The classical Vlasov theory of isotropic thin-walled sections is extended to sections made of composite laminates. The shear deformation of the corss section is accounted for in the formulation of the theory by including the shear properties of the walls in the warping function. All possible elastic couplings such as extension-torsion, bending-torsion, and bending extension are included in the present model. The St. Venant torque arises naturally by considering the walls of the beams as thin-walled shells. In the present model, the tangential stresses are not assumed to the zero, instead, the tangential force and moment resultants are assumed to be zero. To validate the present model, one- and two-cell FRP pultruded composite box beams were tested under tip torsional loads, and their structural response in terms of angle of twist was recorded, which compared well with the analytical model.
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