Bridge loading

A road bridge must carry its own weight and everything that crosses it over its life. In India the loads and their combinations are given by IRC 6. Traffic is represented by standard vehicle trains: Class A, a train of wheeled vehicles for normal design, placed lane by lane; Class 70R, a heavy tracked and wheeled military-type load; Class AA, similar heavy loads used for certain bridges; and Special Vehicle loads for very heavy trailers on designated routes. The designer places them in the worst positions for each member.

Moving vehicles hit joints and bumps and make the deck bounce, so live load is increased by an impact factor. For Class A on concrete bridges it falls with span, from about half for very short spans to well under a quarter for long ones; steel bridges have their own formula. Footpath loads, braking forces, centrifugal forces on curved bridges and the effect of vehicle collision on parapets and piers are added.

Environmental loads include wind, temperature (both uniform change and the gradient through the deck depth), earthquake, water current, buoyancy, earth pressure on abutments and, in some places, snow and ice. Construction stage loads, erection equipment and creep and shrinkage effects also count.

Loads are combined into limit-state combinations with partial factors: the ultimate limit state for strength, the serviceability limit state for stresses, cracks and deflection, and others for fatigue and accidental events. Influence lines are used to find the worst positions of moving loads.