Nonlinear analysis of mixed steel-concrete frames. I: Element formulation

A beam-column element for simulating the inelastic behavior of 3D mixed fra me structures comprised of steel, reinforced concrete, and/or composite mem bers is presented. The formulation makes use of the flexibility method for deriving the element stiffness equations. A stress-resultant bounding surfa ce plasticity model provides the sectional properties, which are then integ rated along the length to generate the stiffness matrices. For steel member s, the plasticity model is based on two nested surfaces, whereas for compos ite and reinforced concrete members the inner loading surface is degenerate d to a point. Stiffness degradation of reinforced concrete and composite me mbers is incorporated as a function of dissipated strain energy. Implementa tion and verification of the models for static and dynamic time-history ana lyses of mixed steel-concrete space frames are presented in a companion pap er.