BIAXIAL CURVATURE AND DUCTILITY CAPACITY OF RC COLUMN BASE CROSS SECTIONS

The deformation performance of the base cross sections of reinforced concrete buildings is fundamental when large seismic events occur allowing the structure to have large excursions in nonlinear field and guaranteeing an overall ductile behaviour. It is well known that the axial force acting on columns significantly reduces the curvature capacity of the sections and for this reason the technical codes give design criteria stating a limitation in order to preserve the displacement capacity. It is also recognized that when biaxial bending occur the cross section undergo a loss in strength capacity. Starting the study of from Bresler (1960), which provided suitable expression to predict 3D limit interaction surfaces, several numerical and analytical models were developed to take into account the biaxial interaction in strength. Simultaneously it is noteworthy to point out that the presence of biaxial bending also influences the deformation capacity of sections causing in most of the cases a relevant loss of the curvature and ductility available with respect to the one owned along the principal axes. This important issue is not faced by technical codes and not exhaustively treated in scientific literature as it was done for strength. Moreover nonlinear structural models based on lumped plasticity do not take into account these interaction aspects when defining plastic hinge properties in terms of curvature capacity. The paper presents a numerical study in which the deformation capacity of RC cross sections subjected to axial load and biaxial bending is investigated by means of a fiber discretization. A procedure for the numerical definition of biaxial domains of ultimate curvature, yielding curvature and curvature ductility is provided and the sensitivity of the biaxial deformation performance to some geometrical and mechanical parameters (aspect ratio, concrete strength and confinement efficacy) is discussed.