欄杆型式對人行橋氣動力反應的影響

The Influence of Railing Types on the Aerodynamic Behavior of Pedestrian Bridges

曾煒翔(Wei-Hsiang Tseng)

指導教授 ： 林堉溢 ； 姚忠達

淡江大學/工學院/土木工程學系碩士班/碩士(2018年)

https://doi.org/10.6846/TKU.2018.00610

摘要

根據以往的研究，常見之橋樑斷面模型試驗，通常為車行橋梁斷面，因斷面寬度較大，研究顯示加設欄杆或護欄等等披覆物對整體結構之氣動力行為影響較小。人行橋一般的斷面寬度較小，其欄杆高度占整橋體深度比例非常的巨大，所帶來的影響不可忽視，因此欄杆的給人行橋橋梁帶來的氣動力效應應加以評估，以免會導致橋梁氣動力行為被錯估。
本研究以橋樑斷面風洞試驗量測為主數值分析為輔，模擬人行橋橋樑斷面加上各式欄杆後的氣動力效應，對人行橋斷面加上不同透孔率的柵狀欄杆(70％、50％、40％)、橫向玻璃欄杆(40％、20％)、直向玻璃欄杆(40％、20％)進行風力係數、顫振導數、顫振臨界風速和抖振等實驗，再使用有限元素程式套入斷面模型所得之氣動力參數進行顫振臨界風速分析，與斷面實驗做比較。最後將所有資料同整進行交叉比對。
在風力係數、顫振導數實驗中顯示，氣動力行為會隨著透孔率的增減和欄杆樣式而有所變化，主要影響為透孔率的變化，由於透孔率的減小會從輕薄細長的斷面逐漸轉變為鈍體斷面，玻璃欄杆斷面的H1*在低無因次化風速的時候會產生渦致振動的效應， A2*會隨著透孔率變小隨之負轉正的地方變小。
顫振臨界風速之分析結果顯示，欄杆的透孔率為影響顫振臨界風速主因，欄杆的透孔率減少顫振臨界風速並隨之降低，數值分析也有相同的趨勢，而欄杆形式也會有所影響顫振臨界風速，柵式欄杆會優於玻璃欄杆，直向玻璃會優於橫向玻璃。加設欄杆後的斷面實驗中紊流的不一定會使臨界風速有顯著上升，甚至在部分欄杆斷面有下降的趨勢，因此在推定顫振臨界風速會應綜合各攻角與各流場的資訊。
由顫振臨界風速與抖振之結果中顯示，負風攻角會造成顫振臨界風速下降與抖振效應放大的趨勢，因此類似斷面之橋梁坐落在受下沉氣流影響之地區或經地形風場分析常為負攻角時，應拉高實場扭轉頻率並使用高透孔率之欄杆，如有需要使用玻璃欄杆，建議使用直向玻璃欄杆。
由本文研究所得之結果顯示，欄杆型式對於橋樑結構系統的氣動力行為、顫振臨界風速、抖振反應有相當大的影響，因此人行橋欄杆效應在整個橋梁抗風設計必須審慎考量。

關鍵字

人行橋 ； 風柵 ； 欄杆 ； 孔隙率 ； 風攻角 ； 氣動力反應 ； 紊流 ； 斷面模型試驗

並列摘要

Most bridge section model tests usually focus on the bridges for vehicles. Previous studies have shown that adding railings or grids has less influence on the aerodynamic behavior of these types of structures because of their large section widths. However, the pedestrian bridge has a smaller section width, and the height of the railing accounts for a large proportion of the total height of the bridge. Therefore, the impact of railings on the aerodynamic behavior of the pedestrian bridge cannot be ignored.

This study mainly investigates the influence of railings on the aerodynamic behavior of the pedestrian bridges by using section model tests and a numerical analysis. The aerodynamic wind coefficients, flutter derivatives, flutter critical wind speeds and buffeting responses of section models with different types of railings are studied. The railings with different porosities of grids(70%, 50%, 40%), horizontal glass (40%, 20%), and vertical glass (40%, 20%) are studied.

The results show that the aerodynamic coefficients and flutter derivatives will change along with the variations of the porosities and types of railings. The main influence on the aerodynamic behavior is the change of the porosities. As the porosity decreases, the cross section gradually changes from a streamlined cross-section to a bluff section. The H1* of the models with the glass railings will produce vortex shedding effects at low normalized wind speeds, and the positive A2*will occur at lower wind speeds as the porosity decreases.

The analysis of the flutter critical wind speed shows that the porosity of the railings is the main factor affecting the flutter critical wind speed. The flutter critical wind speed decreases with the reduction of the porosity of the railing. The numerical analysis also shows the same trend. The flutter critical wind speed varies with different types of the railing. For example, grid railings have better performances than the glass railings, and the vertical glass railings have better performances than the horizontal glass railings etc. In general, turbulent flow increases the flutter critical wind speed in most cases as we expected. However, this trend is reversed in some cases.

The results also show that the flutter critical wind speeds decrease and the buffeting responses increase as the angles of wind attack are negative. If the bridges are possibly attacked by winds at negative angles from the wind field analysis, then the torsional frequency of the bridge should be increased and the railings with high porosities should be used. If glass railings are used, the vertical glass railings are suggested.

The results obtained from the experiment shows that the different types of railings greatly affect the aerodynamic behavior of the footbridges. Therefore, we should take the railing effect of the pedestrian bridge into consideration in the wind resistance design.

並列關鍵字

Pedestrian Bridges ； Wind Barrier ； porosity ； Railing ； Angle of Wind Attack ； Turbulence Flow ； Aerodynamic Responses ； Section Model Test

參考文獻

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