Proceedings of the 6th International Conference on the Application of Physical Modelling in Coastal and Port Engineering and Science (Coastlab16) Ottawa, Canada, May 10-13, 2016 Copyright ©: Creative Commons CC BY-NC-ND 4.0

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LARGE-SCALE TESTING OF TSUNAMI IMPACT FORCES ON BRIDGES

PEDRO LOMONACO1, DENIS ISTRATI2, TIM MADDUX1, IAN BUCKLE2, SOLOMON YIM3, TAO XIANG3

1 Hinsdale Wave Research Laboratory, Oregon State University, USA, pedro.lomonaco@oregonstate.edu 2 Civil and Environmental Engineering Department, University of Nevada, Reno, USA, distratii@unr.edu

3 School of Civil and Construction Engineering, Oregon State University, USA, Solomon.yim@oregonstate.edu

ABSTRACT

A comprehensive series of experiments have been carried out of a large-scale bridge superstructure subject to impact forces of tsunami-type broken and unbroken waves. Two 1:5 scaled specimens of a bridge superstructure, composed of a reinforced concrete deck supported by multiple steel girders and steel cross-frames, were designed and built at the University of Nevada, Reno, while the tests were conducted in the Large Wave Flume at the HWRL, Oregon State University. The relatively large-scale selected allowed a realistic modelling of the bridge superstructure, which included elastomeric bearings under each girder and springs at the level of the bent cap to represent the different dynamic properties of the bridge substructure (bent cap and piles). The bridge testing configurations included the effect of water depth, tsunami-wave height, bridge angle, horizontal stiffness, bearing connection properties, use of shear keys, use of diaphragms between girders, box girders, and venting of the bridge deck. The experiments aimed to provide a detailed data base for developing empirical predictive models of tsunami loads on bridges, improving the understanding of the wave-structure interaction for a semi-rigid specimen, as well as to become a valuable benchmark series for validation of numerical models of non-linear hydrodynamics and wave-structure interactions.

KEWORDS: Bridges, Tsunami forces, Wave-structure interaction, Coastal structure’s resilience.

1 INTRODUCTION

Bridges are an essential part of the transportation system along the coast, and have a significant role providing escape routes and access to coastal communities before, during and after a tsunami, a hurricane, or almost any coastal hazard event. The 2011 Great East Japan Earthquake and resulting Tohoku Tsunami impacted more than 300 bridges along the Japan’s coast, and about 50% were destroyed and washed away. According to the surveys, most of the bridges survived the earthquake but were completely destroyed by the Tsunami, indicating that the current design specification cannot provide bridges with sufficient strength to resist the tsunami loads (Yim and Azadbakht, 2013). Historical reports of modern-design bridge damage and destruction by tsunamis can also be found as well, e.g. the 1946 and 1957 Tsunamis that hit Kauai washed several bridges at Wainiha and Kalihiwai, isolating the communities along the North shore of the island (Pacific Worlds, 2001). This also indicates that, in spite of the awareness of the problem, there is a lack of full understanding of the process, and the difficulties in the implementation of design codes and recommendations for building tsunami-resilient bridges, particularly since some communities are vulnerable to tsunamis, although do not necessarily belong to a seismic hazard zone.

Generally speaking, two main bridge failure mechanism are observed during tsunamis: 1) failure of the connections attaching the bridge superstructure (i.e. the bridge deck and supporting girders) to the bridge substructure (i.e. the bridge piers and/or abutments), and 2) scour around bridge abutments and supporting piers, failure of the foundation, failure of the bridge piers, and failure due to debris impact. In this study, the mechanisms inducing the first type of failure are investigated. Figure 1 presents two examples of damaged bridges by Tsunamis, where the connections attaching the bridge superstructure have failed (first type of failure).

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Figure 1. Remaining piers of the Koizumi Bridge in Japan (left) after the Tohoku Tsunami in 2011, and the Kalihiwai Bridge in

Kauai (right) after the 1957 Aleutian Island Tsunami (from EERI, 2011 and Pacific Worlds, 2001, respectively).

The behaviour of bridges under the action of hurricane waves and storm surges has been studied over the past few years (cf. Yim and Azadbakht, 2013), but fewer studies are found regarding tsunami loading and the dynamic response of the bridge superstructure. So far, the studies have covered on-site surveys analysing the failure mechanisms, small-scale experiments with rigid bridge models, or numerical model simulations with limited validation where the bridge dynamics might not be included.

Bradner et al. (2011) presented an experimental setup for a large-scale bridge superstructure model subjected to regular and random waves over a range of water depths. In their study, a description is made of a unique series of experiments conducted on a realistic 1:5 model scale of a bridge superstructure, where the experimental setup allowed direct control of the stiffness of the horizontal support system to simulate different dynamic properties of the bridge substructure (i.e. bent cap and piles). As a result, it was found that the different dynamic responses of the bridge had a significant effect on the measured peak pressures and resulting forces, particularly on the phase and magnitude of horizontal loads. Moreover, the direct measurement of wave forces could be compared with integrated pressure measurements given the large-scale of the model. This is particularly relevant due to air entrapment under the bridge, which can significantly affect impact pressure measurements in small-scale models, since peak pressures do not scale in accordance to Froude criteria (Note: this is mainly due to the spring-like effect of air compressibility, similar to the dynamic similarity limitations found in oscillating water column wave energy converters, Falcão and Henriques, 2014).

The experimental setup from Bradner et al. (2011) served as a reference for the physical model testing described herein, where some mechanical elements and instruments were used again for coherence, simplicity and comparison purposes.

Hence, the ultimate goal of this work is to execute a comprehensive series of experiments to assess tsunami-wave impact forces on bridges under realistic conditions, which should include the dynamic response of the bridge, identifying the effect of different parameters, e.g. tsunami-wave height, bridge angle relative to the incident wave, bridge superstructure horizontal stiffness, bearing connection properties, use of shear keys to support the girders, use of diaphragms between girders, opening gaps between girders (box girders), and venting of the bridge deck. For comparison purposes with the results presented by Bradner et al. (2011), the bridge specimens were subject to a range of regular wave conditions as well. However, the discussion regarding regular wave tests lies beyond the scope of the present paper.

The data base will allow the development and application of time-domain numerical models describing the dynamic structural response of the bridge under the action of tsunami-waves. The experiments should also provide high quality data for hydrodynamic model calibration and validation, including detailed time series of the free surface elevation, and vertical profiles of the dynamic pressure and flow velocity at different locations along the experimental facility. Furthermore, the data base ought to be the foundation for future development of semi-empirical models, design codes and recommendations for building new and assess existing tsunami-resistant and resilient bridges along the coast.

This paper presents a detailed description of the large-scale experiments of tsunami impact forces on bridges, summarizing an overview of the testing facility, the experimental setup, instrumentation, and test program. The paper also includes a sample set of the high quality measurements along with some interpretation of results.

2 TEST FACILITY AND WAVE FLUME BATHYMETRY

The experiments were conducted in the Large Wave Flume (LWF) at the O.H. Hinsdale Wave Research Laboratory (HWRL) at Oregon State University. The flume is 104.24 m long, 3.66 m wide, and 4.57 m deep. The maximum depth for tsunami-type wave generation is 2 m, while the maximum depth for short (wind) wave generation is 2.7 m. The LWF is equipped with a piston-type dry-back wavemaker with a 4.2 m maximum stroke hydraulic actuator assembly, capable of

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generating short- and long- regular and random waves, as well as tsunami-type waves (solitary waves), in a wide range of periods and heights. The maximum regular wave is 1.7 m height, with a period of 5 s and at 2.7 m water depth. The maximum tsunami (solitary wave) is 1.4 m height at 2 m water depth. The flume is also equipped with a powered carriage with full cross-shore traverse and a bridge crane with 6 tons capacity; as well as another carriage with a powered vertical instrument deployment frame; and finally a light-weight carriage for observation, video recording and lightning applications. Further, the flume incorporates a movable/adjustable bathymetry made of 20 square configurable concrete slabs.

The flume includes a series of bolt-hole vertical patterns to assemble test specimens as well as to support the concrete bathymetry slabs. Each bolt-hole pattern has a 3.66 m (12 feet) cross-shore separation, starting 10.62 m from the wave board, the flume incorporates 22 patterns, also known as “bays”.

For these experiments, the bathymetry was configured with the concrete slabs to comprise an impermeable 1:12 slope 7.3 m long, followed by a 40.25 m long horizontal section, and then another 18.3 m long 1:12 slope to dissipate waves. The top of the horizontal section is 0.84 m from the flume bottom, hence, with 2 m depth at the wavemaker, the horizontal testing section has a water depth of 1.16 m.

Figure 2 presents a longitudinal cross-section of the wave flume with the experimental setup, including the modeled bathymetry and the test specimen. Details of the model bridge specimens are included in the following section.

Figure 2. Longitudinal cross-section of the Large Wave Flume depicting the modeled bathymetry and the model bridge specimen.

2.1 Wave propagation and bridge specimen location

As the wave propagates over the installed bathymetry, and depending on the wave height and available depth, it shoals and breaks forming a bore that dissipates energy. If the depth remains constant once the wave has broken, the wave will reform when the bore height to depth ratio is about 0.5. Otherwise, it will continue breaking until it reaches the shoreline (e.g. for a sloping bottom). The breaking location and the phase velocity of the bore can be theoretically estimated with relatively high accuracy. However, the transient process from the onset of breaking, wave overturning, plunge point, splash-up (wave bouncing), the secondary plunge and, eventually, the formation of a quasi-steady state bore, has not been described completely.

Peregrine (1983) presents a detailed description of the different phases encountered by a plunging wave, including a short discussion about the time and distance travelled by the jet until it plunges ahead. He also indicates the occurrence of several splash-up cycles before the bore actually forms. Another example is the work of Tulin and Waseda (1999), who present a more complex description of steep breaking waves formed by the modulation of wave groups. However, no information has been found about the time required by the wave to evolve from the incipient breaking point to the establishment of the quasi-steady bore, and more importantly, the description of the distance covered by the wave during this process has not been reported explicitly, particularly for a solitary wave. Nevertheless, several numerical and analytical models are capable of producing these results, although the resolution of the available models is insufficient for the accuracy required in this work.

For the bathymetry configuration installed in the LWF, it is expected that the wave profile evolves from the deeper section to the shallower testing section, steepening by shoaling and, if the wave height is large enough, start the process of breaking as described above. Yet, it is unknown the distance the wave would travel from the onset of breaking until the stable formation of the bore and, additionally, the quasi-steady bore height is also unknown.

For example, considering a simplified definition of the wave celerity, c, of a shallow water wave (or Solitary wave) as:

𝑐𝑐 = �𝑔𝑔(ℎ + 𝜂𝜂) (1)

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Where: g is the acceleration of gravity, h is the local water depth, and η is the instantaneous surface elevation height,

with 2 m depth at the wavemaker, the largest tsunami-type wave profile in the Large Wave Flume might be travelling at 5.8 m/s, and at ~5 m/s once the wave have entered the horizontal testing section. As soon as the breaking process starts, the wave profile keeps travelling at a very high but unsteady speed. The change in water depth creates a local variation of momentum at the bottom that is transported vertically, reconfiguring the velocity and pressure profile, adapting them to the corresponding depth. Onset of breaking and a cascade of instabilities will then develop under the right conditions, particularly if the horizontal velocities at the crest are higher than the phase velocity, producing a plunging wave. However, this process takes time (and space) since is governed by the inertia of the fluid.

For the design and execution of the bridge tests, the location of the specimens along the flume depends on the tsunami-wave breaking point and the stable formation of a bore with enough height that will actually hit the superstructure of the bridge. To determine the range of wave conditions and water depths to be tested, the breaking point and location of the quasi-steady bore, should be simulated beforehand.

Indicative numerical model simulations have been performed, providing an estimate of the breaking point and bore formation but, as indicated previously, the details of the bore formation and its height obtained from the numerical model are still uncertain and not accurate enough, and due to the high phase speed of the breaking wave as well as the limited dimensions of the flume, a more precise estimation was required.

Hence, a series of preliminary undisturbed wave tests (in the absence of the structure) were performed in the flume with different wave heights and water depths to assess the onset of breaking point, the location of the quasi-steady bore, and its height, confirming visually the estimations found with the numerical model. It should be noted that physical modeling of these conditions remains far more efficient than the numerical computations, mainly due to the computational time required for each simulation, as well as the uncertainties introduced by the relatively coarse discretization of the domain.

Different solitary waves were executed where, according to the simple solitary wave breaking criteria introduced by Munk (1949), the wave should break when its height to depth ratio (i.e. the breaking index) is larger than 0.78. Considering the preliminary test conditions, breaking should happen between bay 4 and 5, just at the beginning of the bathymetry installed in the flume. However, it was observed that the onset of breaking actually happens further down, between bays 8 and 9, and the quasi-steady bore is formed after bay 12.

Alternate breaking criteria which incorporate a continuous beach slope (e.g. Grilli et al., 1997, Eq. 4 and 5) estimate that the wave breaking index is 2.2 to 2.4 for a 1:12 sloping bottom, with a breaking depth not exceeding 0.63 m, indicating that the solitary wave should not break at the horizontal testing section (with a water depth of 0.96 to 1.16 m, for the tested cases). This discrepancy supports the previous analysis showing that Munk’s breaking index actually triggers the breaking process, which is delayed and happens much later due to inertia effects, as interpreted by Grilli’s breaking index.

The results of the preliminary tests allowed the definition of the optimal location of the bridge model along the flume, where the quasi-steady bore formation and height was observed, ensuring the proper testing of the tsunami impact forces on the bridge specimens. The straight bridge was installed between bays 14 and 15, while the skewed bridge was installed between bays 14 and 16.

3 TEST SPECIMENS

The test specimens were designed and fabricated by the University of Nevada, Reno, with a model scale of 1:5, representing a bridge superstructure composed of a reinforced concrete deck slab, supported by standard I steel girders and cross frames. Following Bradner et al., (2011) the geometric scale of 1:5 was again selected to (1) allow for the largest possible test specimen with a representative length to span the width of the flume, (2) take advantage of the existing mechanical elements for testing, and (3) enable direct comparisons between both tests campaigns.

In this case, the reaction frame developed previously was salvaged and retrofitted, since it was designed to permit the test specimen to move along the axis of wave propagation. By this means, the bridge substructure flexibility was incorporated in the reaction frame, and can be configured for testing different realistic stiffness constants. The horizontal flexibility of the prototype structure was modeled by a pair of elastic springs installed between the bent caps and the end of the anchorage blocks.

As indicated previously, one of the parameters to be analyzed on the dynamic response of the bridge, is the tsunami-wave incident angle, since several examples can be found where the bridge and the tsunami bore presents an oblique angle, particularly if the bridge crosses a channel (or a river) where the margins are made of vertical concrete walls. Under these circumstances, the resulting flow is highly three-dimensional as well as the response of the bridge, where the structure may portray asymmetric vertical and horizontal overturning moments.

Therefore, two bridge specimens have been designed, both spanning the full width of the flume, where the longitudinal

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axis of the first specimen is perpendicular to the longitudinal axis of the flume (straight bridge) while the second specimen depicts a 45 degrees angle (skewed bridge). The straight bridge was located in the horizontal section, being its centerline 31.1 m landward of the foreshore bathymetry, 60 m from the wavemaker (see Figure 2). The skewed bridge specimen centerline was located also in the horizontal section, 32.9 m landward of the foreshore bathymetry, 61.8 m from the wavemaker.

Figure 3 presents a longitudinal view of the straight bridge specimen and reaction frame. Additional details on the design of the reaction frame can be found in Bradner et al. (2011).

Figure 3. Longitudinal view of the straight bridge model and reaction frame.

Figure 3 includes the representation of the horizontal section of the bathymetry, the reaction frame, linear guide rail and rollers, load cells, bent cap, shear keys and the bridge specimen. As seen, the bridge specimen rests on top of 4 load cells (LC) which are connected to the bent cap. The bent cap rests over another 3 load cells and its flexibility is restrained by the horizontal bar attached to the anchorage blocks. To modify the horizontal stiffness, the bar is substituted by springs, while the load cell remains to measure the transmitted forces. The load cells below the bent cap rest on 3 rollers, which slide over the rail that is fixed to the reaction frame. Details on the nomenclature of the load cells depicted in Figure 3 are presented in the following section.

In addition, the flexibility of the bridge connections to the bent cap was modeled explicitly using elastomeric bearings under each girder (not shown in Figure 3). By comparing the response of the same model with rigid connections and a very stiff substructure, the influence of wave-structure-interaction on tsunami forces could be studied.

According to the design presented in Figure 3, the test specimen can be subdivided in three major structural elements: (a) the reaction frame, fixed to the concrete side walls of the flume; (b) the bent cap supported by three elastomeric bearings atop of its corresponding load cells and connected to the reaction frame by high precision ball bearing rollers and a linear guide rail; and (c) the bridge model formed by the concrete deck, 4 steel girders (3 steel girders in the case of the skewed bridge), and steel cross frames, supported by axial load cells and restrained by shear keys on both sides. An image of the straight bridge specimen installed in the Large Wave Flume is presented in Figure 4.

To study the dynamic response and sensitivity of the superstructure to the incident impact loads of a tsunami-type wave, different structural elements were changed as part of the test program. On one hand, the horizontal flexibility of the superstructure, and the effect of the connections between the three major structural elements (i.e. the bridge, the bent cap and the reaction frame), provided an insight of the importance of the structural solutions and degrees of freedom incorporated in the prototype superstructure. On the other side, the effect of air trapped and how the developing pressures are released between the different chambers among girders, was included in the test program, allowing the definition of its relation with the destabilizing loads. Table 1 presents the characteristics of the structural elements studied as part of the testing program.

The different spring stiffness indicated in Table 1 correspond to a rigid connection (the bent cap is bolted to the end anchorage block), a semi-rigid connection corresponding to a realistic bridge superstructure, and a soft connection to deliberately exaggerate the horizontal motions. Further, a standard solution found in several bridges considers a rigid connection to the substructure, and is achieved by installing steel spacers. The soft bearing connection, which is an alternative solution, was made of standard elastomeric pads 64 mm in diameter and 12 mm high. Finally, the shear keys indicated in Table 1 prevents the horizontal displacement of the steel girders relative to the bent cap, and is again a solution found in some bridges.

SWL

Test Frame (W18x76)

50K LC20K LC 50K LC

10K LC

Roller Linear Guide Rail

Bridge deck

Steel bridge girderShear keyUNR LC

h (1.16m,1.06m)

Adjustable slab

0.84 m

2.554 m

Wave

X

Z

Cross frames

Bent cap

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Figure 4. Image of the straight bridge specimen installed in the LWF. Below the bridge, the bent cap is observed in red, resting

on top of the axial load cells and the reaction frame.

Table 1. Characteristics of the structural elements studied as part of the testing program.

Structural Property Parameter Tested values Horizontal flexibility of the superstructure Total spring stiffness, K [kN/m] ∞ (fixed), 458, 107

Bent cap connection to the substructure Elastomeric bearing Rigid (steel spacer), soft Bridge connection to bent cap Shear key Installed, not installed

Air and water flow between cross frames Diaphragm Installed, not installed Air and water inflow between girders Box girder Installed, not installed

Under-pressure development Deck slab venting, Porosity [%] 0, 0.85, 1.70

The diaphragms are non-structural elements to prevent air and water flowing perpendicular to the wave propagation

direction, i.e. parallel to the steel girders. Again, depending on the solution implemented in a given bridge design, diaphragms may be present, and its role on the development of destabilizing forces is still unknown. During the tests, plywood diaphragms were fixed by attaching them to each of the cross frames.

Moreover, the effect of the water inflow and air-induced pressures was studied by closing the compartments bounded by the steel girders, forming a box girder. Effectively, the box girder changes the cross-sectional profile of the bridge superstructure and prevents water diversion, turbulence and air entrapment, so it is expected to have a significant contribution on the resulting wave loads. In this case, the box girders were formed by attaching precut plywood panels and closing the compartments between the girders.

Finally, the pressure development under the deck slab can be controlled by allowing the air-water mixture to flow through the deck. This is done by drilling a series of vent holes, changing its permeability. During the tests, the deck slab permeability was changed by drilling 64 mm diameter holes in a uniformly distributed pattern. The first permeability case had 18 vent holes, while the second case was made of 36 vent holes.

4 INSTRUMENTATION

The study of tsunami impact forces on bridges requires detailed measurements of the wave hydrodynamics, pressure distribution and loads acting on the bridge, as well as the dynamic response of the specimen. Those measurements are necessary not only to understand the behavior and response of the bridge, but also to allow the calibration and validation of numerical models simulating the process, used for further understanding of other mechanisms, as well as to simulate reliably different configurations or wave conditions not tested previously.

Wave hydrodynamics were measured during the tests with 13 resistive-type wave gauges to capture the tsunami-wave propagation before and after the bridge, including an array of 4 gauges installed to estimate the incident and reflected waves during the regular wave tests. Besides, 5 acoustic probes were installed to describe the bridge overtopping process, and 2 pressure gauges were co-located at 2 of the 4 velocity profiles measured with 16 Vectrino+ ADVs. Table 2 presents the instrument layout used to capture the wave propagation dynamics along the wave flume. Co-located pressure gauges and velocity meters in Table 2 include the elevation of the probe from the local bottom. Interestingly, some ADVs included

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measurement at locations above the still water line, where the velocity intensities are typically unknown.

Table 2. Instrument layout along the wave flume for measuring hydrodynamic parameters.

Bay 3 4 6 7 8 9 9-10 11 12 13 14-15 17 Distance to

wavemaker (m) 17.73 21.49 28.80 32.44 36.10 39.66 47.09 50.74 54.38 68.92

Wave gauge (3) - Acoustic probe - - - - - - - - - - (5) - Pressure gauge (elevation from

bottom, m)

- - - 0.384 - - - - - 0.371 - -

ADV (elevation from bottom, m)

- 0.771 1.694 2.309 2.604

- 0.403 1.004 1.629 1.926

- - - - - 0.393 1.000 1.297 1.612 1.912

- 0.666 0.984 1.288

Pressure and loads acting on the bridge have been measured with 12 pressure gauges (P) installed over and under the

bridge deck as well as on the bridge girders, and 16 submersible axial load cells (LC), see Figure 5. Pressure gauges are intended to capture the instantaneous pressure time series which can be integrated to assess the horizontal and vertical forces and compare them with the load cell measurements.

Figure 5. Instrument layout to measure dynamic pressures and loads acting on the bridge specimen. P labels pressure gauges, LC

load cells, and A corresponds to the multi-axis accelerometers.

Finally, the dynamic response of the bridge was measured with 3 bi-axial accelerometers (longitudinal and vertical) fixed on the bridge deck (A in Figure 5), and 5 position transducers to measure the horizontal and vertical displacements of the bridge (not shown). The accelerometers provide information of the structure vibrations during and after the tsunami-wave impact, and the variations across the flume (along the bridge axis). The position transducers were deployed at different locations during the test program, allowing the assessment of the bridge horizontal displacements, including the asymmetries of the bridge motions, as well as vertical deformations.

P13P9

P10

P4,P3P11,P14

P6

P7P12

P8

P9P10 P3, P5

P4 P11

P14

P12P6

P13P7

P8

Wave

LC 10 LC 16 LC 15 LC 14

LC 9 LC 11 LC 12 LC 13

A1

A2

A3

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5 TESTING PROGRAM

The experiments were organized according to the different geometries and structural elements of the test specimens. Primarily, the test program was divided in two large groups, corresponding to the test specimens themselves, i.e. straight or skewed bridge. Initially, the straight bridge (ST) was installed and tested thoroughly, yielding up to 10 configurations of the structural elements (see Table 1). Each configuration was tested under a series of different wave conditions (solitary and regular waves) and water depths. Overall, the straight bridge was subject to 217 tests.

Later, the straight bridge model was removed and the skewed bridge (SK) was installed. Due to the characteristics of the skewed bridge, and availability of time, the skewed bridge was tested under only 4 different configurations, and also subject to solitary and regular waves at different water depths. Overall, 71 tests were conducted with the skewed bridge.

Table 3 presents the characteristics of all configurations and hydrodynamic conditions executed during the test program.

Table 3. Structure element characteristics and hydrodynamic conditions of the full Test Program (see Table 1 for details on the structure element characteristics).

Brid

ge

Orie

ntat

ion

Test

nam

e

Sprin

g st

iffne

ss

Elas

tom

eric

be

arin

g

Shea

r key

Dia

phra

gm

Box

gird

er

Ven

ting

Solit

ary

wav

e he

ight

(1 )

Reg

ular

wav

e he

ight

(2 )

Reg

ular

wav

e pe

riod

Wat

er d

epth

at

the

wav

e m

aker

#Tes

ts

[kN/m] [%] [m] [m] [s] [m]

Stra

ight

ST1 ∞ Rigid Yes No No 0.0 0.36-1.40 0.42-0.80 2-3 1.9-2.2 42 ST2 ∞ Soft Yes No No 0.0 0.36-1.40 0.40-0.60 2-4 1.9-2.2 32 ST3 458 Soft Yes No No 0.0 0.36-1.40 0.40-0.60 2-4 1.9-2.2 23 ST4 107 Soft Yes No No 0.0 0.36-1.40 0.40-0.60 2-4 2.0-2.2 22 ST5 ∞ Soft Yes Yes No 0.0 0.36-1.40 0.40-0.60 2-4 1.9-2.2 23 ST6 ∞ Soft Yes Yes Yes 0.0 0.36-1.40 0.40-0.60 2-4 2.0-2.2 19 ST7 ∞ Soft Yes Yes No 0.85 0.36-1.40 0.40-0.60 2-4 2.0-2.2 17 ST9 ∞ Soft Yes Yes No 1.70 0.36-1.40 0.40-0.60 2-4 2.0-2.2 17 ST10 ∞ Soft Yes No No 1.70 0.55-1.40 N/A N/A 2.0 7

ST11 (3) ∞ Soft No No No 0.0 0.36-1.40 0.40-0.60 2-4 2.0-2.2 15

Skew

ed SK1 ∞ Rigid Yes No No 0.0 0.36-1.40 0.40-0.60 2-4 2.0-2.2 24

SK2 ∞ Soft Yes No No 0.0 0.36-1.40 0.40-0.60 2-4 2.0-2.2 17 SK3 ∞ Soft No No No 0.0 0.36-1.40 0.40-0.60 2-4 2.0-2.2 20 SK4 107 Soft No No No 0.0 0.36-1.40 N/A N/A 2.0 10

(1) Solitary wave heights included 0.36, 0.42, 0.55, 0.70, 0.90, 1.00, 1.20, and 1.40 m for 2.0 m water depth at the wave maker for all configurations, and 0.46, 0.52, 0.65, 0.80, 1.00, 1.10 and 1.30 m for 1.9 m water depth at the wave maker for configurations ST1, ST2, ST3 and ST5.

(2) Regular wave heights included 0.40 and 0.60 m, and wave periods 2.0 and 4.0 s for 2.2 m water depth at the wave maker for all configurations except ST10 and SK4. Only configuration ST1 included cases with 0.42 and 0.80 m wave height, and 2.0 and 3.0 s wave periods for 2.0 m water depth at the wave maker.

(3) In the test sequence, ST11 was executed after ST6 for convenience in the perforation of the deck slab venting holes.

6 DATA ACQUISITION AND PROCESSING

For the Tsunami bridge impact forces, data were collected on two separated DAQ modules, one (master) sampling at a “standard” rate of 50 Hz, gathering data from the instruments capturing the hydrodynamic conditions, while the other DAQ module (slave) sampled at a rate of 10,000 Hz, intended to measure the impulsive structural response of the bridge, i.e. impact forces, pressures, accelerations and displacements. Digital communication with Vectrino systems is done over RS-232 serial ports. The HWRL DAQ modules are synchronized by the master DAQ which also generates the Vectrino triggering pulses. Hence, the time stamp for all instruments are the same within the DAQ sample rate.

Preliminary data analysis and post-processing was performed on all recorded channels, which included the updating of the metadata to incorporate the instrument locations, the application of calibration coefficients and offset (if applicable), despiking and data cleanup, where the data is now expressed in physical units, referenced to the same coordinate system, all times series have the same duration, and the time origin corresponds to the wave machine start pulse.

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7 EXPERIMENTAL DATA AND OBSERVATIONS

The amount of information gathered during the test program created a valuable database which allows detailed analysis on the hydrodynamics of a tsunami-type wave, and the structural response of bridge specimens subject to wave-induced impact forces. Moreover, the different structural configurations provide information on the sensitivity of the major elements tested. Further post-processing and analysis is still undergoing, and will be subject of several future publications. The thorough analysis and interpretation of the results is not part of the present paper, which is limited to the description of the tests performed.

Further post-processing and analysis may include studies of wave propagation, breaking evolution and bore formation characteristics; description of velocity profiles below and above the still water line and its relation to the surface elevation phase; correspondence between the dynamic pressure and the surface elevation for non-linear waves; importance of the impact spike observed in pressure measurements at the structure specimen and its relation to the integrated forces transmitted to the supporting structure; as well as the sensitivity to the different structural configurations, e.g. the horizontal flexibility of the superstructure, the stiffness of the bent cap connection, the existence of shear keys, and the presence of diaphragms, panels or vents in the air-water flow underneath the structure and its corresponding pressures and loads transmitted to the substructure.

For demonstration purposes, in Figure 6 the time series of selected hydrodynamic and structural parameters are presented. Figure 6 portrays the surface elevation evolution for all wave gauges, including the acoustic probes. Figure 6.b presents a detail of the surface elevation and dynamic pressure at bay 13, while Figure 6.c presents the time series of two pressure gauges on the bridge (P3 and P8) as well as one horizontal load cell (LC10) and one vertical load cell (LC14). The surface elevation time series along the flume provide relevant information like the Tsunami amplitude, celerity, onset of breaking, bore formation, elapsed time bridge impact, and transmitted bore height.

Figure 6. Example times series of hydrodynamic and structural parameters for the bridge configuration ST1 (Straight bridge, solitary wave height of 1.4 m, water depth of 2 m at the wave maker). a) Surface elevation along the wave flume, b) Dynamic

pressure and surface elevation at bay 13, and c) Impulsive pressures and forces on the bridge.

As seen in Figure 6.c, the horizontal force is only positive (in the direction of the wave propagation), but the vertical force presents an upward spike simultaneous with the pressure and forces exerted, then a downward force and, finally, a long lasting upward force replicating the pseudo-hydrostatic pressure time series. In general, it is observed that a rigid structure (no springs, steel spacers, use of shear keys) induce a short spike several times larger than the subsequent pseudo-static time series. Moreover, changing the stiffness of the structural elements modify the behavior of the impact force time series, but not necessarily reducing the measured loads or displacements.

Figure 7 presents a sequence of images of a tsunami-type wave impacting on the straight bridge superstructure, where the bore formation and its impact on the bridge is observed. The images also reveals the large amount of overtopping in the

0.000.200.400.600.801.001.201.401.60

32 33 34 35 36 37 38 39 40 41 42 43 44

Surf

ace

elev

atio

n (m

)

Time (s)

-2.0-1.00.01.02.03.04.05.06.07.08.0

-0.2-0.10.00.10.20.30.40.50.60.70.8

37 38 39 40 41 42 43 44

Pres

sure

(kPa

)

Surf

ace

elev

atio

n (m

)

Time (s)

-15.0

-7.5

0.0

7.5

15.0

0.0

10.0

20.0

30.0

40.0

39 39.5 40 40.5 41 41.5 42

Forc

e (k

N)

Pres

sure

(kPa

)

Time (s)

P3 P8

LC10

LC14

Surface elevation

Pressure

bay 4 bay 6 bay 7 bay 8 bay 9

bay 9-10 bay 11 bay 12

bay 13 bay 17

bay 14-15 1st and last acoustic

10

form of green and white water, the later formed by the complex mixture of water and air.

Figure 7. Sequence of images of a tsunami-type wave impacting on the straight bridge superstructure.

It should be noted that several configurations and wave conditions have been repeated to ensure reliability of the database. The preliminary data analysis and observed results confirms the quality and accuracy of the experiments, ensuring its value for the development of bridge design recommendations and validation of numerical models.

8 SUMMARY AND CONCLUSIONS

Overall, 288 tests have been carried out successfully of two large-scale bridges subject to impact forces of tsunami-type broken and unbroken waves, as well as regular waves. The model specimens represent a bridge superstructure, composed of a reinforced concrete deck, supported by multiple steel girders and steel cross frames. The measured hydrodynamic parameters during the testing included the surface elevation, dynamic pressure, and 3D flow velocity profiles along the flume, while the structural parameters included dynamic pressure around the bridge specimens, horizontal and vertical forces at the bridge connection with the substructure, horizontal and vertical accelerations of the bridge deck at different locations along the bridge span, and horizontal and vertical displacements of the specimens.

The bridges were subject to different tsunami-wave amplitudes, as well as different regular wave heights and periods, at varying water depths. The bridge configurations also included variations on the superstructure vertical connection stiffness, substructure flexibility, superstructure horizontal blocking, lateral and horizontal flow blocking among the bridge girders, and venting of the bridge deck.

As indicated previously, the database not only will provide a deep insight to understand the complex wave-structure interaction, but also will be the base on the development of design codes for design and construction of tsunami-resistant bridges. Furthermore, the database represent a valuable source for development, calibration, and validation of hydrodynamic and fluid-structure interaction numerical and analytical models.

ACKNOWLEDGEMENTS

The work presented in this paper was funded by the Federal Highway Administration under Contract No. DTFH61-07-C-00031. Acknowledgement is made of the oversight given by Contract Officer’s Representatives: Dr. Wen-huei (Phillip) Yen, Mr. Fred Faridazar, and Ms Sheila Duwadi.

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