Nonlinear Analysis of Mixed Steel-Concrete Frames. I: Element Formulation

A beam-column element for simulating the inelastic behavior of 3D mixed frame structures comprised of steel, reinforced concrete, and/or composite members is presented. The formulation makes use of the flexibility method for deriving the element stiffness equations. A stress-resultant bounding surface plasticity model provides the sectional properties, which are then integrated along the length to generate the stiffness matrices. For steel members, the plasticity model is based on two nested surfaces, whereas for composite and reinforced concrete members the inner loading surface is degenerated to a point. Stiffness degradation of reinforced concrete and composite members is incorporated as a function of dissipated strain energy. Implementation and verification of the models for static and dynamic time-history analyses of mixed steel-concrete space frames are presented in a companion paper.