Existing reinforced concrete (RC) buildings de signed prior to the 1970s are vulnerable to shear failure of beam-column joints under earthquake excitations because of insufficient transverse reinforcement in the joint region. The failure of such “unreinforced” joints, especially corner joints, has played a crucial role in building collapses in past earthquakes. The accurate prediction of shear strength and flexibility for these unreinforced beam-column joints is therefore essential to assess the seismic risk of older-type RC buildings characterized by having unreinforced beam-column joints. To predict shear strength, two shear-strength models were developed previously, as a part of this larger study. A practical shear-strength mode l combining the two previously developed models is presented in this report.
To validate the shear strength models and to develop a moment-rotation relationship (backbone relationship) of unreinforced corner beam-column joint, four full-scale unreinforced corner beam-column joint specimens with two orthogonal beams and floor slab were tested under quasi-static cyclic load reversals simulating earthquake loading. The test results show that the joint shear strengths decrease with increase of the joint aspect ratio, and for a certain joint aspect ratio, the joint shear strengths are proportional to the beam longitudinal reinforcement ratio within the range of variables investigated in the test matrix. The proposed three models accurately predicted the shear strengths of the tested specimens. Based on the measured joint responses and visual observations of the test ed four corner joint specimens, a backbone relationship was developed for nonlinear joint macro-modeling. Furthermore, the proposed backbone relationship was modified to be applicable to interior and roof beam-column joints. The strength parameters in the backbone relationships were defined using the proposed joint shear strength models. These backbone relationships were validated by accurate reproduction of the force-drift responses of the tested four corner joint specimens in this study and eight other exterior and interior joint specimens taken from the literature. Using these backbone relationships, nonlinear dynamic simulations were performed on three hypothetical prototype RC building frames. These simulations indicate that consideration of the flexibilities for unreinforced joints is important for seismic assessment of older-type RC buildings characterized by having unreinforced joints.
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