新建含挫屈束制支撐之實尺寸兩層樓鋼筋混凝土構架耐震設計與實驗研究

Abstract

已有許多研究証實在鋼建築結構中，加入挫屈束制支撐(BRB)能大幅提升結構勁度、強度與韌性，並由BRB塑性變形消散地震輸入能量，已被廣泛運用在新建或補之鋼結構建築，但在新建之鋼筋混凝土(RC) 結構中仍較少用。這多由於BRB與RC構件介面採化學錨栓傳力時，施工較困難，且化學錨栓在混凝土中受剪與受拉強度並不高，因此BRB尺寸與抗震效益將受限制。本研究探討如何以預埋之工字型鐵件作為RC構件與BRB之連接介面之耐震設計與分析方法，藉由鐵件傳遞BRB傳至RC梁柱構件表面之正向力與剪力，確保BRB能發展RC結構之性能。 研究試體為一座雙層實尺寸高約6米、寬5米含BRB之RC構架，採用A36鋼材BRB與之字形配置，BRB接合板之設計考慮BRB軸力與梁柱開合效應，梁柱構件滿足ACI318耐震設計規範，並對試體進行四次擬動態試驗與反覆載重試驗。本研究目的包含：(1)提出RC構件與接合板在側位移下之梁柱開合效應力量分析方法，並與試驗結果比較。(2)提出 RC構件不連續區之檢核方法與(3)預埋鐵件之設計與施工方法，並透過構架試驗探討其力學行為。(4)透過試驗驗證梁柱構件與BRB之降伏發生次序。(5)提出新建RC構架加裝BRB之設計流程，並透過試驗探討BRB於RC構架之效益。本研究與黃潔倫同學合作，試體反應預測與試驗結果模擬請詳見黃同學所撰論文。 由試驗結果可知，在50/50等級地震作用下，試體二樓最大層間側位移角(IDR)為0.23%，試體尚未進入非線性；於10/50等級地震作用下，BRB與梁端皆降伏，最大IDR發生在二樓為1.0%。在首次2/50等級地震試體最大IDR發生在二樓為2.5%，此時構架反應依然穩定，殘餘之IDR為0.47%；以同樣的2/50級地震再進行測試，試體之勁度與強度反應與首次2/50級作用差異很小，顯示設計適當之BRB-RCF滿足性能導向耐震設計目標，且在經歷大震之後依然能保持良好性能。試體於反覆載重試驗IDR達1.4%一樓柱底已發生彎矩降伏，BRB與梁柱構件之降伏發生次序和試體設計要求相同。當試體達IDR3.5%時，側向強度尚未下降，且維持穩定的遲滯消能行為；至IDR3.5%第三圈時因頂梁主筋斷裂，二樓層間剪力下降15%。反覆載重試驗進行至IDR4.5%時，一樓上部接合板才發生挫屈，但也導致一樓BRB外鋼管亦發生整體撓曲挫屈。當一樓頂接合板之有效長度係數K採2.0時, DCR將達1.05； 而且一樓BRB外鋼管之設計DCR也高達0.95，接合板挫屈之後牽動BRB外鋼管發生整體撓曲挫屈，實驗與此分析結果相符。在50/50級地震作用下，一樓與二樓BRB之最大水平剪力占整體構架水平剪力分別為52%與71%。ETABS模型中RC構件採0.7倍慣性矩折減可得出接近之剪力比例值。至10/50與2/50級地震時，一樓與二樓BRB之最大水平剪力占整體構架水平剪力分別為60%與70%。試驗證實BRB能提供整體構架良好側向勁度。本試驗進行至IDR 2.75%時，一樓與二樓BRB之累積塑性變形容量CPD分別超過476與680。四次擬動態實驗中，一與二層遲滯能量消散比例介於60至94%，證實BRB能消散大部分的地震能量。本研究證實所提鐵件施工與設計方法可行，且在試驗程中皆無發生降伏。試驗結果也證實RC構件不連續區之設計方法能避免破壞。接合板與梁或柱接合介面之剪應力與拉應力除了BRB與接合板銲接處之應變計讀值異常外，簡算所得與試驗反應結果相符。本研究提出含BRB構件與所提接合方式之新建RC構架耐震設計方法。

Buckling-restrained braces (BRBs) have been widely used nowadays in steel structures as it can provide high stiffness, strength and ductility, thereby effectively absorbing seismic input energy. Researches on effectively using BRBs for seismic retrofit of existing RC buildings have been reported. It has been found the construction of BRB and RC member interfaces are often difficult, mostly due to the tensile and shear strengths of post-installed anchors in concrete are limited, the size and effectiveness of the BRBs are restricted. In addition, researches on applying BRBs for new RC building constructions are somewhat limited. This research investigates the seismic design and analysis methods for using the proposed I-shape steel embedment as the interface for the BRB and RC members. Steel embedment must be designed to transfer the BRB normal and shear forces in order to improve the seismic performance of the RC frame buildings. In this study, a full-scaled two-story RC frame with BRBs (BRB-RCF) is tested with four hybrid tests and cyclic loading test. A36 steel BRBs are arranged in zigzag configuration. The design of gussets incorporates the BRB axial and RC frame actions, while the beam and column members comply with ACI318 seismic design provisions. The tasks of this study include: (a) analyze the frame action effect on gussets using the equivalent strut model; (b) develop the design method for the D-region; (c) develop the design and construction methods for the proposed steel embedment; (d) develop the simplified and refined analysis procedures for the BRB-RCF; (e) develop the complete design procedures for the BRB-RCF using the proposed steel embedment. This study is in close cooperation with Ms. Jie-Lun Huang. Details of the specimen response predictions and simulations can be found in Ms. Huang’s thesis. Under the 50/50 hazard level earthquake, the maximum 2nd story’s inter-story drift ratio (IDR) was 0.23%, while all members remained elastic. During the 10/50 level earthquake, BRBs’ and beam ends’ yielding occurred. In the 2/50 earthquake, the maximum 2nd IDR was 2.5%. After three hybrid tests, the specimen’s lateral force vs. deformation responses still remained very stable and the residual IDR was 0.47%. After the same 2nd 2/50 earthquake was applied as an aftershock, the specimen’s stiffness and strength remained pretty much the same, suggesting the BRB-RCF specimen have performed very well under the four, from small to very large, earthquake load effects. During the subsequent cyclic loading test, plastic hinge formed at the 1st-story column base when the IDR reached 1.4%. At this IDR level, all BRBs and RC members yielding have occurred and the sequence agreed well with the predictions. When both two stories reached an IDR of 3.5%, the lateral force vs. deformation response of the specimen was still very stable. In the 3rd IDR=3.5% cycle, because of all bottom bars in the two top beam ends have fractured, the 2nd story shear reduced by 15 %. Up to the 1st IDR=4.5% cycle, 1st story top gusset buckled first, leading to the subsequent flexural buckling of the 1st story BRB to occur also. This is consistent with the predicted results when the gusset’s effective length factor K is assumed 2.0, DCR is 1.05; and steel casing’s DCR is 0.95 for the 1st story BRB. In the 50/50 earthquake, the ratios of peak BRB shear and BRB-RCF shear are 52% and 70% for the 1st and 2nd story, respectively. Similar ratios can be obtained if a factor 0.7 is applied on the gross moment of inertia for RC members in the ETABS elastic model. In the 10/50 and 2/50 events, the ratios become about 60% and 70% for the 1st and 2nd story, respectively. These indicate that BRBs can provide a high lateral stiffness. At the end of 3rd 2.75% IDR cycle, the cumulative plastic deformation CPDs were 476 and 680 for 1st- and 2nd-story BRBs, respectively. The hysteresis energy dissipated ratio in the four hybrid tests are ranging between 60-94% for the 1st and 2nd stories, confirming that BRBs can effectively dissipate seismic input energy. This study demonstrates that the proposed design and construction methods for the steel embedment are effective. No failure of the steel embedment or gusset is observed in the tests. Test results confirmed that the ACI provisions and the softened strut-and-tie model can be effectively applied to prevent the D-region failure. Test results confirm that the gusset force demands are consistent with the predictions. This study proposed the construction and design procedures of the BRB-RCF using the proposed BRB-to-RC member connections.