Overview Of Buckling Restrained Braces (BRB) And Its Buckling Condition

Buckling Restrained Braces (BRB) is a type of braced frame technology designed to enable structures to withstand cyclical lateral loadings for example in the case of earthquakes or strong winds. The BRB system was first developed and tested in Japan in the 1980’s. The system was initially designed where flat steel braces were sandwiched between precast concrete panels to provide effective lateral restraint. The effort to eliminate the buckling failure mode of slender elements led to a collaborative effort between Professor Wada of the Tokyo Institute of Technology with Nippon Steel Construction, resulting in the current model of the BRB system. The main components of a BRB are the steel core, the de-bonding layer, the filler material and the casing. The steel core is the primary load bearing component which resists both tension and compression axial forces subjected to the BRB by lateral forces. The de-bonding material separates the steel core and the filler material hence allowing the steel core to freely move and expand due to tension and shorten due to compression within the filler material. The filler material is used between the steel tube and steel core to resist buckling stress. It can be mortar, reinforced concrete or grout. A steel tube which is the casing envelops the inner member and restrains the steel core from buckling.

BRBs braces have energy dissipative behavior that is improved from that of Special Concentrically Braced Frames (SCBFs). The BRB system is able to restrain buckling and its symmetrical hysteretic response hence less redistribution of loads and deformations in BRB. As the braces do not buckle laterally, damage to adjacent non-structural elements is minimized. The BRB system has a relatively low post-yield stiffness which may still lead to the concentration of damage in one level, even though the brace capacities can be balanced throughout different storeys. BRBs also rely on ductility and must be replaced after usage during a major earthquake.

Buckling Restrained Braces are incorporated in beam–column structures to serve as seismic response controlled members. They are designed to plasticize themselves hence becoming hysteretic dampers when a moderate or strong earthquake occurs. BRBs can absorb significant amount of energy during cyclic loading, for example in case of an earthquake, hence achieving a high level of ductility with stable hysteresis loops. In such a damage-controlled structure post-earthquake inspection of only the seismic-response-controlled members enables the continuous use of entire buildings without the need for demolition. The braces then yield in compression and tension to absorb and prevent the build-up of energy in the structure and reduce the loads on the foundation.

Since the behaviour of a buckled brace is very unpredictable, plenty of research has been done to analyse the BRB behaviour under different loading circumstances so as to help improve the design of the BRB. The design of a Buckling Restrained Brace varies from the type of filler material to be used, connection and cross-sectional area of the steel core as well as the casing. These different designs have been researched and experimented over the years to improve on the behaviour of the BRB.

Overall buckling condition

The initial state, the state that the brace member comes in contact with the restraining member and the state immediately after overall buckling of the BRB. It is subjected to monotonic eccentric loading. The initial deflection is assumed to be the same with the restraining member and the brace member. The brace member causes the lateral deflection as the compressive load increases from the initial deflection state and comes in contact with the restraining member. The lateral deflection increases up to the deformation mode with increase in the load. The contact force acting in the restraining member from the brace member becomes a hoop stress. However, before overall buckling occurs, the total contact force acting on the upper and lower restraining members is balanced, and the lateral deflection of the restraining member is small. In the case of the restraining member with low rigidity, as the load increases, the lateral deflection of the restraining member happens together with occurrence of the overall buckling, and the contact force acts only on the upper restraining member. Though the magnitude of the contact force is not easy to be calculated, it can be considered that almost the same contact force at the contact point of the brace member and the restraining member is acting in the longitudinal direction of the restraining member. The lateral deflection of the restraining member grows as long as the axial compressive force increases, and the yielding at the centre of the restraining member is considered the ultimate state of BRB.