Large-Scale Experiments of Tsunami Inundation of Bridges including Fluid-Structure-Interaction
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Recent major earthquake events that occurred in the Indian Ocean (2004), Chile (2010) and Japan (2011) generated tsunami waves of significant heights, which inundated nearby coastal cities causing extreme destruction and loss of human lives. Many coastal bridges were inundated by the tsunami and although they were able to withstand the earthquake, they were damaged by the subsequent waves. In particular, the tsunami inundation damaged 81 bridges on the coast of Sumatra in 2004 and 252 bridges in Japan in 2011 according to on-site investigations (Unjoh 2007 and Maruyama 2013a respectively). The main damage occurred in the connections of the superstructure to the substructure causing the bridge deck to be unseated and washed away. This damage pattern was observed for different types of bridges including steel-truss bridges, I-girder composite bridges, PC-girder bridges and box-girder bridges. These unforeseen events demonstrated the vulnerability of bridges to tsunami inundation. The main objectives of this study were to (a) understand the tsunami inundation mechanism of coastal bridges, (b) evaluate the accuracy of existing simplified predictive equations for tsunami loads, (c) identify the difference in the bridge response when subjected to unbroken solitary waves and more realistic turbulent bores, (d) investigate not only the total waves forces but also the distribution of these forces in each bearing and connection in order to determine the max force that each connection has to withstand, (e) shed light on the physics of the dynamic wave-structure interaction and how it is affected by the dynamic characteristics of the bridge, (f) gain an insight into the role of air-entrapment and nonlinear wave-air interaction for bridges with diaphragms, (g) examine the tsunami forces for different types of bridges including I-girder bridges with cross-frames and diaphragms as well as box-girder bridges, (h) investigate possible mitigation strategies, such as air-vents in the deck, and (i) develop a high quality database that can be used for validation of CFD and FSI models, and development of recommendations and design guidelines for establishing tsunami-resilient bridges.To this end, advanced fluid-structure interaction (FSI) analyses, which considered both the hydrodynamics and structural dynamics, were conducted in LS-DYNA using High-Processing Computing (HPC). Three different wave types and four different bridge configurations were simulated in the analyses and interesting results were obtained. To complement these analyses and advance the state-of-the-art large-scale hydrodynamic experiments were conducted in the Large Wave Flume of the O.H. Hinsdale Wave Research Laboratory at Oregon State University. Twelve configurations of a 1:5 scale I-girder composite bridge, several wave heights between 0.36m and 1.40m, two water depths and a total of 270 runs were tested in the LWF in order to meet the objectives of the project. The results of the study demonstrate (a) the complexity of the tsunami inundation mechanism with the existence of four different phases, among which a phase with a large overturning moment and a distinct rotational bridge mode at the time of the first impact of the tsunami wave on the bridge where the impulsive horizontal and uplift tsunami loads are maximized, introducing the largest tension in the offshore bearings for most of the waves (Phase 1), a phase with a pure uplift of the bridge and a governing translational bridge mode as all the chambers of the bridge become inundated and the quasi-static component of the uplift force is maximized (Phase 3), introducing the maximum tension in many bearings, and a phase with a downward force when the wave hits the top side of the deck, introducing significant compression especially in the onshore bearings (Phase 4), (b) the dependence of the tsunami forces on the wave type with the bores introducing larger horizontal forces than vertical ones and the solitary waves the opposite, (c) the insufficiency of the current research approach of examining the tsunami effects on bridges via the calculation of the total tsunami forces on the deck and the need to examine the forces in each connection and bridge member in order to really understand the effects of the complex wave-structure interaction, (d) the significance of the inertial forces and the bridge dynamic characteristics on the fluid-structure interaction and the forces introduced in the connections, shear keys and substructure, with the very stiff bridge configurations witnessing larger connection forces than the applied load for many bore heights due to dynamic amplification, (e) the increase of the total uplift forces in bridges with diaphragms due to the air-entrapment and the complex effect on the bridge connections due to the nonlinear wave-air interaction, which is also different for solitary waves and bores, (f) the variation of the tsunami loads for different types of bridges, with the box-girder bridge witnessing uplift forces up to 5 times larger than the ones applied on an I-girder bridge with cross-frames, and (g) the effectiveness of air-vents in the bridge deck as a mitigation measure against tsunamis as well as their limitations, the importance of the distance of the vents from the diaphragms and the girders forming the chambers, and the existence of significant 3D effects even in the case of 2D wave propagation with impact of the waves normal to the bridge span.