增設挫屈束制段之斜撐構件耐震行為研究

Abstract

同心斜撐構架系統以斜撐構件提供整體的側向勁度與強度，並在結構系統進入非線性後為主要的消能構件，在以往的地震中發生過許多的破壞情況，其常見的破壞模式為斜撐經往覆的拉壓行為導致挫屈、降伏，導致該樓層強度與勁度急遽衰減，造成軟弱層(Soft Story)的破壞模式，而斜撐構件常見的缺點有低週疲勞壽命不足、受壓之挫屈強度過低，以及斜撐構件兩端之接合角隅板與樑柱接頭發生破壞，或引發樑、柱構件扭轉等非預期性破壞現象。本研究將傳統斜撐分為多段，以消能元件連結這些斜撐段，目的是希望以類似挫屈束制斜撐（BRB）的方式，將傳統斜撐的消能區塊拉長，消散部分外力給予的能量，並改善斜撐構件的韌性。 本研究設計六組斜撐並進行靜態往覆加載實驗，探討不同補強方式對H型鋼斜撐構件行為的影響，試體分為C與R兩大類，C類是將傳統斜撐的中段切除，換上兩消能鋼板作為連接，再以圍束元件束制，達到避免挫屈的目的，而R類則將傳統斜撐中段的翼板切削，一樣以圍束元件束制住。根據實驗的結果分別探討各試體之指標強度、遲滯行為、累積消能關係、韌性容量、圍束長度與消能和韌性的關係、層間位移角與應變硬化和拉壓比的關係，以及面外位移量。而研究結果顯示，C類以及R類的試體皆呈現飽滿的遲滯迴圈，並將改善傳統斜撐的行為改造成BRB的行為，然而破壞模式也與BRB類似，在較大的變形作用下斜撐在核心斷面轉換段會發生面外挫屈，最後導致疲勞斷裂，而實驗透過縮小圍束元件與核心元件之間隙可以明顯改善此破壞模式，實驗結果也顯示大部分試體的韌性容量皆大於規範規定的200數值，各試體的拉壓比最大值為1.28而應變硬化則大多小於1.3，研究建議儘可能延長圍束消能段有助於提升斜撐韌性。

Steel braces efficiently provide lateral stiffness and strength to the system of Concentrically Braced Frame (CBF). Being the primary members to dissipate energy by their nonlinear behavior, conventional buckling braces are subjected to premature failure leading to damage to the neighboring structural components and mechanism of soft story. The most common failure modes in steel braces are associated with low-cycle fatigue. In this research, we applied the concept of buckling restrained brace (BRB) and renovated the conventional braces by changing part of the brace to a buckling-restrained segment to spread out the energy dissipation region and increase their ductility. Test plan included six wide-flange specimens under static cyclic loading. The specimens were categorized into two types, namely Series C and Series R. For Series C specimens, the mid segments of the braces were cutoff and replaced with two sets of restrained core plates. For Series R specimens, the sections of the mid segments were changed to the one with reduced flange width and restrained elements. Test variables included the length and material of the mid segment. Based on the test observations and data analyses, the performance parameters of different specimens were discussed including the hysteretic behavior, strength (yielding strength, maximum tensile strength and maximum compression strength), energy dissipation, cumulative plastic ductility, effect of restrained length, strain hardening factor, compression strength adjustment factor and out-of-plane displacement, etc. The results showed that the reinforcement strategies in both Series C and Series R specimens were able to enhance the maximum compression strength and energy dissipation capacity of the braces. The typical hysteretic behavior was similar to that of BRB, but the failure modes were also similar to those of BRB. Under large demands of axial deformation, the proposed braces buckled in the region close to the transition area of core element and finally ruptured because of low-cycle fatigue. The impact of such failure modes was significant reduced by reducing the gap between the steel core and the restrained element in some specimens. Test results also showed that most of the specimens had cumulative plastic ductility greater than 200, which was the requirement in the code. The maximum ratio of compression to tension among all the specimens was 1.28 and the strain hardening ratios were less than 1.3 for most of the specimens. The test results also suggested that increasing the length of restrained segment was able to increase the capacity of energy dissipation and ductility of the brace effectively.