A distributed plasticity model for concrete-filled steel tube beam-columns with interlayer slip

A fiber-based distributed plasticity finite element formulation is presented to perform three-dimensional monotonic analysis of square or rectangular concrete-filled steel tube (CFT) beam-columns. This stiffness-based beam-column element fomulation accounts comprehensively for all significant geometric nonlinearity exhibited by CFT beam-columns as part of composite frame structures, and the steel and concrete constitutive models account for the significant inelastic phenomena which are seen in CFT experiments. In addition, the finite element formulation accounts for slip between the steel and concrete components of the CFT by incorporation of a nonlinear slip interface. This formulation is able to capture behavior ranging from perfect bond to immediate slip. The calibration and verification of the slip formulation are presented, and the finite element model is verified against experiments of CFT beam-columns subjected to monotonic loading. Results from a preliminary study are presented on the effect of slip on CFT beam-column and composite frame behavior. A related paper extends this formulation to cyclic analysis of composite CFT frames, and provides details of the steel and concrete constitutive models. (C) 1998 Elsevier Science Ltd.