Seismic Retrofit of Existing Substandard Reinforced Concrete Beam-Column Joints with Hybrid NSM Steel Bars and EB GFRP Wrap Technique

Abstract

Earthquakes are responsible for the death and injury of many thousands of people each year. These deaths primarily occur because the buildings in which they are housed do not have sufficient seismic resistance and therefore collapse killing the occupants. Although we have made tremendous progress in understanding how earthquakes impact buildings and developed building stock with very high levels of collapse resistance, in low-income countries, a considerable proportion of the building inventory still has major structural deficiencies, such as poor-quality concrete and improper detailing of reinforcement. Such deficiencies mean that these structures are still at significant risk of failure during earthquakes. To remedy this situation, it is necessary to retrofit these at-risk structures; however, to do this requires cheap and effective retrofitting technologies. Existing retrofitting techniques do exist, but they are often time-consuming to install, expensive or impractical and therefore restrict how quickly the building stock of a region can be strengthened. This study investigated the behaviour of unstrengthened and strengthened full-scale exterior reinforced concrete (RC) beam-column joints (BCJs) under cyclic load. The retrofitting schemes considered were the Externally Bonded (EB) Glass Fibre-Reinforced Polymers (GFRP) sheet, and the Near-Surface Mounted (NSM) U-shaped steel techniques. Four specimens were designed, built and tested. All four had the same initial specification, with the first specimen being a control which represented a typical connection that had been designed to an outdated Turkish building code, the second specimen was strengthened with u-shaped steel bars using the NSM method. The third specimen was retrofitted with GFRP sheets using the EB method, and the fourth specimen was strengthened using a hybrid method incorporating both of the above-mentioned techniques and developed as part of this thesis. The experimental findings showed that the retrofitted beam-column joints had significantly improved strength, had slightly improved stiffness, and the EB and hybrid methods had significantly greater energy dissipation and ductility compared to the control specimen. According to the experimental results, the specimen strengthened using the NSM method showed an increase in strength of 15.3% (but no noticeable increase in ductility) the EB method showed an increase in strength of 21.8% and an increase in ductility of 29.7%, while the hybrid method showed an increase in strength of 37.0% and an increase in ductility of 31.6%. For all retrofitted specimens, there was also a shift in the failure from the joint region to the beam ends and significantly less damage within the connection, especially for EB and hybrid methods. This change in performance (strength, ductility and shifting of plastic hinge to the beam) would protect a structure from failing due to soft storey collapse. Finally, a finite element (FE) model was developed using ABAQUS software. This model was capable of simulating the connection loaded with a monotonic load and showed good agreement with the envelope curve of the experiments and therefore can considered to be a suitable design technique for these types of connections.