Effect of Axial Load on Shear Behavior of Short RC Columns Under Cyclic Lateral Deformations

The influence of axial loads on the behavior of reinforced concrete short columns under cyclic bidirection deformations was studied. The experimental program consisted of ten tests of short column specimens had the same geometry and reinforcement. The specimen had a 12 in. (30 cm) square cross section with a shear span of 1.5 times the thicknessof the cross section. The shape of the specimen was selected to simulate a short column framing into a stiff floor system.

The lateral loading was controlled by monitoring deformations. Two basic histories were selected: one with displacement variation in only one direction and the other with displacement variation in only one direction and the other with displacements applied alternately in orthodgonal directions. Axial loads were held constant in some tests and varied in others. One level of constant compression and three levels of constant tension were considered. Two sequences simulated loads varying from tension to compression.

The main effect of axial compression was to increase shear strength and to accelerate the shear deterioration at higher levels of deformation as compared with the deterioration of a specimen without axial loads.

In tests with constant tension, the shear capacity and the stiffness were reduced but the shear deterioration was decreased as a reduction in shear and stiffness but only during that part of the loading history where tension was imposed.

In order to develop a design approach for columns falling in shear, a parameter was defined for identifying shear behavior. Guidelines for satisfactory performance in terms of energy dissipation were established. The procedure developed appeared to reflect adequately the influence of the main variables as indicated by the limited test data available.