The Engineering Book: From the Catapult to the Curiosity Rover: 250 Milestones in the History of Engineering

1940

Tacoma Narrows Bridge

Leon Moisseiff (1872–1943)

Imagine that you are an engineer designing a bridge. You follow the best practices of the day, but then the bridge collapses in spectacular fashion. When this happens, engineers learn that their equations are not perfect. The result: best practices are updated, equations are modified, and the world of engineered structures becomes a little bit safer. This is exactly what happened with the Tacoma Narrows Suspension Bridge, designed by Leon Moisseiff, when it disintegrated in 1940.

Suspension bridges were quite common in 1940. Moisseiff himself had designed the Manhattan Bridge in 1909 and the Golden Gate Bridge in 1937. These were gigantic bridges with gigantic spans, and Moisseiff became a highly regarded expert on designing suspension bridges.

So when Moisseiff designed the Tacoma Narrows Bridge across the Puget Sound near Seattle, it probably seemed straightforward. The bridge would need towers, the suspension cables, anchors for the cables, the suspenders, and the deck. There were budget constraints, so when Moisseiff proposed a less expensive design using smaller girders to support the deck instead of the 3x larger trusses that engineers had used in the past, his design won out for two reasons: 1) it was less expensive to build because it contained less steel, and 2) the thinner deck cross section was aesthetically pleasing. Moisseiff himself called the Tacoma Narrows “the most beautiful bridge in the world.”

The problem: engineers did not fully understand aeroelastic flutter. If you have ever experienced a taught rope vibrating in strong wind, you have seen the phenomenon. The bridge’s girders were not stiff enough, and in a strong wind it began to flutter too. In this case, the deck acted as an immensely long and heavy rope, so it fluttered with huge up and down motions that caused it to rip itself apart. If you watch the videos of the bridge before the collapse, the amount of motion seems impossible.

Although fortunately there were no human casualties, engineers analyzed the failure. Since then, the decks of suspension bridges must be stiff enough to prevent this problem.

Pictured: The Tacoma Narrows Bridge collapsing.