Spatial failure mechanisms and performance analysis of coastal bridges under extreme waves

Abstract

Climate change could drive the warming of air and sea temperatures and cause more extreme weather events (e.g., storms, surges, and flooding), inducing tremendous challenges to coastal communities. Coastal bridges, especially for low-lying ones, are susceptible to increased risks and consequences under the climate change scenario. For instance, the 2005 Hurricane Katrina stroke the coasts of Alabama, Mississippi, and Louisiana and destroyed hundreds of highway bridges. The total cost to repair and rebuild the damaged bridges reached over $1 billion. More recently, the 2011 Great East Japan Tsunami destroyed more than 250 coastal bridges. The extreme waves caused by these disasters often hit the bridge superstructure, demolish bearing connections, and wash the deck away. Reconstruction of the damaged bridges and retrofit of existing ones rely on an accurate calculation of the wave-induced loads and structural responses. Therefore, an in-depth understanding of the wave-bridge interaction and prediction methods of extreme wave loads are critical to meet the challenges of a fast-evolving seascape of extreme waves and coastal climate change scenario. Although there were some numerical investigations on the structural performance in the previous studies, most of them were conducted based on simplified methods, e.g., a two-dimensional (2D) numerical model which assumes a uniform cross-section along the longitudinal direction of the superstructure, simplification of structural details like overhangs and diaphragms, and static analysis by applying peak wave loads. It is necessary to develop a detailed model to investigate the wave-bridge interactions and provide more accurate and flexible results. To reach this purpose, this thesis establishes a three-dimensional (3D) Computational Fluid Dynamics (CFD) model to reproduce the wave-bridge interaction during extreme natural hazards. Compared with 2D models, the 3D model has better performance in simulating load distributions on the structure, flow and escape of trapped air beneath the deck, and structural components (girders and diaphragms), thus providing more accurate results. To lend confidence to the established model, laboratory experiments are conducted at the hydraulics laboratory of the Hong Kong Polytechnic University as well. In addition, most of the studies focused on the maximum wave force on the deck, while the time series effects during the wave-bridge interaction and dynamic structural responses attracted less attention. The bearing (or constraint) performance under the wave impacts, which plays an important role in the connection between superstructure and substructure, has not been investigated thoroughly. Since the uplift wave forces are opposed to the traditional downward traffic loads, the bearing responses under wave impacts are rather complicated. Thus, it is vital to study the bearing performance during the wave-bridge interaction utilizing a more sufficient model. By importing the time histories of wave load distributions from the CFD model into a 3D Finite Element (FE) model for the investigated bridge, dynamic structural responses are calculated. Potential failure mechanisms are identified, and different levels of limit states are defined. Furthermore, investigations on the adaptation strategies to mitigate hazard-induced risks on coastal bridges are limited due to the lack of a systematic probabilistic assessment framework. Existing ones often neglect the complex wave-structure interaction process and the component performance. To address this issue, this thesis develops a new component-level overturning failure mode of a bridge subjected to hurricane waves by considering the overturning moment, bearing damage, and uncertainties in structural capacity and demand. On this basis, different adaptation actions are tested and robust strategies are proposed by considering the deep uncertainties associated with climate change scenarios. Overall, this thesis mainly investigates the structural performance and spatial failure mechanisms of coastal bridges under extreme waves based on experimental, numerical, and metamodel methods. Specifically, 3D CFD and FE models are established to simulate the wave-bridge interaction and investigate the dynamic structural responses. Laboratory experiments are conducted to improve and validate the simulation results. Based on the numerical results, a new fragility model considering overturning moment and bearing performance is developed. Limit states, as well as the probabilistic structural capacity and demand, are determined. Multi-criteria robust optimizations are performed to find out the optimal adaptation strategies during the bridge service life.