脊背橋動態反應之量測與分析

Abstract

現地試驗是瞭解結構物實際動力特性最直接的方法。本文主要是對一脊背橋(Extra-dosed Bridge)進行多項現地動態試驗，將試驗資料透過系統識別方法求得脊背橋之結構特性，再將所得結果與有限元素分析結果進行比對。經由微動量測及衝擊試驗，識別出橋體的動力特性，包含自然振動頻率，阻尼比及振動模態；並利用弦理論分析估算鋼纜之預力值；最後，藉由動態車流載重試驗利用公式直接計算脊背橋之位移動態放大係數。 研究結果顯示：(1) 微動試驗及衝擊試驗均識別出8組模態頻率，所識別出以X、Y、Z各方向為主的第1模態頻率分別約為1.48Hz、1.74Hz和0.88Hz；(2) 不同現地試驗所識別出的頻率與振態大部分非常吻合；但與有限元素分析所得者有較明顯差異；試驗所得X向及Z向主振之最低頻率明顯高於有限元素所得，而Y向主振之最低頻率值則呈相反趨勢；(3) 位於第二跨中央外側護欄處所估算之位移DAF較其他測站為大；(4) 現地試驗中，位移放大係數在時速低於60km/hr時，有隨車速增加而遞增之趨勢；該趨勢比有限元素分析所得者明顯；(5) 經由弦理論公式所估算的鋼索張力，與荷重計所量測之值相差4.36%。

Dynamic tests in situ are necessary for determining the dynamic characteristics of a structure. This work reports dynamic tests for an extra-dosed bridge and identifies dynamic characteristics of the bridge. The modal parameters of the bridge are identified via a continuous wavelet transform to process the measured dynamic velocity responses of the bridge from ambient vibration tests and impulse tests. The identified dynamic characteristics are also compared with those obtained from finite element analyses. The dynamic amplification factors (DAF) of vertical displacement along the bridge deck are directly determined for different controlled traffic loads. A cable tension is identified through the measured lowest natural frequency of the cable based on the string theory. The identified cable tension is also compared with the value directly measured from an installed load cell. The following facts are found from this study. (1) Totally, there are eight modes identified from the ambient vibration tests and impulse tests. The results identified form different tests are consistent, and the lowest natural frequencies corresponding to the modes dominated in longitudinal (X), transverse (Y), and vertical (Z) directions are around 1.48Hz, 1.74Hz and 0.88Hz, respectively. (2) There are significant differences between the identified natural frequencies and those obtained from finite element analyses even though they may have similar mode shapes. The identified lowest natural frequencies in X and Z directions are larger than those obtained from finite element analyses; but the opposite trend is found for the lowest natural frequency in Y direction. (3) The station near the edge of deck in the second span has larger DAF than the others. (4) The values of DAF increase with the increase of car speed when the car speed is less than 60km/hr. (5) The identified cable tension is 4.36% larger than the measured.