Abstract： By the traditional bar simulation method, only the shear lag effect of box girder with constant section can be analyzed. To improve the applicability of the existing method, a new equivalent method of analogy bar areas and governing differential equations of shear lag effect was derived for the variable box-section girder. It was further validated by the test results of a cantilever beam made of Perspex. The effects of girder depth and web thickness on the shear lag of box girder, including shear lag coefficients and span lengths of positive shear lag region, were discussed. The reason why the shear lag effect of variable box-section girder was weaker than that of constant section girder were illustrated by analyzing the distribution of shear flow in web and flange of box girder along span direction. The results show that the variation of girder depth and web thicknesses will reduce the shear lag effect of box girder, which is attributed to the reduction of shear stress level in webs of variable box-section girder. It is the firstly increasing and then decreasing trend of shear flows in flange of box girder that cause the variation of positive and negative shear lag phenomenon. The span length of positive shear lag region in variable section box girder will increase, but the maximum shear lag coefficient may decrease. It is recommended that the height and web depth should be adjusted to avoid the drastic variety of shear follow in the web of box girder. The constant or gradually changing shear flow in box-girder webs may effectively depress the shear lag effect of box girder.
