Base shear is defined as the horizontal seismic demand integrated at base. Which statement best describes the standard approach to estimating base shear for bridges?

• A. It is computed solely from the mass of the structure and gravity loads.
• B. It is estimated using weather patterns and wind speed.
• C. It is calculated from the static distribution of all loads without considering dynamic effects.
• D. It uses response-spectrum or pushover analyses to determine lateral forces and then distribute them through the structure.

Answer: (D)

Seismic base shear for bridges comes from how the structure actually responds to earthquake ground motion, not from static gravity loads alone. The realistic way to estimate this lateral demand is to use analysis methods that capture the dynamic nature of shaking.

Two widely used approaches are response-spectrum analysis and nonlinear pushover analysis. Response-spectrum analysis applies a site-specific seismic spectrum to the bridge’s modal modes to estimate the peak lateral forces the structure would experience, and then combines those modal forces to obtain the base shear. Pushover analysis, on the other hand, applies increasing lateral loads until the structure reaches its nonlinear capacity, producing a base-shear versus displacement curve that reflects how the bridge would behave in strong shaking. In both cases, the computed lateral forces are then distributed through the structure based on its stiffness and capacity of members, giving the forces in individual elements and supports.

The other options miss the essential dynamic aspect: seismic demands arise from inertia and amplification during ground motion, not from gravity loads, wind, or a purely static distribution of all loads.