Seismic behavior of ultra short columns of square tubed high strength reinforced concrete

The seismic behavior of square tubed high strength RC (STRC) ultra short columns was studied by testing three STRC columns and one common RC column under combined axial constant compression and lateral cyclic load, with the test parameter being the axial load ratio (0.35, 0.45 and 0.55). The test results indicated that the STRC ultra short column exhibited much higher shear strength, ductility, deformation ability and energy dissipation ability than common RC columns, due to the fact that the steel tube confined the high strength concrete effectively and the web of the tube resisted the lateral shear force. The failure mode of the column with the axial load ratio of 0.35 was flexure failure, while the failure mode of the two columns with the axial load ratios of 0.45 and 0.55 was shear failure. The shear strength increased as the axial load ratio of the columns increased for STRC ultra short columns, while the ductility and ultimate deformation ability decreased as the axial load ratio of the columns increased. The results from elastic-plastic analysis of the steel tube indicated that, during the lateral loading process, the steel tube did not yield for the STRC columns with flexural failure and the steel tube could possibly yield after the peak load for the STRC columns with shear failure. A design equation for calculating the shear strength of STRC columns and design advices were proposed based on the test and analysis results.