Time-dependent analysis of shear-lag effect in composite beams

Taking into account the long-term behavior of the concrete, a model for ana lyzing the shear-lag effect in composite beams with flexible shear connecti on is proposed. By assuming the slab loss of planarity described by a fixed warping function, the linear kinematics of the composite beam is expressed by means of four unknown functions: the vertical displacement of the whole cross section; the axial displacements of the concrete slab and of the ste el beam; and the intensity of the warping (shear-lag function). A variation al balance condition is imposed by the virtual work theorem for three-dimen sional bodies, from which the local formulation of the problem, which invol ves four equilibrium equations with the relevant boundary conditions, is ac hieved. The assumptions of linear elastic behavior for the steel beam and t he shear connection and of linear viscoelastic behavior for the concrete sl ab lead to an integral-differential type system, which is numerically integ rated. The numerical procedure, based on the step-by-step general method an d the finite-difference method, is illustrated and applied to an example of practical interest.