In this paper, a new numerical model for the evaluation of the seismic behaviour of existing reinforced concrete (RC) structure with considering the effect of variation of applied axial load on columns is presented. Focus is given on developing accurate and practical models for simulating nonlinearities in joint core as well as column under varying axial load. For exterior as well as interior joints, according to experimental and finite element results, principal tensile stresses versus shear deformation relations in joint core are recommended. Moreover, to consider the effects of stirrup on the principal tensile stress-shear deformation relation, an incremental procedure is proposed based on the Mohr theory. According to the proposed numerical model, complex nonlinear behaviour of joint core is simulated by two diagonal axial springs so that the effect of variation of axial load is also considered. The properties of these springs are determined based on the proposed principal tensile stress-shear deformation relation in the joint core. Moreover, the shear and flexural nonlinear behaviour of RC beams and columns is also simulated by rotational springs. A simplified methodology is developed to consider the effects of axial load variation on shear and flexural nonlinear behaviour of RC columns. To demonstrate the capability of the proposed numerical model at structural level, two RC frames with various failure modes are investigated. The results confirm the ability of the model in predicting the nonlinear behaviour of the frame, which can provide an alternative method for engineers in practice.
