Improved method for shear lag analysis of thin-walled box girders considering axial equilibrium and shear deformation

The conventional method (CM) for shear lag analysis of thin-walled box girders supposes that the neutral axis coincides with the centroidal axis of the whole cross section, and thus neglects the axial equilibrium condition. An analytical method considering axial equilibrium and shear deformation, which is referred to as PM, is proposed for overcoming the defect of the conventional method. The longitudinal displacement of the web is introduced to satisfy the axial equilibrium condition and locate the neutral axis automatically. The shear deformation is considered according to the Timoshenko beam theory. Three independent shear lag functions are adopted for describing different shear lag intensities of the top, bottom and cantilever slabs. The governing differential equations for the displacement variables are deduced by means of the virtual work theorem and solved with the relevant boundary conditions. The analytical solution is then derived for the distance from the neutral axis to the top fiber of the cross section. The accuracy of the proposed method is validated by comparisons with the available experimental data as well as the finite element analysis results. Moreover, the distances from the neutral axis to the top fiber, axial forces, and stress difference ratios are analyzed for typical thin-walled box girders to quantify the influence of the axial equilibrium condition. Finally, an extensive parametric study is conducted to examine the effects of various geometric parameters on the stress difference ratio under different load types. The results show that the proposed method can provide good predictions for both deflections and axial stresses, and neglecting axial equilibrium leads to considerable underestimations of axial stresses especially at the top flange-web junctions over the interior support of the continuous box girder.