How is a response-spectrum approach used to estimate seismic demand for a bridge?

• A. Use a purely static approach with a fixed lateral force equal to the peak expected ground acceleration.
• B. Apply a time-history analysis with multiple earthquake records.
• C. Compute spectral accelerations at the structure’s fundamental period, scale by damping and mass, and derive lateral forces for design checks.
• D. Estimate lateral forces based on wind loads.

Answer: (C)

The response-spectrum approach estimates seismic demand by translating ground motion into spectral accelerations for the structure’s vibration characteristics, then turning those accelerations into lateral forces for design checks. For a bridge, you first identify the fundamental period of the structure and choose an appropriate damping level. The spectrum provides Sa values, which are the peak accelerations a single-degree-of-freedom system with that period and damping would experience under the ground motion. Multiply this spectral acceleration by the effective seismic mass to obtain an equivalent lateral force (base shear). That force is then distributed along the bridge according to a reasonable mode-shape or design rule to estimate the demands on different members.

This method captures how the bridge responds dynamically to shaking, including resonance effects and how longer-period structures interact with ground motion, without needing full time-history simulations. It sits between a simple static force estimate and a full time-history analysis, offering a practical yet physics-based way to estimate seismic demands.

Wind loads are not used for seismic demand, and a purely static fixed force or a time-history approach would either miss dynamic amplification or require more detailed data, respectively.