longitudinal waves

The nearly 6,000-foot-long Tacoma Narrows Bridge, known as Galloping Gertie, opened up on July 1, 1940, and collapsed just four months later. Winds reached 42 miles per hour on that fateful day, which proved too intense for the structure. There were a number of causes, but the basic problem was that engineers hadn't yet learned to account for wind loads in their designs. During the planning phase, the engineers reduced the proposed depth of the concrete and steel girders beneath the roadway from 25 to 8 feet. This loosened the stiffness of the road, and made it much more susceptible to wind. In fact, before the collapse, local residents had noticed that less intense gusts could cause the bridge to move. But those movements involved longitudinal waves – one end of the bridge rose, the other fell, in a less dramatic fashion than what we see in one of the early scenes in this clip.

Prior to the collapse, though, the wind induced torsional movement. In other words, the road started to twist. While the center line stayed stable, one side of the roadbed rose and the other dropped. When this twisting motion peaked, the sidewalk on one side was 28 feet higher than the opposite one.

Eventually, this twisting motion proved too much for the structure. The cables started to snap, and chunks of the bridge fell into the water below. Finally, the entire center collapsed. With this mass gone, the sections on either end sagged dramatically, dropping more than 40 feet. Nowadays wind-tunnel testing is fairly standard for bridge designs. When engineers drew up the plans for Gertie’s replacement, which has been standing for more than 50 years, you can bet they spent a lot more time factoring in the breeze.—Gregory Mone