System performance and reliability of bridge pile foundations in inland waterways subject to vessel impact

Recent collapses of large bridges due to vessel impacts on foundations raise several important questions: (1) What were the load paths through which the impact load was transferred through the structure to the foundation? (2) How did the collapse of a bridge propagate from the individual piles impacted to a whole pier, then to the superstructure and adjacent piers? (3) How can the robustness or redundancy of foundations be improved so that the failure of one structural element does not lead to the collapse of the entire bridge? These questions must be answered in the present effort to retrofit a large number of bridges in inland waterways and to establish more rigorous design criteria for new bridges against vessel impact. A foundation system with a large safety factor may not be truly safe if the foundation lacks the necessary system reliability such that failure of one of its elements would lead to the failure of the entire foundation system. This was witnessed in the June 2007 Jiujiang Bridge collapse due to vessel impact. The principal objective of this project is to investigate the system reliability and failure paths in multi-pier bridge pile foundations under various levels of vessel impacts. This will be accomplished by (1) conducting extensive centrifuge model tests to study the effects of vessel impact on individual piles, individual pile groups, and multiple pile groups connected by a bridge superstructure; (2) studying the foundation system reliability under vessel impact based on stresses and movements measured in the tests; (3) performing numerical analysis of the system dynamic reliability of the model foundations and the foundations involved in the Jiujiang Bridge collapse. Measures to improve the system reliability of bridge pile foundations against vessel impact will be proposed. An advantage of this research project is to conduct a large number of affordable impact model tests as a means of “parametric” study. Based on extensive stress and movement measurements and high-speed video images from these tests, many complex issues of system reliability surrounding bridge pile foundations under vessel impact, such as failure modes of the system elements, load paths, and the stresses and deformations required to initiate the failure of key system elements and the entire system, can be investigated on a physical basis along with numerical analyses.