Selective weakening retrofit for existing R.C. structures - concept, validation and design example
A summary of the current research towards the development and validation of a counter-intuitive seismic retrofit strategy for non-ductile reinforced concrete (RC) frames, termed as selective weakening (SW) retrofit, is herein presented. The SW retrofit is conceived with the aim for wide-implementation, economical, and noninvasive structural retrofit solution for non-ductile (pre-1970s) RC frame structures. Contrary to the misconception that seismic retrofit must involved strengthening (force-based approach) SW retrofit relies upon targeted weakening of structural elements in order to achieve a ductile failure mechanism, thus explicitly enforcing capacity design philosophy within a displacement-based retrofit strategy. In this research, the SW retrofit is implemented to RC frames and more specifically to exterior beam-column (b-c) joints. The bottom longitudinal reinforcements of the beam are cut at the interface with the column and/or external horizontal pre-stressing is applied to the joint. A more desirable inelastic mechanism can be attained within the b-c connection, leading to improved global seismic performance for the RC frame. Experimental validations of the SW retrofit solution of exterior b-c joints are summarized to complement the conceptual development of the SW retrofit. Insights from quasi-static tests on nine 2/3-scaled exterior b-c joint sub-assemblies (with and without retrofit) are discussed in respect to different retrofitting and as-built parameters. Lastly, a simple hand-calculation retrofit design procedure for a SW retrofit on b-c joints is presented. The results demonstrate the viability of such a simple but structurally efficient seismic rehabilitation strategy.