This paper proposed a shear lag coefficient based on the deflection of box-girders to overcome the disadvantage of the traditional stress-based shear lag theory.Replacing the shear lag function by the additional deflection

the longitudinal displacement expression was established for cross section of flanges of the box girders.The deflection function was developed for the box girders under shear lag effect by means of the variational approach.The shear lag coefficients were defined using the real deflection and the additional deflection of box girders and from this the influence of different supporting conditions on shear lag effect was analyzed.Investigations show that the traditional shear lag coefficient based on stresses can not reflect properly the shear lag effect of box girders.It can also be concluded by applying the shear lag coefficient based on deflection proposed in this paper that the shear lag effect is obviously related to supporting condition and is maximum at the clamped end

minimum at the free end

and intermediate at the simple supported end.The further the distance from a cross section to the supported end

the less the shear lag effect on the section of box girder.