台灣典型集合住宅底層多為商業空間或公共空間，少有隔間牆，二層以上為住宅單元，配有大量隔間牆，牆量差異常導致軟弱底層問題。本研究於4、8及12層三種樓層數結構模型中設定6種牆體配置模式，包括：純構架、含連續到底磚牆、含磚牆軟弱底層、含連續到底RC牆、含RC牆軟弱底層及含連續到底RC牆+磚牆軟弱底層，使用ETABS結構分析軟體及TEASPA耐震評估工具進行非線性側推分析，根據分析結果探討不同牆體配置對低、中、高層RC構架受震行為之影響，最終歸納軟弱底層之類型。
含磚牆軟弱底層模型、含RC牆軟弱底層模型及10倍磚牆量含連續到底RC牆+磚牆軟弱底層模型之底層符合現行規範之軟層定義，層間變位集中於底層；以局部樓層設置塑鉸執行側推分析檢討弱層結果顯示純構架及大部分軟弱底層模型底層為弱層，然而設有塑鉸磚牆軟弱底層模型底層卻非弱層，似不合理，顯示此弱層檢核方法仍有進一步探討空間。TEASPA耐震評估側推分析結果顯示磚牆或RC牆皆使構架彈性勁度及最大強度提升，含磚牆模型中，設有塑鉸磚牆早於構架破壞，磚牆破壞後結構行為接近純構架，若磚牆不設塑鉸或是含RC牆軟弱底層模型，牆體之作用會持續至構架破壞，導致性能點屋頂位移及耐震性能下降，即一般認為的軟弱底層效應。含連續到底RC牆+磚牆軟弱底層模型中的磚牆行為則類似外伸支撐架（outrigger），將中央連續RC牆之懸臂式轉動變形傳遞至外側跨間，導致外側柱之軸力升降。
不設塑鉸磚牆軟弱底層模型、含RC牆軟弱底層模型及10倍磚牆量含連續到底RC牆+磚牆軟弱底層模型底層柱軸力皆呈現顯著升降，以抵抗2層以上牆體旋轉，軸力升降幅度與樓層數呈正相關，且破壞位置集中於底層並導致耐震性能顯著下降，本研究根據上述模型歸納出三種軟弱底層模式：單跨間軟弱底層、多跨間軟弱底層及外伸支撐架式軟弱底層。單跨間牆體軟弱底層因牆體底部僅有兩道支撐柱，柱軸力升降幅度最大，底層柱設置PM塑鉸可反映軸力彎矩互制下彎矩強度降低，而導致提前崩塌；多跨間牆體軟弱底層因有較多跨間柱分攤平衡力偶，柱軸力升降幅度較低，底層柱之PM塑鉸彎矩強度未明顯下降，崩塌模式為層間變位集中於底層導致底層柱因剪力或彎矩明顯高於上部樓層柱而先行破壞；外伸支撐架式軟弱底層之柱軸力升降幅度同樣較低，2層以上不連續到底牆量較高或者樓層數較低時，中央連續牆體底部傾向發生剪力破壞，反之則傾向發生撓曲破壞。

Many typical RC residential buildings in Taiwan have soft story at bottom stories caused by the wall volume difference. Bottom stories are usually for commercial or public uses with small number of walls, while upper stories are residential units with a large number of walls. The objective of this study is to generalize different types of soft story. Therefore, six types of wall configurations in elevation were set up in numerical models of 4, 8, and 12 stories, including: bare frame, fully masonry-infilled frame, soft story masonry-infilled frame, RC wall-frame, soft story RC wall-frame, and soft story masonry-infilled RC wall-frame. Nonlinear pushover analysis and TEASPA earthquake assessment are conducted.
Three types of soft story are generalized. The first type is soft story with single-span walls, which has only two columns below to resist the rotation of the walls. The two columns bear huge axial force change, which leads to sharp decrease of moment strength of PM hinges, and structures collapse due to the failure of bottom-story columns. The second type is soft story with multi-span walls, which has more than two columns to resist the rotation of the walls. The axial force change is not as severe as that of the first type; therefore, the moment strength of columns does not decrease to a serious degree. This type of structures collapse due to the extreme story drifts of bottom story which result in great amount of shear and moment that assumed by columns. The last type is outrigger soft story, with continuous walls behaving like cores and discontinuous walls behaving like outriggers. The outrigger-like walls transmit the rotations of core wall to the exterior columns, leading to axial force change. When the amount of discontinuous walls above 2 stories is high or in low-rise structures, shear failure tends to occur at the base of the continuous walls, and on the contrary, flexural failure tends to occur. Furthermore, the amount of axial force change in bottom-story columns has a positive relation with the number of stories.

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