Distinguish buffeting and vortex-induced vibration and their implications for bridge design.

• A. Buffeting is high-frequency, VIV is low-frequency.
• B. Buffeting is low-to-mid frequency random wind loading; vortex-induced vibration is resonant oscillation from vortex shedding.
• C. Buffeting occurs in earthquakes.
• D. Vortex-induced vibration is always beneficial to stability.

Answer: (B)

Buffeting and vortex-induced vibration describe two different ways wind interacts with a bridge, and recognizing their distinction is essential for proper aeroelastic design. Buffeting is a random, broadband loading caused by turbulent wind fluctuations; the resulting lift and drag forces contain a range of frequencies rather than a single, clean tone, so the bridge responds to a spectrum of excitations and fatigue design must account for stochastic loading and dynamic amplification. Vortex-induced vibration, on the other hand, arises from a regular pattern of vortices shed behind a bluff bridge element; this creates forces that oscillate at a specific shedding frequency that can lock in with the structure’s natural frequency, producing resonant, potentially large-amplitude oscillations if damping is low. In design practice, buffeting drives probabilistic or spectral dynamic analyses and fatigue considerations, while VIV guides measures to avoid resonance—such as modifying natural frequencies, increasing damping, or altering geometry to disrupt vortex shedding. The statement that buffeting is low-to-mid frequency random wind loading and vortex-induced vibration is resonant oscillation from vortex shedding captures these core differences.