Analytical and numerical assessment of seismically vulnerable corner connections under bidirectional loading in RC framed structures

Abstract

Modern reinforced concrete (RC) framed structures are designed to resist earthquake loading using the capacity design method which divides structures into dissipative and non-dissipative zones to ensure structural integrity under earthquake loading. However, there are still many existing reinforced concrete (RC) buildings which were not designed with modern seismic codes and are consequently seismically vulnerable. The majority of these buildings are vulnerable to earthquake loading due to the provision of insufficient transverse reinforcement within beam-column joint regions. A great deal of research has been devoted so far to explaining the response of joints in planar frames. However real RC buildings are 3D structural systems in which beam-column connections are subjected to complex stress states. This study mainly focuses on the 3D response of RC corner beam-column joints under seismic loading which is yet to be fully understood. The research develops a novel biaxial joint model for calculating biaxial joint shear strength based on an evaluation of existing experimental data of 3D biaxially loaded specimens. The proposed biaxial hysteretic model is an extension to the existing uniaxial Pivot rules and the three parameter model using an elliptical interaction curve. The proposed biaxial model considers the effect of variations in the angle of loading between the corner connection framing beams. Some previous experimental tests of corner beam-column connections have been conducted under a fixed loading angle of 45 degrees. However, tests performed with varying angles of loading are more representative of real seismic events. This variation can cause significant degradation in joint strength under cyclic loading. The proposed biaxial joint model models biaxial strength degradation using a biaxial damage index which is defined in terms of the dissipated hysteretic energy and the ultimate displacement. Joint degradation in one direction is assumed to be influenced by the degradation in the other direction. The proposed biaxial model is validated using experimental data available in the literature. The research investigates the influence of joints on the overall response of reinforced concrete framed structures under seismic loading. Comparisons are made between numerical models of RC framed structures with rigid joints, flexible uncoupled uniaxial (uncoupled) edge and corner joints, and flexible uniaxial joints at edges and biaxial (coupled) joints at corners. The comparisons are made for regular and irregular buildings, designed for gravity and wind loads, which are intended to be representative of the many existing structures subject to but not designed for seismic loading. The numerical models are implemented in ADAPTIC, which is a general program for nonlinear static and dynamic analysis of structures. ADAPTIC is subsequently used to simulate the response of 2D and 3D framed structures under seismic loading. The aim is to examine the influence of beam-column joints on the response of 2D and 3D framed structures under seismic loading. The research provides insight into the effect of providing a detailed representation of corner joint response in the seismic assessment of existing reinforced concrete framed structures.