Buckling-Restrained Braces and Their Implementation in Structural Design of Steel Buildings

For a structural engineer, selection of the lateral load-resisting system for a building is a critical decision. While it is easy to understand the requirement to select beams and columns to support the gravity loads (i.e., self-weight of the building itself and the live loads related to building occupancy), a lateral system is required for any building to resist horizontal loading. Lateral loads routinely considered in design are wind and earthquake, although blast, flood, and tsunami are also possible. The lateral system can be especially critical when the building is to be constructed in a region where significant seismic activity is expected. In the case of a steel building, a wide variety of lateral resisting systems are available. These tested and approved lateral systems have the capability to maintain structural integrity in the face of significant lateral loads and inelastic behavior. One of the key elements to steel structures resisting seismic loads is a properly designed energy-dissipating fuse element. This critical element yields and subsequently dissipates energy during an earthquake without causing instability or collapse of the structure. In moment-resisting frames, the beams and columns resist both gravity and lateral loads although typically only the perimeter frames resist lateral forces. In this case, the beam elements are the fuse where inelastic behavior is expected. The challenge with these systems is that repairing or replacing beams after an earthquake is extremely difficult and costly. The other common lateral resisting system for steel buildings is a braced frame. The three most common braced frame systems are concentrically braced frames (CBF), eccentrically braced frames (EBF), and buckling-restrained braced frames (BRBF). Representative schematics for all three steel brace systems are shown in Fig. 1. Buckling-restrained braces (BRB) are a somewhat recently developed structural device that has a balanced force-deformation behavior (i.e., nearly equal strength in tension and compression) and are one of the few manufactured, rather than fabricated, products in common use in the structural steel construction industry. This entry will briefly explore the three common types of braced frames and then discuss in greater depth the mechanics, performance, and design of BRBs and the BRBF system.