Prestressed bridge girders

Most road and metro viaducts in India are carried by prestressed concrete girders. For spans of about 20 to 40 m, precast pre-tensioned or post-tensioned I-girders are cast in a yard, lifted onto bearings and made composite with a cast-in-place deck slab. For longer or curved spans, single-cell or multi-cell box girders are cast in place on falsework or built from precast segments joined by post-tensioning, by span-by-span or balanced cantilever erection.

Design follows IRC 112 for concrete bridges, with loads and combinations from IRC 6, including vehicle classes, impact, braking, temperature gradient and earthquake. The girder must be checked at every stage: transfer in the yard, lifting and transport, erection before the slab is composite, the composite section under the deck and wearing coat, and full service with live load and temperature. Long-term creep and shrinkage redistribute stresses between precast girder and in-situ slab.

Serviceability usually decides the prestress. A common requirement is limited or no tension at the bottom fibre under the frequent load combination, so that cracks do not open and let water reach the strands. Ultimate flexure and shear, including web crushing in thin box webs, are then verified. Durability detailing, cover, duct grouting quality and drainage are as important as strength.

Inspection and maintenance close the loop. Grout voids and corroded strands have caused failures worldwide, so modern practice emphasises full grouting with tested materials, sealed anchorages and, increasingly, monitoring.