Role of beam-column joints in the seismic response of non-conforming RC frames

The seismic response of non-conforming Reinforced Concrete (RC) buildings can be significantly affected by the behavior of beam-column joints involved in the failure mechanisms, especially in typical non-conforming buildings. Many retrofit strategies for existing joints are available, but commonly accepted tools to model and assess non-conforming joints – that are the start-point for retrofit – are not available. Furthermore, conventional modeling approaches consider only beam and column flexibility, although joints can provide a significant contribution also to the overall frame deformability. In this study, the attention is focused on exterior joints with no transverse reinforcement and a possible approach to model them in RC frames is presented. First, experimental tests on unreinforced exterior joints performed by the Authors, together with other tests with similar features and failure modes from literature, are employed to calibrate the joint panel deformability contribution to reproduce numerically the experimental joint shear stress-strain behavior under cyclic loading. After a validation phase of the proposal, and the choice of a proper joint shear strength model from literature, a numerical investigation on the influence of joints on the seismic behavior of a case study RC frame designed according to an obsolete technical (widespread in Italian and Mediterranean building stock) is performed. Structural models that (i) explicitly include nonlinear behavior of unreinforced beam-column joints or (ii) consider joints as elements with infinite strength and stiffness (as in the case of retrofitted joints or in new designed structures) are built, and their seismic performance are compared. A probabilistic assessment based on nonlinear dynamic simulations is performed to evaluate the seismic response at different damage states accounting for uncertainties in ground-motion.