Design and performance of long buckling-restrained braces with rectangular cores

概要

Buckling-restrained braces (BRBs) are often used in tall and large buildings. These applications tend to require exceptionally long BRBs, with 10~20m workpoint lengths (Lwp) and commensurate core yield lengths (Lp). However, the influence of the core and debonding interface dimensions on the compressive overstrength factor (B) and the low-cycle fatigue capacity is not yet clear for long BRBs. Previous experiments have indicated that longer BRBs exhibit greater compressive overstrength factors and lower cumulative inelastic deformations (CID) at fracture. This is due to friction at the higher mode buckling wavecrests between the core and restrainer assembly, which generates a highly nonuniform axial strain distribution and may have a significant effect on performance.

This paper investigates the adverse effects of the core yield length on the compressive overstrength, axial strain, and fatigue demands using finite element analysis. Simplified 2D shell models are used to parametrically analyze a range of core dimensions with yield lengths up to 14m, while detailed 3D solid models are used to examine the performance for representative long BRBs. Higher mode buckling and friction are shown to significantly amplify the compressive strain at the core ends, increasing the potential for core binding. Midspan tensile strains during the subsequent tensile cycles are also amplified due to strain ratcheting, reducing the fatigue capacity. From these results, the effect of the core dimensions and debonding interface properties on B are discussed, leading to the following findings:

1 The friction component of compressive overstrength (Br) is confirmed to increase linearly with the friction coefficient (a), debonding gap-to-core thickness ratio (Sw / tc), and core slenderness (L), while the associated compressive strain amplification increases the susceptibility of long BRB to friction-induced core binding.

2 Compressive strains (sc) at the core ends increase linearly with the average axial strain (E) and Br. The associated

3 Poison expansion may cause the core to locally bind to the restrainer in long BRBs, greatly increasing as a function of the confining strength and stiffness of the restrainer. Therefore, a thicker strong axis debonding gap is required in long BRBs in order to prevent friction-induced core binding. Tensile strains (Er) at mid-span during the subsequent tensile cycle increase linearly with Be, but with the square of
§. When Be is large this may result in premature necking at midspan and is considered to be the primary cause of the reduced fatigue capacity observed in long BRBs.

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