Cyclic behaviour of double-tube buckling-restrained braces for boiler steel plant structures

An inner-stiffened double-tube buckling-restrained brace (BRB) is proposed for boiler steel plants in the low-to-medium seismicity regions. The test specimen is of 15.0 m length, consisting of a 15.0 m inner tube with inner stiffeners at both ends and a 12.2 m outer tube for lateral deformation restraint. The full-scale test was conducted to reproduce the cyclic behaviour of the proposed double-tube steel BRB in the plant structures at a practical manufacturing level. The structural performance of the proposed BRB under extremely low cycle fatigue (ELCF), including the strength, deformation capacity, hysteretic response and the failure mode, was investigated. The finite element (FE) model incorporating a material fracture model was established and verified, which could predict the fracture initiation and propagation accurately. The parametric study was carried out to further assess the influence of the stiffener arrangement, material property, gap size and controlling metric of the imperfection on the cyclic behaviour of the proposed BRB. The experimental and numerical investigations demonstrated that the proposed braces could achieve a stable and saturated hysteretic curve. The failure mode of the test specimen was the stress concentration induced fracture which initiated at the stiffener tips of the inner tube. On the basis of the parametric study, the optimized BRB could achieve a high equivalent ratio of critical viscous damping at the loading amplitude of 1/75 L.