近年來國內已出現興建或設計於山區之長跨徑纜索支撐橋梁的實例，尤其是人行景觀橋。因複雜地形影響，建於山區之長跨徑纜索支撐橋梁，其氣動力反應除應考慮風速外，大風攻角的影響更不容忽視。一般地形橋梁考慮風攻角的範圍約在正負六度內，然而複雜山區風攻角可能高達正負十五度以上。在此大風攻角下，全橋模型實驗有其困難度，再者，全模型試驗耗時長、費用高，對於這類工期短、造價較低的山區人行懸索橋也並不合適。因此本文即藉由斷面模型風洞實驗來探討不同風攻角及不同紊流強度對橋梁之顫振臨界風速及抖振的影響。 本論文主要分為數值分析及斷面模型實驗。數值分析部分是建立一台灣典型人行懸索橋之數值模型進行模態分析，再以此結果作為製作斷面模型及風洞實驗架構的模擬準則；實驗部分是利用斷面模型風洞實驗，分別量測在不同風攻角及紊流強度下之顫振臨界風速及抖振反應，以及探討垂直向與扭轉向頻率比的提升對顫振臨界風速的影響。 顫振實驗結果顯示風攻角對橋梁臨界風速有很大的影響，隨著風攻角的增加顫振臨界風速會急遽下降，尤其是在風攻角大於6.5°後會有明顯下降的趨勢，在平滑流場下+8.5°之顫振臨界風速較+6.5°下降了約23%，在負攻角部分風攻角-8.5°較-6.5°下降了約40%，當風攻角到10°以上遞減趨勢趨於平緩；而紊流強度的增加對顫振臨界風速之影響在各風攻角下皆有明顯的上升，符合以往文獻所敘述。 在不同垂直向與扭轉向頻率比的實驗結果中顯示，頻率比的提高對於顫振臨界風速的提升有極大的影響。實驗結果中風攻角為-5°時，頻率比1:3.17之顫振臨界風速較1:2.1提高67%，頻率比1:3.4時較1:2.1更提高了超過100%以上。 抖振反應分析部分，隨著風攻角的增加抖振反應有明顯增大的趨勢，並且隨著風攻角的提升，反應增加的趨勢越顯著，其中在風速30m/s時，垂直向抖振反應風攻角+5°之反應較風攻角0°增加了42%，風攻角+10°之反應較風攻角0°增加了84%，拖曳向反應風攻角+5°較0°增加了34%，風攻角+10°較0°增加了75%，扭轉向反應風攻角+5°較0°增加了19%，風攻角+10°較0°增加了178%。 由本文研究所得之結果顯示，大風攻角對於橋樑結構系統的臨界風速及抖振反應有相當大的影響，因此座落於山區中或複雜地形之橋梁設計必須審慎考量。 Some cable-supported bridges have been designed or built in mountainous areas in Taiwan. Due to the complex terrain, not only the variations of wind speed but also the possibility of large angle of wind attack should be considered in the investigations of the aerodynamic responses of the cable-supported bridge built in these areas. The angle of wind attack for the bridge built in the complex terrain has been found in the literature as large as fifteen degrees or more. At the large angle of wind attack, the difficulties have risen in full bridge model tests. Therefore, the measurement of the section model responses is the only way to investigate the aerodynamic behavior of the bridges built in the complex terrain. Effects of large angles of wind attack on the flutter wind speeds and buffeting responses of bridges are investigated in this thesis. A typical pedestrian suspension bridge is chosen as the example for the investigation.
The experimental results show that the flutter wind speeds decrease with the increases of angles of wind attack. The trend is more obvious as the angle of wind attack is larger than 6.5 degrees. The flutter wind speed may decrease 40% as the angle increases from 6.5 degrees to 8.5 degrees. As we expected, the increases of turbulence intensities will increase the flutter wind speeds for all angles of wind attack. The increase of the torsional -vertical frequency ratio of the section model will increase the flutter wind speed significantly. The results also show that the increase of angles of wind attack increase the responses buffeting in all directions. At the wind speed of 30m/s, the vertical response at the angle of 10 degrees increase 84% compared to that at the angle of zero degree. The large amount increases are also found in drag and torsional directions. Since the large angles of wind attack may occur for the bridges built in the complex terrain, the aerodynamic behavior of these bridges should be carefully examined.
