A new theory is proposed to definition the shear-lag warping displacement functions of box girders.The distribution law of shear flow of box girder under vertical bending is calculated.Warping displacement functions corresponding to mechanism of shear lag effect is defined according to shear deformation of flange plates.In order to verify applicability of new functions,shear lag of a simple supported girder with concentrated force acting on mid span is analysed concerning two models,which one is that top an...