含平板狀核心挫屈束制支撐圍束單元局部外突破壞行為研究

Research on the Local Bulging Failure of Buckling-Restrained Brace Restraining Members with Flat Steel Core

張捷安(Chieh-An Chang)

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(2014年)

https://doi.org/10.6342/NTU.2014.01453

摘要

挫屈束制支撐(BRB)構架已成為應用廣泛的耐震建築結構系統。在維持BRB的強度、勁度與消能特性的前提下，希望縮減斜撐厚度以減少所佔空間，增加建築可使用樓板面積，薄型挫屈束制支撐(nBRB)成為重要的課題與挑戰。
本研究主要探討nBRB之力學行為與耐震設計，其中平板型核心單元的高模態挫屈會對圍束單元造成側向外推力，一旦圍束鋼管的強度不足以抵抗側向力，則容易發生局部外突破壞， BRB之抗壓強度將大幅降低。本研究由高模態挫屈波長、滑動單元厚度與核心鋼板受拉時因柏松效應所減少的厚度等幾何關係，計算BRB在最大軸壓力下對圍束單元造成的側向正向力。保守起見，假設圍束單元中水泥砂漿僅提供力量傳遞的作用而不提供抗局部外突的強度，使圍束鋼管承受所有側向外推力，本研究利用板的極限定理計算圍束鋼管抵抗局部外突破壞的強度，分別假設均佈面載、三角型面載與線載三種不同對圍束鋼管的側向外推方式，提出預測nBRB圍束鋼管是否會發生局部外突破壞的檢核公式。
過去已曾進行14組nBRB構件試驗，本次試驗再設計8組nBRB試體於國家地震工程研究中心進行構件試驗，主要改變核心板寬度、加載歷時形式與水泥砂漿強度。其中加載歷時包括遞增、遞減、固定與單圈歷時，核心最大壓應變達3.5%，而固定歷時的最大應變量為3%。試驗結果顯示試體核心壓應變均達3.5%才發生圍束鋼管局部外突破壞。核心高模態挫屈波長約為9至10倍核心板厚。不同加載歷時形式會影響局部外突破壞發生的時機，本研究所提之設計方法可預測受極端單圈歷時試體的局部外突破壞；使用95MPa抗壓強度之無收縮水泥砂漿能夠延長nBRB的壽命；試體核心板寬與外鋼管寬之比值(寬度比)越高，設計方法越保守，本研究建議設計寬度比應大於0.3。
有限元素模型分析結果顯示，本研究所提檢核公式可有效預測側向正向力大小，證實核心板抗壓強度與高模態挫屈幾何關係為影響側向正向力的關係；水泥砂漿的剪應力分佈範圍也與檢核公式假設之傳力機制相同。本研究所提檢核公式能保守預測圍束鋼管是否會局部外突破壞，故本設計方法適用於核心為平板型薄型挫屈束制支撐之耐震設計。

關鍵字

挫屈束制支撐 ； 薄型挫屈束制支撐 ； 高模態挫屈 ； 局部外突

並列摘要

Buckling-restrained braced frame systems are widely used for seismic buildings in recent years. In order to increase the usable floor space in the building, developing the thin BRB (nBRB) becomes an important task and challenge.
The seismic design methods and performance evaluation of nBRBs are the priorities of this research. When the nBRB using flat core plate is subjected to a large compressive strain, the high mode buckling wave will form. The wave crests would be in contact with the mortar and create lateral forces on the restraining members. The local bulging failure may occur if the steel casing is not strong enough to resist the lateral force. In this study, the lateral force induced by the maximum compressive force of the BRB is computed from the geometry of the high mode buckling considering the buckling wavelength, unbonding layer thickness, and a reduced thickness of the core plate due to Poisson effect. The compressive strength decreases severely when the bulging failure occurs. In order to achieve a conservative design, it is assumed that the infill mortar contributes no resistance but spreads lateral forces out into outward pushing forces on the inner surface of the steel casing. The resistance of the steel casing wall preventing the local bulging failure is calculated by the limit analysis of plates. Three local bulging load stages are considered including uniformly distributed area load, triangularly distributed area load, and line load acting on the inner surface of steel casing, respectively.
Fourteen nBRB component tests have been conducted in the previous studies. In this research, eight component tests on specimens varying in the core plate width, cyclic loading procedures, and the mortar strength are conducted at NCREE. There are four different types of loading procedures. Such as increasing, decreasing, and singular with the maximum compressive strain of 3.5%. The constant loading procedure applies the cyclic core strain under 3%. Test results show that the local bulging failures occur after exceeding the maximum core strain of 3.5%. The high mode buckling wavelength is about 9 to 10 times the core plate thickness. It is observed that different loading procedures affect the local bulging failure instants. The proposed design method satisfactorily predicts the occurrence of the local bulging failure of the nBRB under extreme loading procedures. The design method is conservative when the core plate width to the steel casing width ratio increases. It is recommended that the core plate-to-casing width ratio be greater than 0.3 for nBRB. Test results confirm that using 95Mpa-high compressive strength mortar can enhance nBRB’s life.
Finite element model (FEM) analysis results indicate that the lateral outward pushing forces from the buckled core plate can be effectively predicted by the proposed design method. The maximum compressive strength of the nBRB and the geometry of high mode buckling waveform can be conveniently incorporated into the calculation of the lateral force. The FEM shear stress distributions in the mortar are similar to the assumptions made for the mortar loading path. Based on the tests and analysis on the 22 nBRB specimens, it is confirmed that the proposed design method can be applied for the seismic design of nBRBs to prevent the local bulging failure of steel casing.

並列關鍵字

buckling-restrained brace (BRB) ； thin BRB ； high mode buckling ； local bulging

參考文獻

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