考慮槽接式挫屈束制支撐兩種挫屈模式之面外穩定性研究

A study of Out-of Plane Stability considering Two Buckling Modes of WES-BRBS.

蕭宇東(Yu-Tung Hsiao)

指導教授 ： 蔡克銓

國立臺灣大學/工學院/土木工程學研究所/碩士(2022年)

https://doi.org/10.6342/NTU202201775

摘要

挫屈束制支撐(Buckling Restrained Brace, BRB)為常見之消能構件，能經濟且有效地提升結構之勁度、強度、韌性與消能行為，已廣泛用於耐震結構系統。但當BRB或端部接合發生挫屈造成面外不穩定時，BRB軸向勁度與強度將大幅縮減，喪失消散能量之功能。國內常用之槽接式挫屈束制支撐(WES-BRB)可使用Brace on Demand (BOD)進行設計，檢討圍束鋼管、連接段與接合板穩定性時，未考量彼此之間的耦合效應，以及施工誤差及初始面外變位缺陷等影響穩定性的參數。2017年陳力維延伸Takeuchi教授之理論，提出WES-BRB之穩定性評估模型，但須利用有限元素模型分析接合板旋轉勁度，較不便工程應用。2018年歐易佳提出簡化模型，使用虛擬側向力及塑性分析法探討面外穩定性，但假設過多參數。紐西蘭Zaboli博士根據實驗結果之挫屈型式，探討接合板及漸變段同時發生塑角之情況。本研究由Zaboli博士之文章發想，根據WES-BRB之配置特性及可能發生之兩種挫屈型式建立模型，分別為圍束單元與接合板(RGF)、及接合段與接合板(NGF)之耦合挫屈。考慮圍束單元撓曲行為、軸彎互制效應、二次效應及初始面外缺陷量，使用虛功法建立面外穩定性檢核式。其中使用簡化公式計算未加勁及邊緣加勁接合板之面外旋轉勁度，與有限元素模型分析所得差異均在8%以內。為驗證檢核式，本研究設計兩組WES-BRB試體，兩試體實驗結果皆發生面外挫屈，其中一組發生了NGF，是WES-BRB實驗中沒發生過的面外挫屈行為。本研究綜整本研究、陳力維及歐易佳共八組試體，將各試體尺寸及實際量得之初始面外缺陷代入檢核式進行評估驗證，各組試體之端部初始面外偏移程度皆不同，但結果顯示預測之面外挫屈強度與實驗所得，誤差均在4%以內，証實本研究所提之挫屈模型與強度檢核式，皆能準確預測試體的反應，表示可供BRB設計之應用。設計BRB時，初始缺陷量以BRB工作點間長度1/500、接合板長度1/50及核心與圍束單元間之間隙貢獻總和來保守估計。

關鍵字

挫屈束制支撐 ； 圍束單元撓曲效應 ； 初始缺陷 ； 面外挫屈 ； 二次效應 ； 反覆加載試驗 ； 穩定性分析模型 ； 接合板面外旋轉勁度

並列摘要

Buckling Restrained Brace (BRB) is a cost-effective energy-dissipating component that has been widely used in seismic structural systems. However, when the out-of-plane (OOP) instability of BRB occurs, the axial strength, stiffness, and the energy dissipation capacity of the BRB will be greatly reduced. The welded end-slot buckling restrained brace (WES-BRB), commonly used in Taiwan, can be designed using Brace on Demand (BOD), it checks three independent stability requirements for the steel casing, the connection zone and the gusset, respectively. Nevertheless, these three stability checks do not consider their coupling effects and initial imperfection effects on the stability. Chen et al. adopted Takeuchi’s procedures and proposed a buckling model considering flexural restrainer and gusset rotations in 2017. However, these procedures require the finite element model (FEM) analysis in order to compute the gussets’ rotational stiffness. Later in 2018, Ou et al. proposed a simplified model, and applied notional load and plastic analysis methods to access the overall OOP stability of WES-BRB. This model assumes a large number of parameters. Recently in 2021, Zaboli et al. proposed a simplified direct analysis method which considers gusset buckling mode and BRB-gusset interactive failure mode. Based on the geometric conditions and two possible OOP buckling modes of WES-BRBs, this study proposes two models considering flexure restrainer, P-M interaction effect, second-order effect and initial imperfections. The two failures modes are restrainer-gusset interactive failure (RGF) mode and neck-gusset interactive failure (NGF) mode. The OOP rotational stiffness of gusset can be calculated from a simplified equation. The differences on the results from the simplified procedure and FEM anayses are lower than 8%. In order to validate the proposed OOP buckling models, two full-scale BRB specimens each of 7.3m long with a 988-kN nominal yielding strength, with two different steel casing wall thickness and OOP end drifts were tested. In order to verify the effectiveness of the proposed models, this study combines other six specimens tested by Chen et al. and Ou et al. A total of eight specimens with varying restrainer stiffness, gusset thickness, with/without edge stiffeners or OOP drift demands are examined. It is demonstrated that the proposed models effectively predict specimens’ OOP buckling strength with errors less than 4%. It is concluded that the initial imperfection can be conservatively estimated as the sum of 1/500 of the length between two work points, 1/50 of the gusset length, and the contribution of the typical gap of 2mm between the steel core and restrainer in the WES-BRB design.

並列關鍵字

Buckling Restrained Brace ； flexural effect of restrainer ； initial imperfection ； out-of plane buckling ； second-order effect ； cyclic loading test ； stablility model ； rotational stiffness of gusset

參考文獻

1.American Institute of Steel Construction (AISC), “Seismic Provisions for Structural Steel Buildings (AISC 341-16)”, AISC, Chicago, Illinois, 2016.

2.American Institute of Steel Construction (AISC), “Specification for Structural Steel Buildings (AISC 360-16)”, AISC, Chicago, Illinois, 2016.

3.A.C. Wu, P.C. Lin, K.C. Tsai, High-mode buckling reponses of buckling-restrained brace core plates, Earthquake Engineering Structural Dynamics 43 (2014) 375–393.

4.Chen, L.W., Tsai, K.C., Tsai, C.Y. and Wu, A.C. (2019), “Evaluating out-of-plane stability for welded BRBs considering flexural restrainer and gusset rotations”, Journal of Constructional Steel Research, 2019; 159:161-175.

5.Chuang, M.C., Tsai, K.C., Lin, P.C. and Wu, A.C. (2015), “Critical limit states in seismic buckling-restrained brace and connection designs”, Earthquake Engineering Structural Dynamics, 2015; 44:1559-1579.