臺灣博碩士論文加值系統

台灣地區的橋梁承受洪水及地震等多重天災，基樁因河床淘刷而裸露，基礎的勁度與強度降低；在地震力作用時，橋梁的耐震性能可能違背最初設計，改由基礎控制。
國內橋梁基礎裸露嚴重，又處於地震帶，工程界極需對這些橋梁進行耐震性能評估。本篇論文以容量震譜法評估不同河床沖刷深度下橋墩的耐震性能。研究針對橋墩的有限元素模型進行多自由度側推分析，模型採用纖維梁-柱桿件來模擬鋼筋混凝土橋柱和基樁，並利用土壤彈簧來模擬在基樁上的土壤壓力。在多自由度的側推時，施加在上部結構力量和施加在樁帽上力量的比值是透過橋墩的模態分析來決定，當橋柱或基樁降伏之後，由於側向勁度顯著改變，兩個側推力的比值隨之調整。側推分析所求得的受力-位移曲線用於建構橋墩的容量震譜，容量震譜中橋墩耐震性能點定義為橋柱已先達到性能極限(damage-control limit)或是基礎已先達到使用性能極限(serviceability limit)。
本研究透過比較不同基礎裸露深度下的性能點地表加速度，了解河床沖刷對橋墩耐震性能的影響。分析結果顯示，若橋墩在設計時即具備足夠的基礎強度，其耐震性能在沖刷深度不大時，持續由柱的破壞極限來控制，性能點地表加速度隨著沖刷深度增加而變大；一旦沖刷深度超過臨界值，橋梁的抗震性能改由基礎的使用性能極限來控制，性能點地表加速度隨著沖刷深度增加而急遽下降。

In current practice, the foundation of most bridges is strategically designed to remain elastic even under severe seismic demands. A bridge foundation is typically equipped with a large strength, so that inelastic deformations caused by earthquake excitation would occur in the column. However, for bridges located in regions exposed to flood and seismic hazards, pile exposure resulting from riverbed scour reduces the lateral stiffness and strength of the foundation. Once the scour depth exceeds the critical level, the strength of the foundation becomes insufficient to protect the piles from damage during an earthquake. The seismic performance of a bridge with foundation exposure is completely different from that of the original design. Since many bridges located in seismic-prone regions also suffer from serious riverbed scour, the assessment of their seismic performance becomes important.

In this thesis, capacity spectrum method is used to assess the seismic performance of a bridge at different scour depths. The bridge is originally equipped with sufficient foundation strength and therefore has a satisfactory seismic performance before the onset of river scour. The capacity spectrum of the bridge bent is constructed based on the lateral pushover curve obtained by finite element analysis. The finite element model employs fiber beam–column elements to model the reinforced concrete column and piles, and the beam-on-nonlinear-Winkler-foundation framework to simulate soil–pile interaction. The ratio of the lateral force applied on the superstructure to that applied on the pile-cap is determined by the modal analysis of the bridge bent. During the pushover process, the lateral force ratio is adjusted after the formation of plastic hinges in the structure to account for the effect of changes in stiffness. To ensure a satisfactory seismic performance, the inelastic deformations of the foundation and the column are controlled within the serviceability limit and the damage-control limit, respectively. The seismic performance limit of the bridge is identified as the column or the foundation that first reaches its deformation limit. The seismic demand imposed on the structure is assessed using the acceleration-displacement response spectrum, which is constructed with the consideration of the influence of structural yielding. The maximum seismic demand that the bridge can handle can be defined when the demand spectrum intersects with the capacity spectrum at the point corresponding to the performance limit of the bridge. The influence of riverbed scour on the seismic performance of bridges is assessed by comparing the maximum allowed seismic demands of the bridge at different scour depths. Preliminary results highlight that considering the bridge is originally designed with sufficient foundation strength, its seismic performance is first governed by the damage-control limit of the column. The maximum allowed seismic demand increases as the scour depth increases. However, riverbed scour reduces the lateral strength of the foundation and increases the potential of foundation damage during an earthquake. Once the scour depth exceeds a critical level, the seismic performance of the bridge is governed by the serviceability limit of the foundation. The maximum allowed seismic demand decreases when the scour depth increases. Some design implication is also discussed in the thesis.

致謝 i
摘要 ii
Abstract iii
目錄 v
圖目錄 vii
表目錄 xii
第一章 緒論 1
第一節 前言 1
第二節 研究動機與目的 1
第三節 論文大綱 2
第二章 文獻回顧與探討 4
第一節 模型建立 4
第二節 側推分析 4
第三節 耐震分析 5
第三章 橋梁的有限元素模型 6
第一節 OpenSees軟體介紹 6
第二節 結構模型 6
第三節 結構載重放置證明 11
第四節 模型斷面材料性質與節點桿件 11
第五節 土壤彈簧設定 14
第六節 群樁效應 17
第七節 工址、地盤選定、反應譜 18
第四章 側推分析 22
第一節 質量矩陣、勁度矩陣假設 22
第二節 計算側推比例 28
第三節 側推力增加比調整 33
第四節 側推分析結果 39
第五章 結構分析 41
第一節 容量震譜、等效降伏 41
第二節 性能目標地表加速度 44
第三節 需求震譜 46
第四節 分析流程圖 48
第五節 結構耐震分析I (以橋梁基礎9根樁第三類地盤為例) 49
第六節 結構耐震分析II (地盤、軸力比、基樁根數改變) 50
第七節 結構耐震分析III (基樁裸露與耐震能力折減的關係) 54
第八節 結構耐震分析IV (材料預期強度與原始強度比較) 56
第九節 結構耐震分析V (側推力增加比更換比較) 57
第六章 結論與建議 59
第一節 結論 59
第二節 建議 60
參考書目 62
附錄一 64
附錄二 70
附錄三 89
附錄四 99

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