Prompted by the observations and concerns, seismic design requirements for braced frames have changed considerably and the concept of BRB frames has been introduced in which buckling of braces is prevented [19,20]. Different parts of BRB shown in figure 3 are sleeve and core. The core consists of central yielding zone,C (reduced section) and transition zones, B and A (larger area than the yielding zone and similarly restrained) on either side of yielding zone; and connection zones that extend past the sleeve and connect to the frame, typically by means of gusset plates.
The sleeve is the outer part of BRB which covers the core. The basic principle in the construction of BRB is to prevent buckling of a central steel core by encasing it over its length in a steel tube filled with or without concrete or mortar. BRB provides a slip surface between the steel core and the surrounding concrete, so that axial loads are taken only by the steel core. The materials and geometry in this slip layer must be carefully designed and constructed to allow relative movement between the steel element and the concrete due to shearing and Poisson’s effect, while simultaneously inhibiting local buckling of the steel as it yields in compression. In BRB, the basic structural framework is designed to remain elastic during seismic response and all the seismic damage (yielding) occurs within the braces. This class of steel braces dissipates energy through stable tension-compression yield cycles as shown in figure 2. The comparative buckling behavior of conventional brace and BRB is shown in figure 4. The concrete and steel tube encasement provides sufficient flexural strength and stiffness to prevent global buckling of the brace, allowing the core to undergo fully reversed axial yield cycles without loss of stiffness or strength. The concrete and steel tube also helps to resist local buckling. The stable hysteretic behavior of a properly detailed BRB contrasts with the behavior of bracing elements in typical conventional braced frames (CBFs). The BRB has ability to independently control strength, stiffness and yield displacement or ductility by varying the cross-sectional area of the steel core, the yield strength of the steel and the length of the core which is allowed to yield. The invention on BRB started in early 80’s and its testing took place in mid 80’s. It was implemented in Japan in 90’s. Because of its good response, this technology was transferred in US in 1998 whose testing and simulation took place in 1999 and then safely implemented in important projects after 2000 [5]. The development of BRBd is shown in figure 5. The use of BRB at some important project is shown in figure 6.