Strongback對同心斜撐構架受震行為影響

Abstract

同心斜撐構架（Concentrically Braced Frame，CBF）系統為現代常用的結構系統。此構架型式缺點為當某一層樓斜撐構架中的斜撐發生降伏或挫屈，即會造成該層樓的側向勁度與強度降低，樓層變形會集中於此層樓，即弱層(Soft Story)現象。造成該樓層嚴重破壞，可能使整體建築無法修復而無法繼續使用。本研究的主要目的即為以strongback改善CBF中的弱層現象，使結構物的層間變位均勻分布於各個樓層。 本研究為個案分析，研究方法為建立三層與十二層的SCBF構架，分別加入不同強度與勁度的strongback，再分別進行靜態與動態的非線性行為分析，以觀察短周期結構物與長周期結構物對加入不同強度勁度的strongback影響。研究變數為α(勁度變化因子)與β(強度變化因子)，分析結果比較靜態非線性行為DCF(Drift Concentration Factor)、動態非線性行為DCF、各層樓的遲滯迴圈、層間最大變位、層間永久變位與樓層動態歷時，並評估α(勁度變化因子)與β(強度變化因子)對結構物弱層改善的情形。 研究結果顯示，就此個案為例，加入α與β數值較高的strongback可以降低結構物的DCF，並降低結構物的最大層間變位與最大永久變位，使各樓層的最大層間變位與最大永久變位均勻分布。但過低的α與β數值（α<0.0048，β<0.014）對結構物並無影響；過高的α與β數值（α>0.0144，β>0.042）對結構物影響非常小，因此本研究以符合經濟效益與改善結構物弱層情形為前提，建議α=0.0096，β=0.028作為strongback設計之參考數值。

The objective of this study is to improve drift concentration in Concentrically Braced Frame (CBF) structures by adding strongback system. In convention CBF structures, the buckling of the braces leads to severe reduction in system strength and stiffness. Therefore soft story mechanisms followed by large permanent deformation are commonly observed in CBF structures. The storngback system using additional structural components along the height of the building to resist local deformation is able to improve the distribution of the drift. Case studies investigate the effects of strength and stiffness of the strongback system on the behavior of three- and twelve-story CBF systems representing short- and long-period structures. The primary variables to be investigated are stiffness factor α (strongback stiffness/CBF stiffness) and strength factor β (strongback strength/CBF strength). We conducted nonlinear static and dynamic analyses to evaluate the effectiveness of α and β on structural demand parameters such as drift concentration factor (DCF), maximum drift ratio and permanent drift ratio. Analyses results show that strongback system with higher α and β will reduce DCF, maximum drift ratio and permanent drift ratio of CBF systems. Also, strongback systems with α<0.0048 and β<0.014 have only little effects on improving structural behavior. On the other hand, strongback systems with α>0.0144 and β>0.042 are noneconomic. The case studies suggested that α=0.0096 and β=0.028 accounting for both efficiency and economics can be used in the design of the selected CBF systems.