防屈曲支撑外伸连接段破坏失效机理及试验研究

防屈曲支撑（buckling-restrained brace，BRB）外伸连接段的稳定控制是保证其正常发挥耗能作用的关键。基于研究者们试验中出现的BRB外伸段破坏形式及特征，分析BRB外伸连接段的失稳破坏过程及破坏失效机理，研究不同失效模式下外伸连接段失稳破坏的影响因素，进而提出改善外伸连接段稳定性能的端部构造形式。基于理论分析推导不同失效模式下相应外围约束单元与外伸连接段控制截面上的轴力与弯矩计算公式，建立不同失效模式下外伸连接段的稳定分析模型，并通过拟静力试验验证所提出端部构造形式的合理性以及稳定分析模型的有效性。研究结果表明：BRB外伸连接段的失稳破坏为约束单元或外伸段控制截面上的压弯作用效应超过其极限承载力而破坏；外伸段的稳定性与支撑端部转角变形的大小密切相关，且BRB耗能单元的高阶多波屈曲变形是引起支撑端部转动变形的直接原因；满足稳定设计准则的条件下，减小外伸段长度及耗能单元与约束单元间间隙值，增加约束过渡段长度可进一步提高和保证外伸段的稳定工作性能；设计制作的符合文中外伸段稳定条件及端部构造形式的3根BRB在试验过程中未出现外伸段的失稳破坏，其外伸段控制截面上的压弯作用效应均处于弹性范围内。研究结论为BRB外伸连接段的稳定设计及相应的稳定控制提供理论参考。

Failure Mechanism and Experimental Research on the Unrestrained Connecting Segment of Buckling Restrained Brace

The stability control of the unrestrained connecting segment of buckling-restrained braces (BRB) is key to the energy dissipation behavior. Based on the failure modes and failure characteristics of BRB observed in the previous research, the failure process and failure mechanism of the unrestrained connecting segment were analyzed, and the factors influencing the buckling failure under different failure modes were studied to improve the end configurations of connecting segment and enhance the stability behavior. Then the calculation formulas for axial force and bending moment on the control section of the outer constrained sleeve and unrestrained connecting segment under different failure modes were derived based on theoretic analysis, and the stability analysis model of the unrestrained connecting segment was established. According to the proposed end configurations and stability analysis model, BRB specimens were designed and tested through quasi-static tests to verify the reasonability of the end configurations and the effectiveness of the stability analysis model. The results demonstrated that the instability failure of the unrestrained connecting segment was caused by the fact that the compression-bending effect on the control section of the restraint element or the unrestrained connecting segment exceeded its ultimate capacity. The stability of the unrestrained connecting segment was closely related to the end rotation of BRB, which was directly caused by the high-order multi-wave buckling deformation of the BRB core plate. Under the condition that the stability design criteria were satisfied, reducing the length of the unrestrained connecting segment and the gap between core steel and outer constrained sleeve, or enlarging the restrained transition segment could improve the stability behavior. Moreover, no stability failure was found in the BRBs designed by the proposed method, and the stress caused by the compression-bending effect on the control section of the unrestrained connecting segment was in the elastic range. The results obtained in this paper could provide a theoretical reference for the stability design and stability control of the unrestrained connecting segment of BRB.