橋梁沉箱基礎耐震設計之理論分析模式發展

本研究以理論分析之角度，考慮沉箱基礎之非線性（韌性）行為，發展橋梁沉箱基礎之初步耐震設計流程。首先基於溫克模型 (Winkler model) 提出埋置於三種理想完全彈塑性土壤之剛性沉箱基礎，受到複合側力與彎矩載重下之反應解析解。分析模型僅考慮基礎周圍土壤之側向阻抗，並根據基礎頂部與底部周圍土壤之降伏情形界定出不同的載重狀態與極限狀態，以基礎頂部之力與彎矩平衡求得各載重狀態下基礎反應與極限承載力之解析解，並以此解析解建立側力-彎矩互制平面上之破壞包絡線與載重狀態分區。接著本研究提出修正之解析解以考慮底部剪力與彎矩效應之影響，利用此解析解可建立沉箱基礎頂部之力-位移與彎矩-轉角側推曲線，並模擬縮尺與現地試驗以進行驗證。利用所提出之分析模型，本研究分析耐震設計規範中用以評估基礎性能之檢核項目及容許值與基礎-土壤狀態之關係，以此探討其之合適性。繼則進行受震反應分析以求得不同回歸期地震作用下橋梁沉箱基礎之性能反應值，並進行一系列參數研究探討各分析參數對基礎與結構系統性能之影響。最後基於研究結果，提出橋梁沉箱基礎之初步耐震設計流程，分別考慮傳統以基礎為容量保護構件及以基礎為韌性構件之二種設計概念。並針對凝聚性及非凝聚性土壤之沉箱基礎為例，示範設計流程，並比較兩種設計概念所得的沉箱基礎設計結果之差異。

關鍵字

沉箱基礎；複合載重；解析解；基礎性能；耐震設計

並列摘要

From a theoretical point of view, this study develops a preliminary seismic design procedure for bridge caisson foundations considering nonlinear (ductile) behavior of caisson foundations. Analytical solutions of the lateral responses of a rigid caisson in three types of elastoplastic soil under combined horizontal and moment loadings are proposed based on the Winkler beam model. The analytical models that only consider the lateral soil reactions on the lateral wall of the caisson are used to derive basic solutions first, and then the effects of base shear and moment resistances are considered by modifying the loads at the caisson head in the basic solutions. The proposed models are used to simulate model and field tests in the literature for verification. This study utilizes the proposed models to evaluate the relationships of foundation performance indexes in seismic design codes and the foundation and soil states. Seismic response analyses are conducted to determine the performance points of bridge caisson foundations under earthquakes of different return periods. A series of parameter analyses are conducted to investigate the influences of model parameters on the bridge performance. Based on these analyses results, this study proposes preliminary seismic design procedures of bridge caisson foundations based on two different design concepts—regarding foundations as capacity-protected elements and ductile elements. Design examples in cohesive and cohesionless soils are demonstrated and the design results of the two design concepts are compared.