Seismic retrofit of reinforced concrete frames with diagonal prestressing or FRP strips

Performance of reinforced concrete structures during previous earthquakes has indicated that the majority of buildings designed prior to the enactment of modern seismic codes and those designed more recently in regions where code enforcement is difficult to achieve, have suffered seismic damage associated with non-ductile frame construction. Because it is not economically feasible to replace seismically deficient building infrastructure with new and improved buildings, seismic retrofit strategy remains to be the most viable approach to seismic risk mitigation.; Currently, there are retrofit techniques available to stiffen and strengthen non-ductile concrete frames with unreinforced masonry infill panels, while other techniques are used to improve deformability of elements. The available retrofit techniques may be viewed in three categories; (i) those that involve the addition of structural elements, such as steel bracing systems or concrete shear walls to control deformations, (ii) those that involve surface treatment of masonry walls by adding concrete overlays with steel mesh, bonding steel plates on both sides and using fiber reinforced polymer (FRP) composite materials, and (iii) those that involve reinforcing masonry walls internally. The first category of retrofit methods, while stiffens and strengthens the frames, often results in a change of dynamic properties of structures by increasing mass and reducing period of the structure.; The main objective of the proposed research is to develop new and improved seismic retrofit techniques for non-ductile reinforced concrete frame structures with and without masonry infill walls. The techniques to be investigated involve the application of surface bonded and FRP anchored diagonal FRP sheets and externally applied diagonal prestressing.; The experimental and the analytical results indicate that the two techniques used in the research were able to control the structure deflection.; The initial prestressing in the cables has effect on the structure drift demand. If it is high, the strands yield and may even rupture during loading and result in high drift demand, as opposed to non-prestressed strands that may continue remaining elastic. However, the situation reverses during small drift demands, where, increased prestressing results in reduced drift demands as long as the strands remain elastic. This implies that there exists an optimum level of prestressing.; Also the results indicate that CFRP strips can be used to effectively control lateral drift during strong earthquakes, protecting non-ductile frame buildings.