Applying the elementary beam theory to thin-walled flexural members can result in inaccurate estimation of the stresses along the flange and underestimation of the beam deflection. This is due to shear lag effects, which causes uneven normal stresses in the flanges. The potential problems associated with the non-uniform stress have been identified in many engineering structures, including box beams in bridges and T-shaped members in building structures. Shear lag effects in thin-walled flexural members have been considered in design of buildings and bridges through a parameter defined as effective flange width, which has been established based on elastic analyses. As ultimate limit state design being widely adopted, the effect of material nonlinearity on shear lag needs to be quantified.This study investigated nonlinear shear-lag effects in thin-walled flexural members for bridges and buildings through analytical and experimental means. The variation analysis method was adopted in this study after various analytical approaches in the literature were examined and their limitations were identified. The existing variation analysis method was modified by adopting a Taylor series function to represent the uneven longitudinal displacement in the flanges. The prediction of normal stresses and member displacements by the proposed variation analysis was compared with the results of experimental tests of rectangular box girders in the literature and the tests of two steel box beams in this study The comparison indicated that the proposed analysis method can greatly improve the prediction of elastic shear lag effects.The proposed variation analysis was then extended to model the nonlinear shear lag of steel box beams. An effective modulus was formulated, which was modified from the elastic modulus considering both elastic and plastic deformations in a box beam. The extension also implemented a post-yielding Poisson's ratio determined through coupon tests. Two simply supported steel box beams were loaded beyond full yielding of the flanges. Comparison of the measured strains and the predicted nonlinear shear lag effects indicated that the proposed method can accurately capture the strain, stress and deflection of steel box beams.A series of parametric studies were conducted to investigate critical parameters for modeling shear lag effects. The results of parametric studies were used to evaluate the existing design provisions for box girders in the current AASHTO LRFD Specifications. Modifications to the existing AASHTO LRFD Specifications were proposed for consideration in future revisions.
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