Experimental and Analytical Studies on Old Reinforced Concrete Buildings with Seismically Vulnerable Beam-Column Joints

Existing reinforced concrete (RC) buildings designed prior to 1970s are vulnerable to shear failure in beam-column joints under earthquake loads because of insufficient transverse reinforcement in the joint region. To assess the seismic risk of old RC buildings, the accurate prediction of shear strength and deformability for these unreinforced beam-column joints is essential. Several joint shear strength models are available in the literature but they have been originally developed to predict the shear strength of reinforced beam-column joints. Due to the different shear force transfer mechanism between reinforced and unreinforced beam-column joints, the existing models have little success to assess the shear strength of unreinforced beam-column joints. The ASCE/SEI 41-06 provisions specify shear strengths and backbone curves for unreinforced beam-column joints but the predictions using these provisions are usually conservative compared with many experimental test data collected from the literature. This study is focusing on developing accurate shear strength models and backbone relationships for unreinforced exterior and corner beam-column joints.This study proposes two shear strength models, semi-empirical and analytical, for unreinforced exterior and corner beam-column joints to reflect the influence of two key parameters: (1) joint aspect ratio which is defined as the ratio of beam to column cross-section heights, and (2) beam reinforcement index which is related to the amount of beam longitudinal reinforcement in tension. These key parameters are determined from a parametric study using a large experimental data set of unreinforced exterior and corner beam-column joints from the published literature. The proposed models are validated by accurate predictions of the shear strength for the database specimens. Besides the accuracy of the proposed models, the semi-empirical model has the advantage of straightforward extension to other types of beam-column joints. An important advantage of the analytical model is that for the case of beam yielding followed by joint failure, the analytical model can predict the reduced shear strength without the need for the complexity of ductility consideration.The experimental phase of this study includes testing four full-scale unreinforced corner beam-column joint specimens. These four specimens are designed to investigate the effect of the joint aspect ratio and the beam longitudinal reinforcement ratio. The test results show that the joint shear strengths are reduced with increase of the joint aspect ratio and for each of the joint aspect ratio, the joint shear strengths are proportional to the beam longitudinal reinforcement ratio within the range provided in the test specimens. The shear strengths of the four specimens are accurately predicted by the two proposed models, while the ASCE/SEI 41-06 provisions for shear strength produce conservative estimates of the strengths for the test specimens.Based on the measured joint shear stress-rotation and visual observation of the tested corner beam-column joint specimens, a multi-linear backbone relationship is proposed in this study to reflect the following beam-column joint responses: (1) initial joint cracking, (2) either beam reinforcement yielding or significant opening of existing joint cracks, (3) either existing joint cracks further propagation or additional joint cracks opening at the peak load, and (4) residual joint shear stress and rotation after severe damage in the joint. Corresponding parameters in the backbone relationship are defined from the comparison with test results. The proposed backbone relationship is verified by the simulations for beam-column subassemblies of the tested four specimens and other four planar exterior beam-column joint specimens from the literature. To investigate the effect of beam-column joint flexibility on the lateral response in a structural system level, nonlinear static and dynamic simulations are performed. These simulations indicate that beam-column joint flexibility is essential for older-type RC buildings characterized by having unreinforced beam-column joints. As an extension of this study, progressive collapse analysis for older-type RC buildings will be pursued with the proposed beam-column joint backbone relationships