Shear lag in typical box-girder bridge
For multiple-cell box girders, there are normally two arrangements. First one is that independent cells are connected by their top flanges only while the other one is that the cells are connected both at bottom and top flanges. From structural point of view, it's recommended to adopt the second arrangement. For the case of cells associated by top flanges only, their flanges are heavily stressed in transverse direction owing to flexure that can't be effectively distributed across the cross section.
In structural analysis of bridges shear lag have to be considered in design in some situations. Shear lag occurs when some parts of cross section aren't directly connected. For a box-girder bridgenot all parts of flanges are joined directly to webs so that connected part becomes highly stressed whereas unconnected flanges aren't fully stressed. Particularly for wide flanges of box-girder bridges axial loads are transferred by shear from webs to flanges that result in distortion in their planes. Therefore plane sections don't stay plane and stress distribution in the flanges aren't uniform. Furthermore there is a tendency for longitudinal in-plane displacements of bride deck away from web/flange connection to lag behind those parts of bridge in close vicinity to web/flange connection.
The effect of shear lag causes longitudinal stress at flange/web connection to be higher than mean stress across the flange. Thus the effect of shear lag has to be catered for in the design of box-girder bridges specifically for those with wide flanges.