Seismic retrofit of reinforced concrete beams, columns, and joints with thin sheets and steel plates

This study is aimed to develop a new seismic retrofit technique of reinforced concrete (RC) beams, columns, and their joints with lightweight steel sheets and steel plates, to validate the retrofit technique with testing of two 4/5-scaled beam-column assemblage specimens, and to develop a strut-and-tie model to ascertain the force transfer mechanism of beam-column joints under seismic loads. Because of the introduction of thin steel sheets, weld joints that were used in the conventional steel jacket of RC columns could be costly in field applications. Nailed joints of two types, interlocked and lap-spliced, were therefore proposed in this study. Each type of the nailed joints was evaluated by testing lap-spliced specimens and concrete rings under quasi-static loading. Test results indicated that an interlocked joint of 6 inches wide with 2 rows of 5 nails failed in ductile bearing of nails against steel sheets with or without rotation of nails provided a minimum gap of 0.5 in. is available at both ends of each sheet Similarly, a lap-spliced joint revealed a ductile bearing failure mode when 5 rows of 10 nails were used. The proposed retrofitting technique can significantly improve the seismic performance of beam-column assemblages, especially with post-tension of the beam. In comparison with the unretrofitted specimen that was tested by Ereckson and failed in column shear at displacement ductility of 2.5, the displacement ductility capacity of the two retrofitted specimens was increased to 8 without post-tensioning and 12 with post-tensioning, corresponding to the pullout and fracture of column longitudinal bars, respectively. No brittle shear failure of the retrofitted columns and joints was observed throughout the testing program. The strut-and-tie model can simulate the tension forces in instrumented reinforcing bars that were in good agreement with the test results.