橋梁沉箱基礎側推試驗之模擬分析

Abstract

本研究針對三個於礫石地盤之橋梁沉箱基礎現地側向載重試驗進行模擬分析。三個試驗不同之處在於側向載重施力點位置與基礎裸露深度。分析模型以梁元素模擬橋柱與基礎結構，採用分布塑鉸模型模擬其撓曲非線性反應，並使用六參數土壤彈簧模式模擬沉箱周邊土壤之受力變形行為。分析結果顯示分析模型可以有效模擬橋柱與基礎受側向載重作用下之反應，且沉箱基礎之勁度會顯著地受到載重施加高度與基礎裸露深度的影響。 根據參數研究之分析結果，雖然沉箱基礎埋置於礫石層中，但仍須考量其基礎柔度之影響：相較於固定基盤的情況，在相同側力作用下，基礎柔度會使上部結構因基礎變位產生額外變位，當基礎裸露的深度愈大，基礎柔度的影響會愈顯著。當基礎裸露深度過大時，除其柔度效應顯著外，基礎甚至無法提供足夠之勁度與強度來承載上部結構。此外，對於受側向荷載之沉箱，其前後方土壤正向反力係最主要的土壤側向阻抗來源。 另一方面，在工程結構分析上常以水平位移彈簧、垂直位移彈簧與旋轉之非耦合等效彈簧取代完整之基礎模型，有時甚至以固定基底忽略基礎柔度之影響以簡化基礎行為，本研究亦透過參數分析探討這些簡化方法的合適性。

In this study, three in situ laterally loaded tests of caisson foundations in gravels were analyzed. The differences of these tests were the application position of lateral loads and the exposed length of the foundations. The analysis models used beam elements with plastic hinges to simulate the column and a six-component Winkler beam model to simulate the foundation. Analysis results showed that the proposed analysis models could reasonably predict the behaviors of the caisson and the column. The lateral stiffness of foundation was significantly influenced by the application height of lateral loads and the exposed length of the foundations. According to the results of parameter analyses, although the foundations were situated in a gravel stratum, the caisson with a large exposed length underwent significant foundation flexibility and nonlinearity. A large exposed length may cause a reduction in the lateral stiffness and strength of the foundation. The horizontal soil reactions in front of the foundation provided major resistances to lateral loading. On the other hand, in engineering practice, in order to simplify the complexity of the foundation modeling in structural analysis, a set of uncoupled horizontal spring, vertical spring and rotational spring are often used to replace the complete foundation model, or even the fixed base condition is assumed. In this study, the appropriateness of those simplified methods were investigated.