Seismic Design of Steel Buckling-Restrained Braced Frames

Buckling-Restrained Braced Frames (BRBFs) are one of the newer types of seismic force-resisting systems used in modern building designs. As the two example configurations shown in Figure 1-1 illustrate, BRBFs resist lateral loads as vertical trusses in which the axes of the members are aligned concentrically at the joints. Although the global geometric configuration of a BRBF is very similar to a conventional Concentrically Braced Frame (CBF), the members, connections, and behavior of BRBFs are distinctly different from those of Ordinary Concentrically Braced Frames (OCBFs) and Special Concentrically Braced Frames (SCBFs). The key difference is the use and behavior of the Buckling-Restrained Brace (BRB) itself.

Unlike the standard sections used for braces in OCBFs and SCBFs, the BRB is a fabricated assembly. As shown in Figure 1-2, the most common BRBs consist of a steel core-plate (the yielding element, hereafter called the “core”) that is surrounded by a steel tube casing filled with grout or concrete. Figure 1-2 shows a core consisting of a steel plate. Other core cross-sections, such as cruciform or multiple plates can also be used. The core is axially decoupled from the fill and casing by various means that produce a physical isolation or gap. As the name states, the BRB assembly restrains core buckling under compressive loading and achieves a compressive yield strength that is approximately equal to its tensile yield strength. Therefore, the core area can be sized for designlevel seismic loads based on the yield stress of the core, Fysc, as opposed to braces in conventional CBFs, which are sized based on the critical buckling stress, Fcr, of the section.