A comprehensive mixed distributed plasticity fiber formulation developed at UIUC over the last several years for rectangular CFT beam-columns as part of complete three-dimensional composite frames is being extended to cover circular CFT and SRC beam-columns and thus enable a full range of investigations on this project.
Both the steel and concrete constitutive models are based on comprehensive formulations that account for key characteristics such as, in the steel, gradual reduction in modulus, gradual reduction in elastic zone, ratcheting, overshooting, local buckling and resulting softening, and different material parameters in the corners and flats of cold formed steel tubes; and in the concrete, nonlinear response up until the peak stress, variation in the peak stress with confinement, post-peak softening, post-peak ductility due to confinement, tension stiffening, and crack opening and closure via cycling into tension and back into compression.
The formulation also accounts for the cyclic slip response between the steel and concrete, and can track behavior ranging from perfect bond to complete slip. This is particularly important in the connection region. The formulation is calibrated to a large number of tests of composite beam-columns, connections, and frames subjected to static and dynamic loading. The formulation is implemented in a special version of OpenSees.