Cyclic plastic hinges with degradation effects for frame structures

A model of cyclic plastic hinges in frame structures including degradation effects for stiffness and strength is developed. The model is formulated via potentials in terms of section forces. It consists of a yield surface, described in a generic format permitting representation of general convex shapes including corners, and a set of evolution equations based on an internal energy potential and a plastic flow potential.

The form of these potentials is specified by five parameters for each generalized stress-strain component describing yield level, ultimate stress capacity, elastic and elastoplastic stiffnesses, and a shape parameter. The model permits gradual changes in stiffness and strength parameters via damage-based degradation.

The degradation effects are introduced in the energy and flow potentials and result in additional evolution equations for the corresponding strength and stiffness parameters. The cyclic plastic hinges are introduced into a six-component equilibrium-based beam element, using additive element and hinge flexibilities.

When converted to stiffness format the plastic hinges are incorporated into the element stiffness matrix. The cyclic plastic hinge model is implemented in a computer program and used for analysis of some simple structures, illustrating the characteristic features of the cyclic response and the accuracy of the proposed model.