Parts of a bridge

A bridge can be read from the top down. The superstructure is everything that spans: the deck that carries traffic, the girders, slabs, boxes, trusses, arches or cables that carry the deck, the wearing coat, crash barriers, railings, footpaths and drainage. It is designed for its own weight, the weight of surfacing and services, traffic, temperature and wind.

Bearings sit between the superstructure and the substructure. They transfer vertical and horizontal loads while allowing the deck to rotate and, at free bearings, to slide as it expands and contracts. Expansion joints at the ends of each continuous unit let the deck move without cracking the road surface. Both are replaceable parts with lives shorter than the bridge.

The substructure holds the bearings at the right level. Piers support intermediate spans; abutments at the ends support the last spans and retain the approach embankment, with wing walls or return walls to hold the fill at the sides. Pier caps spread bearing loads; in rivers, piers are shaped to reduce scour and resist debris and current.

Foundations carry everything into the ground: open footings on rock or firm soil, piles, or wells in deep scourable riverbeds. Approaches, the embankments or viaduct spans leading to the bridge, together with approach slabs, complete it. Design in India follows IRC 6 for loads, IRC 112 for concrete, IRC 24 for steel and IRC 78 for foundations and substructure.