考量等效沖刷載重之橋梁可靠度設計方法研究

Abstract

台灣地理上因板塊擠壓而造成高山林立，河流坡短流急，又因梅雨及颱風等天候影響，年降雨量大，造成雨季時河川流量暴增，因此造成台灣的橋梁沖刷問題非常嚴重。由於台灣西部都市之間交通依賴跨河橋梁聯繫，而跨河橋梁受損也將造成人命及財產的損失。考量現行橋梁設計規範當中，並無法完整考慮橋梁受到沖刷時之安全性，故針對受沖刷橋梁進行研究，以理論方式計算樁基礎橋梁受沖刷後破壞機率，以期將沖刷效應考慮進橋梁設計中。 本研究以文獻中之等效載重概念為主，以不同沖刷深度下之承載力損失量，將沖刷深度轉換為作用於基礎上之等效沖刷作用載重。為了驗證理論，利用樁基礎橋墩靜力承載試驗方式，找出樁基礎於不同埋置深度下之側推承載力，並計算其沖刷前後承載力之差值，並以此試驗結果重新推導等效沖刷作用載重公式。利用修正後之等效沖刷作用載重公式，提出以條件機率之方式，配合研究中定義之可接受破壞因子，計算橋梁樁基礎之側向不穩定性破壞機率。藉由計算不同沖刷深度下的破壞機率，進一步以理論方式建置沖刷易損性曲線。此外，利用橋梁破壞機率計算方法，結合災害分析及統計資料，則可將沖刷效應發展至橋梁設計規範中，故提出考量沖刷效應下橋梁設計的方法，提供未來於可靠度分析與設計時參考。

Taiwan is located in a convergent plate boundary. As a consequence, more than two-thirds of this island are rugged mountains, which caused the rivers are steep and have rapid currents. Also, mainly caused by typhoons and the plum rain season, the amount of precipitation is more than 2500mm in Taiwan. These factors result in severe scour problems for cross-water bridges. As the current AASHTO-LRFD bridge design codes did not consider scour effect in the design procedure, the current codes cannot ensure the safety of bridges under scour along with other hazard events. This study proposed a procedure to evaluate the failure probability of bridges with pile foundations under scour event. Equivalent scour load is used to transfer the scour depth into a load form. By using a series of experimental single-pile bridge lateral loading test, the relation between the reduction of resistance of the bridge pier and the corresponding scour depth is obtained. The revised equivalent scour load formula which can be used for the bridge failure probability estimation is proposed. Also, the scour fragility curves which are related to the flume test result are established. A methodology for bridge design considering scour is also proposed in this study, which could be a reference for the following research on bridge scour issue.