Deformation Capacity of Reinforced Concrete Columns with Varying Axial Load and Diagonal Lateral Load

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Recent earthquake resistant design concept of structures places explicit emphases on the inelastic deformation capacity in addition to the previously accepted resisting capacity. Deformation capacities of reinforced concrete members are determined by shear failure, bond failure, flexural failure or buckling of main bars after flexural yielding of the sections. In the design guidelines for earthquake resistant reinforced concrete buildings based on ultimate strength concept proposed by Architectural Institute of Japan, practical design equations for ductility are presented for the first two failure mechanisms. However, only ambiguous specification for arranging methods are regulated for the last two failure mechanisms.The objective of this paper is to propose an evaluating equation for deformation capacities determined by flexural failure, which may be caused by the compressive failure of the core concrete. The effects of the compressive failure of the core concrete become significant for corner columns subjected to varying high axial load. Therefore, these problems are discussed paying special attention to the effects of the varying axial load. Core sections with the same amount of longitudinal reinforcement both in tension and compression were studied. The ragid-plastic relation was assumed for steel bars and the stress block relation without energy dissipation under cyclic loading was assumed for concrete. Under these assumptions the ultimate curvature of a column subjected to varying axial load can be expressed. Finally, the proposed evaluating equation for deformation capacities determined by flexural failure was foun to be effective to evaluate the deformation capacities of columns with varying axialload.