Effective flange width definition for steel-concrete composite bridge girder

A composite section is made up of a concrete slab attached to a steel girder by means of shear connectors. Under positive bending moment, part of the slab will act as the flange of the girder resisting the longitudinal compression. When the spacing between the girders becomes large, it is evident that simple beam theory does not strictly apply because the longitudinal compressive stress in the flange will vary with distance from the girder web, the flange being more highly stressed over the web than in the extremities. This phenomenon is termed "shear lag." For design purposes, the effective flange width was introduced into national and international design specifications, whereby various effective flange width formulae were derived based on different analytical and experimental results. Accordingly, the effective flange width is generally less than unity, which is not realistic for a small girder spacing. In current effective flange width for mulae, the theoretical derivation is based primarily on a planar stress distribution reflecting shear lag at the central fiber of the concrete. However, this simplification ignores the fact that stresses vary through the thickness. This through-thickness variation needs to be taken into account to produce a more viable representation of effective flange width criteria. Hence, the need for a different definition of the effective flange width becomes apparent. This paper proposes a different method for defining the effective flange width for the composite section, which can be utilized with the results obtained from the finite-element analysis. A three dimensional finite-element model of the composite bridge is verified, and a numerical example illustrating the proposed effective flange width definition is provided.