What wind velocity caused the Tacoma Narrows Bridge collapse?

• A. 15 m/s
• B. 22 m/s
• C. 19 m/s
• D. 25 m/s

Answer: (C)

The key idea here is aeroelastic flutter: when wind interacts with a flexible bridge, the pressure forces can transfer energy into the structure’s twisting motion. If damping is low and the wind speed reaches a critical level, the twisting oscillations can grow each cycle instead of dying out, leading to rapid, unstable motion and eventual failure. For the Tacoma Narrows Bridge, that unstable flutter set in at a wind speed around 19 meters per second (roughly 42 miles per hour). At about this speed, the wind’s pressure distribution around the deck produced a twisting moment that fed energy into the bridge’s torsional mode faster than the structure could dissipate it, causing large, self-excited oscillations that the bridge could not withstand. So, the wind velocity around 19 m/s matches the conditions that triggered the aeroelastic flutter that collapsed the bridge.