支撑屈服后屈曲约束支撑框架损伤集中效应分析

屈曲约束支撑(BRB)屈服后刚度较低，某一层或几层BRB率先屈服会造成屈曲约束支撑框架(BRBF)进入弹塑性阶段后不同楼层间刚度的突变，框架弯矩会重分布，进而影响框架的损伤机制。为此，提出框架基于柱端弯矩比的强柱弱梁计算公式，公式表明：当框架节点下部与上部柱端弯矩同号相等时，柱梁承载力比需求最小为1；当弯矩反向相等时，需求则为无穷大；通过分析楼层间BRB刚度比对两层BRBF弯矩分布的影响，说明BRBF中某层支撑屈服会导致该层节点下部与上部柱端弯矩比发生较大的改变，节点上部柱端弯矩甚至反向增大；综上，支撑屈服后BRBF易出现层间柱铰机制。最后，通过算例验证提出的强柱弱梁计算公式和BRBF弯矩分布规律，同时表明：当BRB-框架刚度比较大或者框架柱梁承载力比较小时，BRBF损伤集中效应较为明显。

Damage concentration effect analysis of buckling-restrained braced frames after yielding of the braces

The post-yield stiffness of a buckling-restrained brace (BRB) is low. The yielding of the BRBs at one story or multiple stories may cause the stiffness mutation of stories after the buckling-restrained braced frame (BRBF) goes into the elastic-plastic stage. Consequently, the frame moment may redistribute with the damage mechanism of the frame being affected. A weak beam-strong column calculation formula is proposed based on the moment ratio of column end. The formula shows that the demand of column-beam strength ratio is at least equal to 1 (the minimum value) when the moments of the upper and lower column ends at frame joint are equal and have the same directions, while the demand is infinite when the moments are equal, but have opposite directions. Furthermore, the influences of the BRB stiffness ratio of stories on the moment distribution of two-story BRBF are analyzed, disclosing that the yielding of the BRBs at one story in the BRBF can lead to significant change of the moment ratio of lower and upper column ends of frame joint at this story, and the moment of upper column end of the joint may even reverse its direction and increase. Therefore, the BRBF is susceptible to inter-story column hinge mechanism after the yielding of braces. Lastly, the proposed weak beam-strong column calculation formula and moment distribution law of BRBFs are verified via examples. Additionally, the examples show that when the stiffness ratio of BRB-frame is relatively large or the strength ratio of frame column-beam is relatively small, the damage concentration effect of BRBF is more evident.