INFLUENCE OF JOINT MODELING ON SEISMIC EVALUATION OF NON-SEISMICALLY DESIGNED RC FRAME STRUCTURES

Under the action of seismic forces, joints of RC frame structures are subjected to large shear stresses in the core. The seismic performance of RC frame structures is often governed by the behavior of its beam-column connections. This is especially true for the case of non-seismically designed frame structures that were constructed following old and seismically poor detailing practices. For correct assessment of seismic performance of such structures, it is essential to consider inelastic behavior and distortion of the joint core. In this paper, the influence of joint modelling on seismic evaluation of nonseismically designed (NSD) RC frame structures is highlighted through examples at sub-assembly and structural level. It is clearly brought out that not considering the inelastic behaviour of beam-column joints can not only result in over-prediction of the failure loads, but also can lead to wrong prediction of failure modes and unrealistically high ductility. The examples clearly demonstrate the importance of modelling the joint inelastic behaviour in order to capture the realistic seismic behaviour of the NSD structures. Furthermore, it is shown that a relatively simple and practical lumped plasticity based model is able to capture the seismic response of the joints of the structures well. INTRODUCTION Traditionally, beam-column joints of the reinforced concrete (RC) frame structures were considered rigid, which was credited to the confining action of the members framing into the joint core. This approach works well to analyse structures subjected to gravity loads since the joint panel is not subjected to large forces in such cases. However, under the action of seismic forces, beam-column joints are subjected to large shear stresses in the core. These shear stresses in the joint are a result of moments of opposite signs on the member ends on either side of the joint core. Typically, high bond stress requirements are also imposed on reinforcement bars of beams framing into the joint. The axial and joint shear stresses result in principal tension and compression that leads to diagonal cracking and/or crushing of concrete in the joint core. These stresses in the joint core are resisted by the socalled strut and tie mechanism (Paulay and Priestley, 1992). To prevent the shear failure of the joint core by diagonal tension, joint shear reinforcement is needed, which is therefore prescribed by the newer design codes. Moreover, these codes prescribe a large anchorage length of the bars terminating in case of exterior joints, so that a bond failure may be avoided. It is now generally accepted that the design and detailing requirements laid by new codes ensure a better seismic performance as compared to those designed as per old codes. This is especially true for the performance of beam-column joints of structures. However, a majority of the structures 1 Post-Doctoral Research Engineer, Institute of Construction Materials, University of Stuttgart, 70569 Stuttgart akanshu.sharma@iwb.uni-stuttgart.de 2 Professor Emeritus, Institute of Construction Materials, University of Stuttgart, eligehausen@gmx.de 3 Professor, Institute of Construction Materials, University of Stuttgart, hofmann@iwb.uni-stuttgart.de