Stress-Strain Relation Models for Square Contined Reinforced Concrete Columns

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In order to evaluate ultimate strength and suctility of flexurally failing reinforced concrete members, the use of axial stress - strain relation models of concrete, which represent behaviors of concrete practically as far as in the range of descending part after maximum stress, will be necessary. In this study, two series of experiments were conducted in order to propose a more practical stress - strain relation model for square confined r/c columns.In series i, 15 square columns, each 150mm square by 530mm high, containing different arrangements of square steel hoops were to concentric axial force only. The stress - strain relation model was proposed based on the results of series 1. The increments of maximum stress and axial strain at maximum stress by hoop confinement were considered. The effects of square confinement were also considered in this model, using reduction factors of core concrete of sections. the reduction factors were calculated as ratios of area of core concrete confined effectively by hoops to that of core concrete confined not effectively.In series 2, 8 one side fixed and another side free type square columns, each 180 mm square by 500 mm high ( loaded portion ), containing same arrangements of hoops with two types of series 1 were subjected to not only high axial force but shear force and flexural moment. Six speximens of series 2 were subjected to eccentric axial force under constant top displacement of 2DY, 4DY and 8DY maintained by a horizontal jack, in which dy is a yielding displacement of tensile main bars. (series 2-1). two specimens of series 2 were subjected to horizontal shear force under constant axial force (series 2-2). The base moment - curvature relations of speciments of series 2 were simulated using the stress - strain relations proposed.