Which phenomenon is considered the most critical for the dynamic stability of suspension bridges due to the risk of collapse?

• A. Damping
• B. Divergence
• C. Vortex Shedding
• D. Flutter

Answer: (D)

The most dangerous dynamic issue for long-span suspension bridges is flutter. It’s a self-excited, aeroelastic instability where aerodynamic forces couple with the structure’s bending and torsional motions in just the right way that energy is fed into the oscillation as the wind blows. When the wind speed reaches a critical value, the oscillations can grow because the aerodynamic input overcomes the structure’s damping, leading to rapidly increasing motions that can culminate in collapse. The Tacoma Narrows Bridge collapse is the classic illustration of flutter turning a wind-induced vibration into a catastrophic failure.

Damping is the mechanism that dissipates energy and helps control motions, so higher damping is beneficial, but it’s not itself a destabilizing phenomenon. Divergence is a static instability caused by lift increasing with deflection, leading to a large deformation under steady wind, but it’s not the dynamic, self-amplifying process that characterizes flutter. Vortex shedding can cause periodic forces and vibrations, but with proper design and damping these are typically manageable and do not inherently drive the catastrophic growth seen with flutter. Flutter uniquely represents a dynamic instability where energy transfer from wind sustains and amplifies motion, making it the primary concern for dynamic stability and collapse risk in suspension bridges.