Mixed formulation of nonlinear steel-concrete composite beam element

This paper presents an inelastic beam element for the analysis of steel-concrete girders with partial composite action under monotonic and cyclic loads. The element is derived from a two-field mixed formulation with independent approximation of internal forces and transverse displacements. The nonlinear response of the steel and concrete component of the girder is based on the section discretization into fibers with uniaxial hysteretic material models for the constituent materials. The partial interaction between concrete deck and steel girder through shear connectors is accounted for by an interface model with distributed force transfer characteristics. A direct state determination algorithm for the implementation of the composite element in a general purpose nonlinear analysis program is presented, and the stability characteristics of the algorithm are discussed in conjunction with the selection of appropriate force and displacement interpolation functions, The validity of the model is established by correlation of analytical results with experimental evidence. Numerical studies are used to compare the model with the classical displacement formulation. These studies confirm the superiority of the proposed model in modeling composite girders under monotonic and cyclic loads that induce stiffness deterioration and strength softening.