Inelastic cyclic behaviour of locally buckled plastic hinges of steel members

Post-local buckling behavior is a very important consideration in plastic and seismic design of steel structures. It describes the structural behavior up to the final collapse state. Past investigations in this area are normally to model the structural members with a large amount of plates or shell elements. The finite strips element approach is considered a more efficient approach. The drawbacks of the above two approaches are the complexity and computational inefficiency when applied to framed structures.; As part of this study two series of experiments were performed in order to evaluate general cyclic behavior without local buckling and monotonic and cyclic post-buckling behavior. The experiments have shown that, during the course of cyclic plastic deformation, the material always tends to follow the cyclic strain path regardless of whether the strain paths are proportional or nonproportional. The moment-curvature relationship exhibits straight line bounds to which the nonlinear portions approach asymptotically. The load surface expanded almost isotropically and approaches a limiting surface. The straight line bounds are approximately parallel to each other. From the local buckling tests, the critical deformation capacity for the cyclic loading condition is lower than 60% of the value for the monotonic pure bending condition. The influence of local buckling on the behavior of pure bending for monotonic and cyclic loading condition is small, but the one under monotonic bending with a large initial axial strain as well as the one under combined bending and axial for both monotonic and cyclic loading condition is serious.; A simple analytical model, based on the stress resultant plasticity model developed earlier at the University at Buffalo for predicting post-local buckling behavior for cyclic and nonproportional loading histories, has been proposed. This model, that is calibrated by the experimental results, utilizes the cyclic post-local buckling moment-curvature relationship, and can describe the strength and the stiffness deterioration which are the important factors in the cyclic post-local buckling behavior of thin-walled structural members. The analytical moment-curvature results compare well with the experimental results in the constant amplitude cycling, and linearized dissipated energy deterioration which is very important in seismic design can be predicted from the proposed model.