A proposal for the numerical modeling of monotonic and cyclic behavior of exterior RC beam-column joints

The vulnerability of exterior beam-column joints in existing RC buildings represents a key aspect to ensure structural integrity of buildings under seismic loadings. Recent seismic aftermaths showed substantial damages resulting from beam-column joints in older-type non-seismically designed frame structures, often leading to undesirable brittle failure mechanisms. RC buildings constructed prior to developing current seismic codes and guidelines generally lack joint transverse reinforcements. As a result, the seismic assessment of existing RC buildings necessarily requires to consider beam-column joints, by correctly evaluating their strength and deformability local capacity and, consequently, their contribution to the seismic global response of structures. In order to realistically predict the seismic performance of such buildings, accurate experimental and numerical studies for assessing the inelastic joint behavior have been recently developed by the scientific community. In the current literature are available different theoretical models for the prediction of the joint shear capacity, mainly obtained from calibration processes involving experimental databases. Although these models represent valuable tools, they are based on different formulas and suggest values, which particularly affect the evaluation of both the shear strength and the deformability. In this context, the current study aimed to improve the existing theoretical and numerical models concerning the seismic response of exterior beam-column joints without transverse reinforcements. To this purpose, a proposal for the numerical modeling of both monotonic and cyclic behavior of exterior RC beam-column joints is developed by using a macro-modeling approach. The beam-column joint element is modeled through the well-known “scissors model” in which the two main mechanisms governing the overall behavior of the RC joints are considered by means of two nonlinear rotational springs in series. In particular, the first spring represents the shear deformation of the joint panel, while the second one represents the “fixed-end-rotation” of the beam due to the debonding of the longitudinal steel rebars at the beam-joint interface. In the model, the two springs are defined by proper moment-rotation constitutive laws selected from the literature. For the spring simulating the shear behavior of the joint panel, various combinations of some literature proposals are taken into account with the purpose to identify the laws better simulating the overall response of the RC joint. The numerical simulations are performed by using the OpenSees software in which the ability of the considered constitutive laws to reproduce the monotonic behavior of RC beam-column joints is assessed by considering an experimental database of cyclic tests collected from the literature; the RC joints included in the database are characterized by different geometric dimensions, material properties and structural details features. By minimizing the error between the numerical and experimental results in terms of force-displacement curves, the models which better approximate the monotonic behavior of the joints are identified. Then, a calibration process is carried out on the shear stress and strain parameters in order to improve the existing multilinear law. The identified laws are also used to perform cyclic analyses, which depend on several parameters describing the unload-reload path, the strength and stiffness degradation and the pinching effect. Starting from a proper damage rule, a calibration process is carried out to define appropriate ranges of these parameters. The numerical simulations provide a satisfactory agreement with the experimental results. Finally, in order to accurately predict and simulate the joint response at structural level, the proposed joint model is implemented in the numerical modeling of a 2D frame case study from the literature. Pushover analyses have been performed and the resulting capacity curve proved that the numerical model is able to accurately reproduce the observed nonlinear behavior of the frame.