Development and validation of an acceptance criteria and damage index for buckling-restrained braces (BRB)

This paper focuses on introducing two methodologies related to buckling-restrained braces (BRBs): one for determining the acceptance criteria generally used in performance-based seismic design (PBSD), and another for assessing a damage index (DI) when BRBs are subjected to seismic actions. Both methodologies are established from the results of experimental test conducted on a series of full-scale BRB specimens. In total 19 full-scale BRB specimens were manufactured with local industry and workforce. 14 BRB specimens were tested using a lowcycle loading protocol, and five more BRB specimens were tested using a high-cycle fatigue loading protocol. The specimens were designed having differential deformation and energy dissipation capacities. For the low-cycle protocol, the axial strain in the BRB core was continuously increased until failure, while for the fatigue protocol, the axial strain in the core was increased from zero to 1.5%, and continued at 1.5% strain until core failure occurred. This paper also describes the results of a series of nonlinear dynamic analyses carried out on a sample building in order to validate the proposed methodologies when subjecting the building to ground motions. The results provided sufficient arguments to conclude that: (1) the proposed acceptance criteria is capable of considering the effect of (i) core plastic length, (ii) cumulative plastic deformation, (iii) and brace failure type (depending on the loading history). (2) The proposed damage index is shown to be capable of considering the effect of the maximum core strain attained as well as the cumulative plastic deformation effect. Moreover, a qualification scale was assigned to the proposed damage index as a tool for evaluating whether the BRB element should be replaced or left on site. Finally, the results suggest that the proposed methodologies can be useful in structural design practice.