Experimental Behavior of Nonconforming RC Columns with Plain Bars under Constant Axial Load and Biaxial Bending

Available capacity models for the assessment of seismic capacity of reinforced concrete columns are mainly based on experimental tests under axial load and uniaxial bending. Furthermore, experimental tests have been mainly performed on columns representative of new constructions. In order to analyze the effects of biaxial bending on the seismic performances of existing r.c. columns with design characteristics non-conforming to present day seismic codes and practices, the paper presents the results of a first stage of tests on r.c columns under both axial load and biaxial bending, which effectively simulate the seismic actions. In particular, the paper presents results of four tests on full scale square columns reinforced with plain bars, subjected to constant axial load and biaxial cyclic actions. Two different cyclic displacement paths were investigated: horizontal displacements with an inclination angle of 45or 30 with respect to the cross section principal axes. The influence of biaxial bending actions on the global behavior of existing r.c. columns in terms of stiffness, strength, and deformation capacity as well as the effects of different horizontal displacements orientation on their performances are herein investigated and discussed; the comparison with experimental outcomes provided by uniaxial bending tests on companion specimens are also presented. Experimental findings showed that biaxial bending actions affect columns rotational capacity more than strength. The rotational capacity reduction owing to biaxial bending actions, even if more significant than the flexural capacity reduction, is not currently taken into account in the available seismic guidelines related to existing r.c. buildings.