A summary of test results for selective weakening and post-tensioning for retrofit of non-ductile R.C. exterior beam-column joints
This paper summarises the experimental results of a counter-intuitive seismic retrofit strategy – Selective Weakening, targeted at improving the seismic performance of poorly detailed (pre-1970) exterior beam-column joints, a critical weakness of non-ductile reinforced concrete frames. The retrofit interventions investigated comprised of: a) selectively weakening the beams (reducing joint shear demand) and/or b) adding external post-tensioning to the beam-column joints (adding horizontal confinement and axial force to the joint) with the aim to change the local inelastic mechanism of the external joints. This would result in improved global displacement, ductility and energy dissipation capacities without significant increase in demand of foundation strength, typical of strengthening retrofit. Extending from previously presented preliminary experimental results, a comprehensive test matrix and its results are presented herein. Nine 2/3-scaled exterior beam-column joints subassemblies – as-built and retrofitted were constructed and tested to investigate the feasibility and effectiveness of the proposed retrofit strategy. Parameters considered in the tests were the levels of external post-tensioning forces, locations of beam weakening, the presence of column lap-splice and the presence of cast-in-situ slab and transverse beams. Experimental results have indicated that joint shear and column lap-splice failures can be prevented or mitigated using a Selective Weakening retrofit approach, leading to the targeted seismic performance. This research is part of the six-year NZ FRST national project on “Seismic retrofit solutions for NZ multi-storey building”.