Beam finite element for thin-walled box girders considering shear lag and shear deformation effects

•A new beam finite element is developed for predicting the performance of shear lag effect.•The warping displacement of the cross-section is defined as the sum of five deformation modes.•Homogeneous solutions of the differential equations are selected as interpolation functions.•Only one more DOF is required to account for the shear lag effect.•The structure performance can be well captured by the B3S beam element.