A cyclic nonlinear model for concrete-filled tubes .1. Formulation

This paper presents a three-dimensional (3D) cyclic nonlinear finite-element model for concrete-filled steel tube (CFT) beam-columns and composite frame structures. This CFT beam finite element is incorporated into a matrix analysis program suitable for conducting monotonic static, cyclic static, or transient dynamic seismic analyses of complete composite unbraced frame structures composed of steel I-girders framing biaxially into CFT beam-columns. The material formulation consists of a concentrated plasticity bounding surface model in 3D stress-resultant (force) space. A polynomial expression developed in previous work to represent CFT cross-section strength is incorporated into this material model to represent the force-space yield surfaces. In addition, new formulations are present for isotropic and kinematic hardening of the loading and bounding surfaces to model strength degradation and stiffness deterioration of CFTs subjected to cyclic loading. Part II of this two-part paper presents the calibration and verification of the CFT element. This formulation is verified against a comprehensive set of experimental data and is accurate for a wide range of CFT cross-section sizes and material strengths.