Explain the difference between static and dynamic loads in the context of bridge response and give an example of each.

• A. Dynamic loads are only due to temperature changes; static loads include only dead weight.
• B. Static loads are constant or slowly varying (dead load); dynamic loads vary with time (traffic-induced vibrations, wind gusts, seismic events).
• C. Static loads are always lighter than dynamic loads, and dynamic loads never cause issues.
• D. Static loads are unpredictable; dynamic loads are controlled by design.

Answer: (B)

Think of loads in terms of how they change over time. Static loads are essentially constant or change very slowly over the time scale of a bridge’s response. The classic static load is the dead load—the weight of the bridge itself and other permanent components like the deck, girders, and attached fixtures that don’t vary much once built. An example of a dynamic load is one that varies with time and can cause vibrations, such as traffic moving across the bridge, which produces changing forces as vehicles pass; wind gusts that push and pull on the structure; or an earthquake that delivers a sudden, time‑varying force.

Because static loads stay essentially steady, they’re analyzed with methods that assume a steady state. Dynamic loads, however, require dynamic analysis that accounts for inertia and damping to predict vibrations, potential resonances, and fatigue. An example of static load: the dead weight of the bridge itself. An example of dynamic load: the fluctuating force from passing traffic causing bridge vibrations.