Experimental and numerical study towards a deformation based seismic assessment of substandard exterior R.C. beam column joints

In reinforced concrete framed structures under seismic excitations the beam-column joint cores are arguably one of the most vulnerable zone. Experimental tests have shown that the structural behavior of poorly detailed joints is decisive for the structural response of older frame buildings. Due to inadequate shear reinforcement in the joint, poor bond properties of longitudinal reinforcement and deficiencies in the anchor- age of reinforcement, a brittle failure mechanism can be expected. In the numerical analysis of r.c. moment resisting frames the joint core is usually considered as rigid and all the plastic rotations are assumed to take place in the beams and/or columns. Although this assumption is reasonable for structures subjected mainly to gravity loads, it may be highly misleading for structures subjected to seismic loads. In the literature several methods to assess the shear resistance of beam-column connections were proposed, but the deformation ca- pacity of joints was not deeply investigated yet. In this study exterior beam-column joints designed for gravity only (or mainly) loads as typical of old code provisions are considered. Experimental investigations were conducted in the laboratory of the Bhabha Atomic Research Centre (BARC) in Mumbai. Three exterior beam- column joints characterized by lack of shear reinforcement in the joint panel and by different anchorage solu- tions commonly used in the construction practice until the beginning of the 1970s were tested. Numerical analyses were carried out with the finite element (FE) Code MASA, developed at the University of Stuttgart and capable of three-dimensional (3D) nonlinear analysis of quasi-brittle materials, like concrete, based on the microplane material model. In both experimental and numerical investigations particular attention was given to the evaluation of the deformation capacity of the joint. The capability to numerically reproduce the joint behavior was discussed and the influence of several parameters such as bond of longitudinal reinforcement of beam and column and shape of the anchored bars were investigated. The results were compared with the available data described in the literature and found in the tests.