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New Zealand and Australian steel structure design standards contain equations to encourage yielding at the ends of steel members rather than along their lengths. This paper evaluates the accuracy of these equations using a commercially available computer program as well as an analytical procedure. The analytical procedure considers non-linear geometric effects and material effects of the member stiffness by considering stability functions in conjunction with residual stress effects. New equations to prevent yielding away from the member ends, which are less conservative than the current code equations, are developed. Simplifications of these equations being considered for adoption into the New Zealand steel structure design standard are described.

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In a current research project the influence that floor slabs containing precast prestressed units have on the seismic performance of plastic hinges in beams of moment resisting frames is being examined. A three dimensional, approximately half scale, one storey, two bay moment resisting frame together with adjacent floor slab has been constructed and tested. The experimental results show that the building code, ACI 318-05, underestimates the flexural strength of beams by a considerable margin. An analytical model has been developed which predicts the flexural, shear and elongation response of plastic hinges in beams subjected to inelastic rotation history and varying axial load levels. This model can be used to predict the interaction between beams in moment resisting frames and floor slabs containing precast prestressed units. Analysis using this model shows good agreement with the experimental results.

This paper describes the development of an analytical plastic hinge element that can predict flexural, axial, shear and elongation deformation in plastic hinges. The element consists of layers of longitudinal and diagonal springs that represent the behavior of concrete and reinforcement. These springs are modeled using nonlinear path-dependent cyclic stress-strain relationships of either concrete or reinforcing bars depending on their position in the cross section and also depending on the mechanisms they are representing. Through a preliminary scrutiny of the analytical results, in-depth qualitative and quantitative information on the causes and consequences of plastic hinge elongation are discussed. An extensive experimental verification of the plastic hinge element is presented in a companion paper.