Experimental study of the effect of mass ccentricity of long span bridges
懸吊式橋樑 ； 風洞實驗 ； 斷面模型 ； 質量偏心 ； 攻角 ； Suspension bridges ； Wind tunnel ； Cross-section model ； Eccentric mass ； angle of attack
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本研究係以實驗方式，在均勻流場之情況下，針對質量偏心效應對懸吊式橋樑氣動不穩定性進行探討。實驗於中興大學農林試驗場內之土木系風洞實驗室中進行，研究以簡單矩形斷面模型作為橋體基本外型，藉由改變來流風速、第一模態(垂直向)與第二模態(扭轉向)之頻率比、質量偏心位置(0%、±5%、±10%)及攻角(0°、4°、8°)下，探討橋體動態反應之影響。 研究以固定第一模態頻率之情況下，改變兩模態之頻率比，依據實驗所得橋體垂直向位移反應與扭轉向角度變化之均方根值分析不同質量偏心相應於不同攻角造成之效應。 研究結果發現，在同一偏心情況下頻率比越高則氣動穩定性愈佳，其顫振臨界風速隨頻率增加上升。而同一頻率比時，攻角0°下游偏心有較佳之氣動穩定性、其次為無偏心，最差為上游偏心。攻角4°則無明顯趨勢；攻角8°時，頻率比1.2及1.35除在偏心量±5%外，氣動穩定性依序為下游偏心、無偏心、上游偏心。 頻率比為1.2時，在上游偏心-10%攻角0°、4°、8°與下游偏心+10%攻角0°之動態反應與其他頻率比之結果有所差異。
In this study, I investigated the eccentricity effect on the aerodynamic instability of a suspension bridge in the uniform flow field. The experiments were conducted in Wind Tunnel Laboratory of the Agricultural Experimental Station of the National Chung Hsing University. I used a simple rectangular cross-section model as a bridge’s basic body shape. The effects on the dynamic reaction of the bridge plate were investigated based on a two-dimensional approach. Measurements of the dynamic reaction of the bridge plate were implemented by changing the flow speed, frequency ratio of the first mode (heaving direction) and the second mode (torsional direction), eccentric mass site (0%, ±5%, ±10%) and angle of attack (0°, 4°, 8°). On the premise, the first mode (heaving direction) frequency was constant. At different frequency ratios, the eccentric mass will cause interaction effects on the bridge. The interaction effects were investigated by analyzing the root-mean-square values of the vertical displacement of the bridge’s body and the changes of the angles in the torsional direction. According to the experimental results, I found that, under the same eccentricity conditions, the aerodynamic stability improved while the frequency ratios increased. The flutter critical velocity increased along with the frequency ratios. Under the same frequencies, when the angle of attack was 0°, the aerodynamic stability was the best when the eccentric mass site was downstream, and then non-eccentric, with the worst when upstream; when the angle of attack was 8°, frequency ratio at 1.2 and 1.35 except when the eccentric value was ±5%, the aerodynamic stability was the best when the eccentric mass site was downstream, and then non-eccentric, with the worst when upstream The dynamic reactions from the conditions while the frequency ratio was 1.2, the upstream eccentricity was -10% and the angles of attack were 0°, 4°, 8°, and that while the downstream eccentricity was +10% and the angle of attack was 0° were different from those of the other frequency ratios.
工程學 > 土木與建築工程