Experimental investigation on seismic performance of reinforced concrete columns under horizontally bi-directional cyclic loading

Bi-directional quasi-static tests on 9 reinforced concrete (RC) frame columns were carried out to study the seismic performance of the columns under horizontally bi-directional seismic loading. The effects of axial compression ratio, volume stirrup ratio, longitudinal reinforcement ratio and loading paths on the mechanical properties of columns were investigated. The mechanical characteristics, bearing capacity, ductility and ultimate drift angle of the columns under bi-directional loading were also analyzed. It is shown that the bi-directional loading has detrimental effect on the seismic performance of RC columns compared with the uniaxial loading. That is, the bi-directionally loaded columns exhibit more severe damage and more degradation in bending capacity and ultimate displacement. With the loading path alternating from uniaxial to cross to rhombic to square ring, strength degradation, damage degree and bending capacity degradation of the column at the same displacement may increase to some extent. It is mainly affected by the stress concentration of the cross section of column under bi-directional loading and the correlation of mechanical properties in two horizontal directions. Analysis also shows that by considering the bi-directional seismic actions, safety redundancy of story ultimate drift angle of the RC frame structure proposed by typical technical standards may be significantly reduced. Hence, the recommendations of structural design and analysis can be proposed based the preliminary experimental results. That is, in the seismic design of RC frame columns under bi-directional seismic actions, the compression ratio should be appropriately decreased, the stirrup ratio for constraint should be increased and the bending capacity of column should be adequately reduced. When the story drift angle is used for judging the structural collapse, the magnitude of the maximum story lateral displacement vector under the bi-directional seismic actions is taken as the judgment displacement, and the judgment criterion of ultimate story drift angle is selected according to the seismic analysis (structure design or analysis of existing structure). © 2019, Editorial Office of China Civil Engineering Journal. All right reserved.