Five Forces Acting on Bridges

Bridge engineers design for five primary load categories:

1. Dead Load: Permanent weight of structure (deck, girders, piers, abutments, railings, utilities). Constant.
2. Live Load: Transient weight of traffic (vehicles, pedestrians, trains), including dynamic impact factor.
3. Wind Load: Lateral and uplift forces from wind pressure on superstructure, substructure, vehicles. Critical for long spans.
4. Seismic Load: Inertial forces from ground shaking. Depends on zone, soil, structure period. Ductility detailing essential.
5. Hydrodynamic Load: Water pressure, buoyancy, scour, debris impact on piers/abutments in rivers/coasts. Includes flood, tsunami, ice.

Additional considerations: thermal expansion, settlement, construction loads, collision (vessels/vehicles), blast. Codes (AASHTO, Eurocode) specify combinations and load factors for LRFD/ASD design.