Tacoma Narrows Bridge Collapse

The Tacoma Narrows Bridge was very well constructed, with carbon steel girders anchored in massive concrete blocks. The bridge was the first to use plate-type girders to help hold up the roadbed. With the earlier designs any wind would simply pass through the truss, but in the new design the wind would be diverted above and below the structure. Shortly after construction finished at the end of June (opened to traffic on July 1, 1940), it was discovered that the bridge would sway and buckle dangerously in relatively mild windy conditions for the area. This vibration caused one-half of the central span to rise while the other lowered. Drivers would see cars approaching from the opposite direction rise and fall, riding the energy wave through the bridge. However, at the time the mass of the bridge was considered sufficient to keep it structurally safe.

The November 7, 1940 failure of the bridge occurred when a never-before-seen twisting mode occurred, from winds at a mild 40 miles per hour. This is a torsional vibration mode, whereby when the left side of the roadway went down, the right side would rise, and vice versa, with the center line of the road remaining still. Specifically, it was the "second" torsional mode, in which the midpoint of the bridge remained motionless while the two halves of the bridge twisted in opposite directions. Two men proved this point by walking along the center line, unaffected by the flapping of the roadway rising and falling to each side.

Eventually, the twisting motion produced by the fluttering increased beyond the strength of the suspender cables. Once several cables failed, the weight of the deck transferred to the adjacent cables that broke in turn until almost all of the central deck fell into the water below the bridge.