An inelastic model for low cycle fatigue prediction in steel braces

This paper presents a nonlinear inelastic cyclic model to predict the effect of low cycle fatigue on the behavior of steel brace elements. This brace element is assumed as a beam-column element with pinned ends and a plastic hinge at mid-span. The kinematic hardening rule, based on the concept of a yielding surface in the theory of plasticity, together with the tangent modulus of elasticity, have been taken into account. Furthermore, a simplification of the linear cumulative damage theory is used to represent the low cycle fatigue deterioration of the brace element during the inelastic cyclic behavior. In the presented method, as an expansion of physical model theory, the governing yield function of the plastic behavior of the brace element is transformed mathematically by different factors. Then the yield surface which is defined in resultant stresses space, is changed and diminished by separately altering the axial load and bending moment terms of the yield function in each step of loading history to manifest low cycle fatigue deterioration. Comparison of the results of the proposed model with two existing experimental results has shown that this model was capable of predicting the low cycle fatigue of brace elements, and in one example, the rupture point of the brace element was predicted well by the model. It is advised that the model should be further expanded to more precisely include the effect of local buckling and distortion of brace elements with different cross-sectional geometries at the plastic hinge.