Deciphering the Tsunami Wave Impact and Associated Connection Forces in Open-Girder Coastal Bridges

Abstract

In view of the widespread damage to coastal bridges during recent tsunamis (2004 Indian Ocean and 2011 in Japan) large-scale hydrodynamic experiments of tsunami wave impact on a bridge with open girders were conducted in the Large Wave Flume at Oregon State University. The main objective was to decipher the tsunami overtopping process and associated demand on the bridge and its structural components. As described in this paper, a comprehensive analysis of the experimental data revealed that: (a) tsunami bores introduce significant slamming forces, both horizontal (Fh) and uplift (Fv), during impact on the offshore girder and overhang; these can govern the uplift demand in connections; (b) maxFh and maxFv do not always occur at the same time and contrary to recommended practice the simultaneous application of maxFh and maxFv at the center of gravity of the deck does not yield conservative estimates of the uplift demand in individual connections; (c) the offshore connections have to withstand the largest percentage of the total induced deck uplift among all connections; this can reach 91% and 124% of maxFv for bearings and columns respectively, a finding that could explain the damage sustained by these connections and one that has not been recognized to date; (e) the generation of a significant overturning moment (OTM) at the initial impact when the slamming forces are maximized, which is the main reason for the increased uplift in the offshore connections; and (f) neither maxFv nor maxOTM coincide always with the maximum demand in each connection, suggesting the need to consider multiple combinations of forces with corresponding moments or with corresponding locations of application in order to identify the governing scenario for each structural component. In addition the paper presents “tsunami demand diagrams”, which are 2D envelopes of (Fh, Fv) and (OTM, Fv) and 3D envelopes of (Fh, Fv, OTM), as visual representations of the complex variation of the tsunami loading. Furthermore, the paper reveals the existence of a complex bridge inundation mechanism that consists of three uplift phases and one downward phase, with each phase maximizing the demand in different structural components. It then develops a new physics-based methodology consisting of three load cases, which can be used by practicing engineers for the tsunami design of bridge connections, steel bearings and columns. The findings in this paper suggest the need for a paradigm shift in the assessment of tsunami risk to coastal bridges to include not just the estimation of total tsunami load on a bridge but also the distribution of this load to individual structural components that are necessary for the survival of the bridge.

Role of Trapped Air on the Tsunami-Induced Transient Loads and Response of Coastal Bridges

“…The two load cells installed on the shear keys and the two on the substructure link measured the horizontal forces at two different levels, while the eight load cells below the bearings and the six load cells below the bent cap measured the vertical forces in bearings and columns, respectively. Using the measured forces, the wave-induced moment was calculated as shown in [28]. Figure 3 shows the time-histories of recorded vertical and horizontal forces, together with the calculated moments, for three selected solitary waves and three bores.…”

“…These large OTMs have also been observed in previous studies (e.g., [11,15,17]) but their temporal variation and effect on the structural components has not yet been fully understood. A detailed discussion of the role of OTM for bridge decks with steel bearings is presented in [28].…”

“…These large OTMs have also been observed in previous studies (e.g., [11,15,17]) but their temporal variation and effect on the structural components has not yet been fully understood. A detailed discussion of the role of OTM for bridge decks with steel bearings is presented in [28]. Figure 4 presents the vertical displacements measured by the two string pots and the cumulative rotation of the deck (angle φ) relative to its initial position calculated from Equation (1).…”

“…Although solitary waves have been traditionally used to simulate tsunamis, the recent studies by Madsen et al [25] and Chan and Liu [26] noticed differences in the wave characteristics (e.g., wave profile and wavelength) of solitary waves and tsunamis. Moreover, the studies by Leschka and Oumeraci [27] and Istrati et al [28], which investigated the loading of solitary waves and bores on vertical cylinders and bridges, respectively, revealed the existence of substantial differences between the forces caused by two wave types. Therefore, despite the simplicity and convenience of solitary waves, using more complex type waves such as bores might be a necessity.…”

Experimental Study on Extreme Hydrodynamic Loading on Pipelines. Part 1: Flow Hydrodynamics

“…They concluded that two different waves result in different flow hydrodynamics, i.e., wave height and flow velocity. Istrati et al [18] investigated the different types of tsunamis, i.e., solitary waves and bores, and their effect on I-grider bridge with cross-frames. They characterized the induced forces from a bore as short-duration impulsive horizontal force at the time of bore impact that is followed by a smaller magnitude and longer duration forces.…”