Experimental and Numerical Validation of Selective Weakening Retrofit for Existing Non-Ductile R.C. Frames

Abstract

The development of cheap, whilst effective and relatively non-invasive structural retrofit techniques for existing non-ductile reinforced concrete (RC) structures still remains the most challenging issue for a wide implementation on a macro scale. Seismic retrofit is too often being confused as purely structural strengthening. As part of a six-years national project on “Seismic retrofit solutions for NZ multi-storey building”, focus has been given at the University of Canterbury on the development of a counter-intuitive retrofit strategy for earthquake vulnerable existing rc frame, based on a “selective weakening” (SW) approach. After an overview of the SW concept, this paper presents the experimental and numerical validation of a SW retrofit strategy for earthquake vulnerable existing RC frame with particular focus on the exterior beam-column (b-c) joints. The exterior b-c joint is a critically vulnerable region in many existing pre-1970s RC frames. By selectively weakening the beam by cutting the bottom longitudinal reinforcements and/or adding external pre-stressing to the b-c joint, a more desirable inelastic mechanism can be attained, leading to improved global seismic performance. The so-called SW retrofit is implemented on four 2/3-scaled exterior RC b-c joint subassemblies, tested under quasi-static cyclic loading at the University of Canterbury. Complemented by refined 3D Finite Element (FE) models and dynamic time-history analyses results, the experimental results have shown the potential of a simple and cost-effective yet structurally efficient structural rehabilitation technique. The research also demonstrated the potential of advanced 3D fracture-mechanics-based microplane concrete modelling for refined FE analysis of non-ductile RC b-c joints. Keyword: Selective weakening, seismic retrofit / rehabilitation, concrete beamcolumn joint, modelling concrete, 1.0 INTRODUCTION AND STATEMENT OF PROBLEM The recent publications of technical guidelines [11, 22], FEMA 356 pre-standard [10] and international standards (Eurocode8-Part 3 [8] the ASCE-SEI 41 standard[2]) represent significant progress and address crucial needs in seismic rehabilitation and retrofit for modern urban society. It is widely recognised that poorly detailed nonductile reinforced concrete (RC) moment-resisting frames pose significant risks to urban population in seismic zones. Research on the seismic performance of pre706 ATC & SEI 2009 Conference on Improving the Seismic Performance of Existing Buildings and Other Structures