Numerical Investigation of Tsunami-Borne Debris Interaction with Coastal Bridges
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Authors
Hasanpour Esyahbanati, Seddigheh
Issue Date
2023
Type
Dissertation
Language
Keywords
Alternative Title
Abstract
The destructive potential of tsunamis has been well-documented in past events such as the 2004 Indian Ocean and 2011 Great East Japan tsunamis, which resulted in extensive damage to coastal regions including the destruction or damage of numerous bridges. These transportation links are particularly vulnerable to damage from tsunami-driven debris, such as boats, vehicles, and shipping containers, which upon impact can remove a bridge superstructure from its supports. The significance of understanding the effect of floating debris on coastal bridges cannot be overemphasized, as transportation infrastructure plays a vital role in post-disaster response and recovery efforts. Despite the availability of data from several debris-related studies, the majority of them have been focused on buildings, and very limited information is available for bridges. Furthermore, the majority of studies have been experimental, as the numerical investigation of complex multi-physics phenomena involving fluid flow with turbulent wave breaking, and non-linear contact between the debris, the trapped fluid, and the bridge, is quite challenging. Accurate quantification of the forces involved in debris-flow-bridge interaction is important for the design of tsunami-resilient bridges. The main objectives of this study were to (a) understand the two-fold effect of debris impact and damming on bridges, (b) shed light on the debris dynamics and debris-fluid-bridge interaction and associated loads, (d) quantify the effect of the debris orientation, (d) explore, calibrate and assess the accuracy and limitations of particle-based (SPH) and/or coupled particle-mesh based (SPH-FEM) methods, (e) investigate the role of debris mass, and (e) provide recommendations regarding simplified prescriptive load equations for debris impact for inclusion in the Tsunami Design Guidelines for Coastal Bridges developed by PEER and recently adopted by the AASHTO Committee on Bridges and Structures. In the present study, a thorough examination of the effect of various factors on debris movement, velocity, and impact force on bridge superstructure was conducted. By utilizing the coupled SPH-FEM numerical technique, it is demonstrated that the trajectory of debris can vary depending on the tsunami flow characteristics, the debris initial orientation, debris mass, and the bridge elevation. Through observation and analysis, three distinct patterns of debris movement around bridge decks are identified and designated as Patterns A, B, and C. Pattern A, the most frequently observed pattern, involves debris impacting the offshore side of the bridge superstructure, followed by movement below the soffit and eventual resurfacing on the onshore side. Additionally, when debris passes below the deck, it may impact the soffit, leading to uplift loads that can surpass the maximum horizontal loads. Pattern B involves debris movement above the deck with or without impact on the top surface. Pattern C -the least frequently observed pattern- involves a debris impacting the offshore side of the superstructure and becoming trapped below the offshore overhang, resulting in repetitive impulsive loads and long-duration damming loads until the end of the inundation. The study also reveals that the debris exhibits both horizontal and vertical velocities at the instant of primary impact, resulting in applied forces on the bridge in both directions simultaneously. Additionally, the research demonstrates the complexity of the debris dynamics and debris-flow-bridge interaction, with some cases resulting in secondary impacts of greater magnitude than the primary impact. The study further demonstrates that the debris initial orientation has a significant effect, with longitudinal debris reaching higher velocities and resulting in larger impact forces than the transverse one. In addition, it is also shown that the debris mass plays a crucial role in determining its movement, velocity, and impact forces. The results of this study indicate that the presence of the debris significantly impacts the flow velocities and pressures on bridges relatively to clear-water tsunami conditions. Specifically, it is found that the presence of debris leads to a consistent increase in total horizontal forces. Through the use of SPH-FEM analyses, the ratio of total forces with transverse debris to total forces without debris (Rx) is found to range between 1.5 and 6.5, with an average value of 2.67. Additionally, the ratio in the vertical direction (Rz) is found to range between 0.9 and 4.7, with an average value of 1.85. Furthermore, it is found that the presence of longitudinal debris leads to an average of 3.64 and 2.13 times larger horizontal and vertical forces respectively, in comparison to cases without debris. These findings highlight the importance of considering the debris in tsunami risk assessment frameworks and the design of bridge structures in tsunami-prone areas. In summary, the findings of this research are expected to have significant implications for the design and construction of bridges in areas prone to tsunamis and in this regard, a preliminary set of prescriptive equations for the debris impact forces is proposed.