本文應用有限元素法針對斜張橋之斜索(cable stay)建立單一纜索元素系統OECS(One-Element Cable System)與多個纜索元素系統MECS(Multi-Element Cable System)之分析模型。前者以單一纜索元素模擬斜拉索,後者以多個纜索元素模擬斜拉索。並以兩個不同結構型式之斜張橋為例,分別進行靜、動態分析比較。 呆重及預力作用下斜張橋之平衡位置可透過初形分析形狀迭代計算,得到滿足精度要求之理想初形位置。OECS與MECS兩者分析所得初形之斜索的索形不同,後者所得結果較接近真實情況。本文基於初形分析所得之平衡位置進一步進行頻率、振態分析,以OECS模擬分析僅可求得斜張橋之整體振態(global mode)。若以MECS模擬,除了可得到斜張橋之整體振態外,亦可獲得斜索與塔、樑間的耦合振態(coupled mode)及單純的斜索局部振態(local mode),此結果與OECS所得之結果有很大的差異。最後進行強震作用下斜張橋結構之動態反應分析。地震荷載選擇含近斷層效應之集集地震(TCU068石岡國小測站),分別輸入不同方向之地震力作用。經過歷時分析計算後,MECS可獲得強震作用下斜索上各結點的動態位移反應。另由分析結果顯示斜張橋受水平順橋向地震力影響較垂直向為大。
To investigate the essential characteristics of cable-stayed bridges, OECS (one-element cable system) and MECS (multi-element cable system) models for the cable stay of bridges are developed by using the finite element method. Initial shape, modal and dynamic analyses are conducted to compare OECS with MECS. The results from initial shape analysis indicate that balance positions of both OECS and MECS can be reasonably estimated based on shape iteration. The prediction of MECS is more appropriate than OECS. On the basis of initial shape analysis, modal analysis is conducted and its results imply that MECS can reveal global, local and coupled modes of the bridge, whereas OECS model can only yield global modes. Dynamic analysis is presented to evaluate the structural response induced by seismic loading. The advantage of MECS over OECS is that the real time history of lateral displacements in cable-stays can be calculated in MECS. The results also show that the structural vibration induced by seismic loading in horizontal direction makes more contributions to that in vertical direction.