boxgirder

A box girder is a structural member whose cross-section is a hollow rectangle or square, forming a closed box. Box girders are used as principal supports in bridges and elevated structures, often providing high bending and torsional stiffness in a relatively shallow depth.

There are two main families: steel box girders and concrete box girders (often prestressed). Steel box girders are fabricated from flat steel plates that are welded or bolted together to form a hollow box, sometimes in multiple cells or bays. Concrete box girders are typically prestressed and cast with a hollow rectangular cross-section; they may be cast in place or built in segments that are post-tensioned or fully prestressed after assembly. In both cases, transverse diaphragms or webs connect the interior of the box to resist shear and distribute loads, while external girders and diaphragms provide stiffness and continuity.

The primary advantages of box girders include excellent torsional rigidity and lateral stability, efficient resistance to bending in deep spans, and the ability to house prestressing tendons or utility conduits within a sealed interior. The closed cross-section reduces wind and dynamic effects and enables relatively long spans with moderate deck depths. Box girders are popular for highway and railway bridges, long-span viaducts, and elevated transit structures.

Design considerations for box girders involve ensuring adequate shear transfer between elements, accounting for torsional effects, and protecting steel or concrete from corrosion and moisture ingress. Construction methods vary: welded steel box girders; plate-assembled or segmental concrete box girders with post-tensioning; and hybrid steel-concrete configurations. Overall, the box girder form combines structural efficiency with versatility for long spans and complex geometries.