一般而言，探討長跨橋梁氣動力行為主要建構在平均風向與橋梁主軸正交的情況下。但就實際風場而言，平均風向鮮少正交於橋軸，而且近期一些研究指出，就某些案例而言，當在特定風向角與風攻角下橋梁之氣動力反應會比零風向角時來得顯著。傳統上處理風向角的問題是採用餘弦法則以及斜風理論等近似理論，然而，這類的分解理論並無法反應出此情況，而且僅適用於較小的風向角下。因此，本計畫針對不同的風向角與風攻角進行考慮，預計發展出一套合理而完整的斜風分析模式。另一方面，由於曲線型橋梁可視為風向角連續變化的情形，因此，此分析模式亦可進一步應用於曲線型的橋梁結構。 本計畫為接續96年期計畫的後續研究，主要工作是建構和執行不同風向角及風攻角的顫振導數及風力係數斷面模型實驗。由於傳統斷面模型實驗是風向垂直於橋軸，因此本階段需要設計新的實驗架構來量測各種風向角及風攻角下的風力係數和顫振導數。同時本計畫也將依相似律建構之斷面模型進行實驗，直接量測不同風向角及風攻角之抖振反應及顫振臨界風速。實驗量測之風力係數和顫振導數搭配96前期計畫所發展的理論，發展出一套適用的分析方法來預測斜風下橋梁的氣動力行為。而後再藉由斷面模型實驗直接量測的結果來驗證分析模式的正確性。同時此分析模式結果將與近似方法分析所得的結果加以比較討論。 Most theories and wind tunnel investigations of aerodynamic behavior of long-span-bridges are established in the case of mean wind direction being vertical to bridge axis which is considered as the worst case. In reality, mean wind direction is seldom vertical to bridge axis. Furthermore, recent researches indicate that in some cases the most significant aerodynamic behavior will happen as the bridge subjected to skew winds with a particular angle of wind attack. In the past, for dealing with the buffeting responses and flutter critical wind speed in the case of the bridge subjected to skew winds, the “cosine rule” and “skew wind theory” were often used. However, these approximate theories can not predict the case mentioned above and are only valid for small yaw angles. For this reason, this project intends to develop a reasonable model to investigate the aerodynamic behavior of long-span bridges subjected to skew winds. The model can also be applied to curved bridges because the yaw angles along the bridge axis are continuously changing for a specific wind direction. This project is the continuing research from the last project (executed from 2006/08-2007/07) in which the derivations of flutter and buffeting theories were developed. The work in this year is to set up the frame of the section model test for different yaw angles and different angles of wind attack. Since wind direction is vertical to the bridge axis in traditional tests, a new configuration capable of measuring the aerodynamic coefficients in different yaw angles and angles of wind attack will be designed. The static wind coefficients and flutter derivatives for different yaw angles will be measured in this period. At the same time, the buffeting responses and the flutter wind speeds for different yaw angles and angles of wind attack will be measured in the wind tunnel test in which the section model is constructed based on the similarity rules. The measured coefficients in conjunction with the proposed model established in the last project will be used to predict the aerodynamic behavior of long-span bridges subjected to skew winds. The validity of the analytical results can be examined by the measured results in the section model test. Also, the results from this approach will be compared with those from the approximate theory.
