建築物受垂直向地震之研究分析

Analysis of Building Response from Vertical Ground Motions

地震之振動可分成水平與垂直兩個方向，一般而言在垂直方向上地震振動幅度較水平方向來得小，故以往在探討建築物耐震能力時，大多考慮地震在水平方向上之影響，對於垂直向振動之分析則鮮有研究。目前在國內外多數耐震規範中，建築物垂直方向之地震力更僅僅係以水平方向地震力做為參考依據，普遍為1/2~2/3倍水平地震力。然而在近年來的一些地震中均發現到等量之垂直與水平向振動，在鄰近斷層之區域垂直向振動甚至有超過水平向振動之情況。
為瞭解垂直向振動對於建築物之樓層反應與實際影響，本研究透過中央氣象局在全台各地所設置之建築物強震監測系統，蒐集建築物地震實測紀錄，針對台灣地區強地動特性分析建築物在垂直方向上之樓層振動反應以及頻率特性。研究結果發現建築物之頂樓垂直向加速度放大倍率平均值約為2倍；其中50公尺以下中低樓層建築物之頂樓垂直向放大倍率為1.78倍，而50公尺以上高樓層建築物之頂樓垂直向放大倍率則為2.52倍。此現象說明隨著建築物總樓層高度愈高，垂直向振動於建築物頂樓之放大程度也愈大，也顯示出垂直向振動反應對於高樓層建築物而言並不可輕忽。

The building structures are subjected to both horizontal and vertical ground motions during earthquakes, but only the horizontal ground motion has been extensively studied and considered. Most of the prevailing seismic codes assume the vertical component of the ground motion to be 1/2 to 2/3 of the horizontal component. However, it was found that the vertical ground motion may equal or even significantly exceed the horizontal ground motion in recent earthquakes. In such situations, most existing code specifications must be considered un-conservative.
In order to understand the building responses from vertical ground motions, many earthquakes records are studied in this paper. System identification methods (ARX Model) are used to analyze the strong motions and to find out the characteristics of the vertical amplification factors and the natural frequencies of buildings. The results show that the vertical amplification factors for high-rise buildings and low-rise building are 1.78 and 2.52 respectively, and the average vertical amplification factor of all buildings is about 2. The result points to an important message, the taller the building is, the greater the effect of vertical vibration on the building will be.