recognize the importance of analyzing a structure aerodynamically, to predict how it will behave in severe winds. This is done using wind tunnel tests on models, and by mathematical and computer modelling.
In 1950, a new 18 million dollar bridge opened on the site of the first Tacoma Narrows Bridge. Tested in wind tunnels, the four-lane deck has 8 metre deep stiffening trusses that form a box, a design that resists torsional (twisting) forces. There are also hydraulic dampers at the ends to control vibrations.