Bidirectional Loading Test of Buckling-Restrained Braces with Gusset Plate Connections

Buckling-restrained braces (BRBs) are a type of seismic device and are employed in steel moment-resisting frames and other building and civil engineering structures. Previous studies have examined the behavior of BRBs toward only the in-plane dimension of the structural plane with BRBs. However, the structural planes with BRBs are exposed to not only in-plane drift but also out-of-plane drift, so the bidirectional behavior of BRBs needs to be confirmed, especially under large deformation during a strong earthquake with motions greater than the seismic design limit. To explore this bidirectional behavior, a cyclic loading test of BRBs under bidirectionally large deformation with as much as over 5% story drift was conducted. Test parameters were the details of the gusset plate connections, the material strength of splice plates, the inserted length of the spread part of the core member into the end of the restrainer, and the loading direction. The test results clarified that out-of-plane buckling of BRBs with their connections occurs when both the inserted part and the splice plates yield, even if the BRBs satisfy conventional design criteria. It was confirmed that out-of-plane buckling occurs earlier if the strength of the splice plates is smaller or the story drift is applied bidirectionally. Furthermore, a method to calculate the bending moment at the splice plates was derived, and its validity was confirmed by comparison to the test results. Finally, based on the above investigation, a design criterion to prevent the splice plates from yielding was devised.