Concept and Implementation of a Selective Weakening Approach for the Seismic Retrofit of Reinforced Concrete Buildings

S. Pampanin, M.G. Ireland and D.K Bull

Department of Civil Engineering, University of Canterbury, Christchurch, New Zealand

Current seismic retrofit strategies generally focus on increasing the strength/stiffness or upgrading mechanical properties of a structure. A typical drawback with this is that the upgraded behaviour might result in an increased demand on the structural and sub-structural elements, i.e. foundation. Herein proposed is a counterintuitive but rational seismic retrofit strategy of selectively weakening a structural system. Such a retrofit strategy is suitable for application to alternative seismic resisting systems and components including walls, beams, columns and diaphragm connections.

As the first phase in the development of selective weakening, the feasibility of such a retrofit strategy is discussed, with particular focus on possible applications to the seismic retrofit of existing reinforced concrete structural walls. The proposed intervention involves splitting the wall vertically and cutting it at the foundation level to change the inelastic mechanism from shear-type to a flexural/rocking-type behaviour. As part of the overall research program, a series of experimental (quasi-static cyclic) tests on 2/3 scaled reinforced concrete walls representing pre-1970 construction practice or retrofitted configurations are under preparation. A summary of the retrofit strategy design and expected behaviour will be herein given.

Recent earthquake events (e.g. Turkey 1999, 2003 and Taiwan 1999) have highlighted the undesirable behaviour of some existing reinforced concrete structures and the need for appropriate retrofit solutions. Currently two alternative approaches for seismic retrofit are conceptually adopted and implemented in practice: the first approach focuses on reducing earthquake induced forces (i.e. modifying the demand) and the second focuses on upgrading the structure to resist earthquake induced forces (i.e. modifying the capacity, Chuang and Zhuge 2005). In order to reduce earthquake induced forces, base isolation or damping devices are commonly added to the structure, whilst upgrading of the structural capacity is usually achieved by intervening on specific elements or by changing the load paths within the structure. A wide variety of different retrofit techniques for existing reinforced concrete structures including the use of advanced materials (i.e. Fibre Reinforced Polymers) have been extensively investigated and successfully implemented. Issues related to costs, invasiveness and the requirement of specialist knowledge are however typical problems encountered. A comprehensive summary can be found in fib bulletin on seismic retrofit (2003a) and on FRP (2001), while some specific approaches will be mentioned in a later section.

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