During an earthquake, what can occur when ground motion resonates with a bridge's natural frequency?

• A. Reduced loads
• B. Large amplifications in response and potential failure
• C. No effect
• D. Immediate settlement

Answer: (B)

When a bridge is shaken by an earthquake, it has natural frequencies at which it tends to vibrate. If the ground motion contains energy at one of those frequencies, the bridge can enter resonance, where the external push aligns with the bridge’s own vibration mode and the motions add up rather than cancel out. This dynamic amplification means the bridge experiences much larger displacements and higher internal forces than in non-resonant shaking, increasing the likelihood of damage or even failure if damping is insufficient or the members are already near capacity. Damping can reduce the peak, but resonance still drives a significantly amplified response.

Other options don’t capture this behavior. Resonance does not imply reduced loads, no effect, or immediate settlement; it specifically involves amplified dynamic response and potential structural failure under those amplified forces.