Seismic retrofit of reinforced concrete frames using buckling-restrained braces with bearing block load transfer mechanism

A new method of retrofitting reinforced concrete (RC) frames with buckling-restrained braces (BRBs) to improve frame strength, stiffness and energy dissipation is proposed. Instead of typical post-installed anchors, load is transferred between the BRB and RC frame through compression bearing between an installed steel frame connected to the BRB, and high-strength mortar blocks constructed at the four corners of the RC frame. This avoids complex on-site anchor installation, and does not limit the allowable brace force by the anchor strength. Cyclic displacements of increasing amplitudes were imposed on two RC frame specimens retrofitted with different BRB strength capacities. In one of the frames, the bearing blocks were reinforced with wire mesh to mitigate cracking. A third RC frame was also tested as a benchmark to evaluate the retrofit strength and stiffness enhancements. Test results indicate that the proposed method efficiently transferred loads between the BRBs and RC frames, increasing the frame lateral strength while achieving good ductility and energy-dissipating capacity. When the bearing block was reinforced with wire mesh, the maximum frame lateral strength and stiffness were more than 2.2 and 3.5 times the RC frame without the BRB respectively. The BRB imposes additional shear demands through the bearing blocks to both ends of the RC beam and column member discontinuity regions (D-regions). The softened strut-and-tie model satisfactorily estimated the shear capacities of the D-regions. A simplified calculation and a detailed PISA3D analysis were shown to effectively predict member demands to within 13.8% difference of the measured test results. Copyright (c) 2016 John Wiley & Sons, Ltd.