ABSTRACT
This chapter focuses on straight steel–concrete composite box-girder bridges. A single box girder with multiple cells is economical for very long spans. Multiple webs reduce the flange shear lag and also share the shear forces. A composite box section usually consists of two webs, a bottom flange, two top flanges and shear connectors welded to the top flange at the interface between concrete deck and the steel section. Steel composite box girders are usually built of three steel sides and a composite concrete deck. Internal diaphragms not only provide warping restraint to the box girder, but improve distribution of live loads, depending on their axial stiffness which prevents distortion. Box-girder bridges should be checked for strength and stability during various construction stages. The compactness of a multiple steel boxes is controlled only by web slenderness. The purpose of the ductility requirement is to prevent permanent crushing of the concrete slab when the composite section approaches its plastic moment capacity.
