A softened membrane model for composite box-girders with corrugated steel webs under pure torsion

•A new model is proposed for the behavior of RC/PC composite BGCSWs under pure torsion.•The shear strain relations between concrete slabs and CSWs employed in the model is proved to be reasonable.•The correction of thickness of shear flow in concrete slabs are considered.•The proposed model is able to predict the entire torque-twist curves and is verified by the experimental results.