Research on the Local Bulging Failure of Buckling-Restrained Brace Restraining Members with Flat Steel Core

Buckling-restrained braced frame systems are widely used for seismic buildings in recent years. In order to increase the usable floor space in the building, developing the thin BRB (nBRB) becomes an important task and challenge.
The seismic design methods and performance evaluation of nBRBs are the priorities of this research. When the nBRB using flat core plate is subjected to a large compressive strain, the high mode buckling wave will form. The wave crests would be in contact with the mortar and create lateral forces on the restraining members. The local bulging failure may occur if the steel casing is not strong enough to resist the lateral force. In this study, the lateral force induced by the maximum compressive force of the BRB is computed from the geometry of the high mode buckling considering the buckling wavelength, unbonding layer thickness, and a reduced thickness of the core plate due to Poisson effect. The compressive strength decreases severely when the bulging failure occurs. In order to achieve a conservative design, it is assumed that the infill mortar contributes no resistance but spreads lateral forces out into outward pushing forces on the inner surface of the steel casing. The resistance of the steel casing wall preventing the local bulging failure is calculated by the limit analysis of plates. Three local bulging load stages are considered including uniformly distributed area load, triangularly distributed area load, and line load acting on the inner surface of steel casing, respectively.
Fourteen nBRB component tests have been conducted in the previous studies. In this research, eight component tests on specimens varying in the core plate width, cyclic loading procedures, and the mortar strength are conducted at NCREE. There are four different types of loading procedures. Such as increasing, decreasing, and singular with the maximum compressive strain of 3.5%. The constant loading procedure applies the cyclic core strain under 3%. Test results show that the local bulging failures occur after exceeding the maximum core strain of 3.5%. The high mode buckling wavelength is about 9 to 10 times the core plate thickness. It is observed that different loading procedures affect the local bulging failure instants. The proposed design method satisfactorily predicts the occurrence of the local bulging failure of the nBRB under extreme loading procedures. The design method is conservative when the core plate width to the steel casing width ratio increases. It is recommended that the core plate-to-casing width ratio be greater than 0.3 for nBRB. Test results confirm that using 95Mpa-high compressive strength mortar can enhance nBRB’s life.
Finite element model (FEM) analysis results indicate that the lateral outward pushing forces from the buckled core plate can be effectively predicted by the proposed design method. The maximum compressive strength of the nBRB and the geometry of high mode buckling waveform can be conveniently incorporated into the calculation of the lateral force. The FEM shear stress distributions in the mortar are similar to the assumptions made for the mortar loading path. Based on the tests and analysis on the 22 nBRB specimens, it is confirmed that the proposed design method can be applied for the seismic design of nBRBs to prevent the local bulging failure of steel casing.