Retrofitting of reinforced concrete moment resisting frames.

A seismic assessment of a reinforced concrete building designed in the late 1950's in New Zealand has revealed several possible problems of behaviour during future severe earthquakes. Typical problems are (l)columns with inadequate flexural strength to prevent column sidesway mechanisms(soft stories), (2)large diameter longitudinal beam bars passing through interior columns with small depth, (3)poor anchorage of longitudinal beam reinforcement in exterior columns, (4)small quantities of transverse reinforcement for shear, confinement of compressed concrete and restraint against premature buckling of longitudinal compression reinforcement in beams and columns, and (5)small quantities of shear reinforcement in beam-column joint cores. An experimental programme was carried out to investigate seismic assessment procedures of existing reinforced concrete frames with poorly detailed reinforcement, and retrofit techniques by jacketing with new reinforcement. Three full-scale beam-interior column joint subassemblages with reinforcement details typical of reinforced concrete building designed in the 1950's were constructed. The beam-column joint core lacked shear reinforcement and the longitudinal beam bars were poorly anchored in the joint core. One of the beam-column joint replicas was tested as-built subjected to simulated severe seismic loading. The test results indicated that beam-interior column joints of early building frames would suffer severe diagonal tension cracking in the event of a major earthquake. The damaged(tested) beam-column joint replica, and the two undamaged(not tested) beam-column joint replicas, were then retrofitted by jacketing with new reinforced concrete and tested under simulated seismic loading. It was found that concrete jacketing was a useful technique for enhancing the stiffness, strength and ductility. Four full-scale beam-column joint subassemblages with reinforcing details of early reinforced concrete frames were also constructed and tested subjected to severe seismic loading. Two of the subassemblages were beam-interior column joint specimens which lacked shear reinforcement. These two subassemblages had different column depth to beam bar diameter ratios and were tested mainly to investigate the effect of the bond conditions along the beam bars passing through the joint core on the seismic behaviour of beam-column joints without shear reinforcement. The other two subassemblages were beam-exterior column joints with limited shear reinforcement and with different arrangements of beam bar hooks in the joint core. In one specimen the beam bar hooks were bent away from the joint core(the tails of the top bars were bent up and the tails of the bottom beam bars were bent down), as was the case in many early frames. In the other specimen the tails of the beam bars were bent into the joint core, as is current practice. Seismic load tests showed that the performance of the beam-exterior column joints with very little shear reinforcement was significantly influenced by the directions in which the tails of the beam bar hooks in the joint core were bent. Beam-exterior column joints of early building frames in which the tails of the hooks of the beam bars are bent out of the joint core would behave unsatisfactory during future severe earthquakes. A theoretical study was conducted to investigate the seismic behaviour of the joints without shear reinforcement. The shear mechanisms of such joints were postulated based on the test results in this study. One approach to assess the seismic performance of the beam- column joints without shear reinforcement is proposed. The approach was based on a limiting nominal horizontal joint shear stress which is a function of the displacement ductility factor imposed on the frame. The seismic behaviour of beams with small quantities of transverse reinforcement was also studied in terms of available curvature ductility factors and shear strengths