HYSTERESIS PERFORMANCE EVALUATION OF SEISMIC RETROFITTING TECHNIQUES FOR SEVERELY DAMAGED RC FRAME BUILDINGS

Abstract

Stiffness irregularity or soft-storey is the most commonly noticed irregularity in many multi-storied RC frame structures, where the lateral stiffness of a storey is lesser than the above existing stories. The column members in such ground stories experience a massive shearing force and storey drifts during the event of a potential earthquake. These earthquake-induced forces lead to inelastic rotations in columns, with an eventual formation of a failure plastic hinge. These generated plastic hinges witness crushing or cracking of concrete along with buckling or yielding of longitudinal and transverse reinforcement. Seismic retrofitting of these damaged reinforced concrete buildings proves to be a challenging task for the structural community. The present research focuses on cyclic performance evaluation of conventional and non-conventional retrofitting techniques for severely damaged RC buildings, and their efficiencies are judged based on the original tested model frame. Single-storey, single-bay RC portal frames are cast and tested under reverse cyclic lateral load conditions till the formation of plastic hinges at the probable locations. Five distinctly damaged RC frames are retrofitted, where the buckled or ruptured longitudinal rebars at the column base are locally restored using the proposed interlinked coupler-box confinement technique. The first model frame, equipped with the coupler-box assemblies, is re-tested to investigate the efficiency of interlinking technique. The local-restored frame sections at the column base and beam-column joints of the second model frame are externally wrapped by carbon-fibre composites, i.e. CFRP strengthening. The global strength and stiffness of the third model frame are enhanced by the addition of hollow circular steel pipes as a lateral bracing system. The global behaviour of strength and energy dissipation potential of the fourth-numbered model frame are enhanced by the addition of a metallic damper with combined flexure and shear yield mechanisms. The fifth damaged model frame is retrofitted with a new discrete yield damper, where the vertical and lateral load-resisting mechanisms function independently. The experimental test program involves retrofitting five individual full-scale RC portal frames that are damaged under consistent vertical loads and reverse cyclic accumulating lateral drifts. The retrofitted frames are re-tested under similar loading conditions, and a comparative behaviour is established with the initial tested bare frame, which characterizes the efficiency of suggested retrofitting techniques. The crack propagation pattern, along with the failure analyses,is studied for the tested model frames. Computational parameters of dissipated energy, equivalent viscous damping, strength and stiffness degradations, and damage index are derived from the hysteresis behaviour of tested frames for a performance correlative study. Peak-oriented hysteretic models with basic and post-capping strength deterioration modes are calibrated from the experimental test results, which can be adopted in the numerical modelling of retrofitted structures. The proposed rebar coupler sleeve complies with the Indian Standard codal regulations for high-performance Class-H couplers. Load-deformation capacity of the proposed coupler-box column is doubled compared to the conventional coupler-stirrup column. Reclamation in the initial strength and stiffness of the original model frame with better post-yield behaviour is noticed in the coupler-box and coupler-box + CFRP retrofitted frames. The initial stiffness and peak lateral strength efficiency are doubled in the brace-fit retrofitted RC frame. The structural configuration of metallic yield damper with the shear plate oriented in the direction of lateral force improves the initial strength and stiffness, and the perpendicularly oriented flexural ADAS plates enhance the energy dissipation potentiality. Thus the metallic damper-fit RC frame experiences a four-fold increase in initial stiffness with a relatively high energy dissipation rate than the other tested portal frames. The proposed discrete damper experiences a proportional gain in strength with an increase in the number of adaptive yield links that undergo synchronous yielding under applied lateral force. The rate of initial stiffness gets doubled in the discrete damper-fit frame, with the energy dissipation rate nearly equal to the metallic damper frame. The vertical load-bearing pedestals remain intact, allowing for uninterrupted gravity load transmission, and the threaded-bolt arrangement supports the easy link restoration process. The performance-oriented parameters derived from the hysteresis behaviour of tested frames establish confidence in restoring the damaged structure’s pre-yield and post-yield behaviour.