Seismic risk mitigation through retrofitting nonductile concrete frame systems

A large proportion of existing buildings across the world consists of non-ductile structural systems. Performance of structures during recent earthquakes has demonstrated seismic vulnerability of these systems. The majority were designed prior to the enactment of modem seismic codes, while some were designed more recently in areas where code enforcement can not be ensured. These structures constitute significant seismic risk, especially in large metropolitan centers. Because it is economically not feasible to replace a large segment of the existing infrastructure with seismically superior systems, retrofitting non-ductile systems remains to be a viable seismic risk mitigation strategy. The objective of this paper is to highlight recent research at the University of Ottawa in Canada on seismic retrofit strategies for such systems. Specifically, projects on i) concrete frames with unreinforced masonry (URM) infill walls retrofitted with surface bonded FRP sheets, ii) non-ductile reinforced concrete frames braced by diagonal prestressing, and iii) the use of active control and smart structure technology for seismic retrofitting non-ductile reinforced concrete frames are presented. Research findings indicate that the application of these retrofit techniques produces good to excellent improvements in strength and inelastic deformability of otherwise seismically deficient non-ductile systems.