Survey of Bridges and Their Damage Mitigation Provision in Recent Tsunamis Paper Number: 12-3007, Author: Samrakshak Lamichhane and Claudia Marin-Artieda, Department of Civil Engineering, Howard University

Introduction

A tsunami consists of a series of ocean waves generated mostly by an

earthquake which causes a sudden sea floor displacement. When a tsunami

hits, bridges located on the affected area are subjected to harmful effects

such as flooding and wave impact; while torrents from tsunami may cause

erosion in the bridge’s foundation. Drag forces and floating objects may

further sweep away untied superstructures and induce large impact forces. In

recent tsunamis, several bridges that were designed to mitigate some level

of damage caused by both earthquakes and tsunamis were severely

damaged, collapsed, or were simply swept away. This project delineate

lessons learned from the performance of bridges and their earthquake-

tsunami mitigation mechanisms in recent tsunamis.

Objectives

To outline lessons learned in recent tsunamis about: (a) failure

mechanisms of bridges and their mitigation measures, and qualitative

identification of the induced forces; and (b) bridge vulnerabilities to tsunamis

and performance limitations of earthquake-tsunami protective systems.

To draw recommendations to improve the resilience of bridges and the

current practices for dual earthquake and tsunami protection of bridges in

tsunami prone-zones. Dual-Hazard: Earthquake and Tsunami

The 2011 Japanese tsunami vastly demonstrated the sensitivity of bridges to

earthquake and tsunami hazard since several bridges that were designed to

mitigate some level of damage induced by both earthquake and tsunami effects

were severely damaged, collapsed, or were swept away. For example: (a) the

Kesen Bridge, although equipped with seismic elastomeric bearings and

dampers, lost the entire superstructure; (b) the Utazu Bridge, although

equipped with seismic cable restrainers, steel stoppers, and reinforced side

blocks, sustained transverse deck movement and uplift; (c) the Numata Bridge,

although equipped with longitudinal stoppers meant to prevent transverse

movement of the deck, had its deck uplifted away; (d) the Koizumi Bridge,

although equipped with sliding bearings, dampers and cable restrainers, lost

two three-bay-continuous spans; and (e) the Shin-Kitakami Bridge, although

equipped with roller bearings, had two of its spans washed away.

Tsunami Forces against Bridges

Hydrostatic forces are generated by submerged bridge

components under hydrostatic pressure. The hydrostatic

forces on bridge components are a function of water

elevation and imbalanced pressure. These forces usually

affect the wing walls, abutments and retaining walls and the

substructure of the bridge.

Buoyant forces are the vertical forces acting through the

center of mass of the submerged bridge components.

Buoyant forces combined with hydrodynamic lifting forces

and trapped air on bridge components can result in important

uplift effects on the substructure of the bridge.

Hydrodynamic forces consist of surge and drag forces.

Surge forces are generated by the impingement of the growing water in front of a tsunami on a structure. Hydrodynamic forces are caused by drag when the

tsunami moves inland with moderate or high velocity and

flows around the bridge. The resultant force acts on the

projected area and in the flow direction and increases proportionally with

the exposed surface.

Debris impact forces are due to floating debris that is carried

by the tsunami’s high-speed waves and that hit the bridge

components. These forces depend on the wave height, the

amount and quality of debris and bridge height.

Tsunami Vulnerability of Bridges Protective Measures to Mitigate Tsunami Effects

Foundation

Bearings

Pressure in

the

bearings

Development of

cracks

Accumulation of

debris

Washout of sediments

near piers

Liquefaction

Kesen Bridge-Route 45- Rikuzen-Takada City. Bridge

equipped with seismic elastomeric bearings and

dampers, lost the entire superstructure (courtesy of

Keigo Suzuki)

Utazu Bridge equipped with restrainer, suffered

broken brackets (courtesy of Mark Yashinsky,

Caltrans Office of Earthquake Engineering)

The redundancy of a bridge can be defined as the overall structural capacity of

the system to take internal and external loading (vertical and lateral) and to proper

distribute them after the failure of some of its components.

Continuity of superstructures provides the redundancy benefits for vertical and

horizontal loading conditions. This measure is also beneficial at mitigating

damaging seismic effects on bridges.

Monolithic connections provide resilience to tsunami effects in a similar way as

superstructure continuity.

Tie downs

Raising superstructure ’s elevation

Uplift and longitudinal restrainers

Deep foundations

Other measures in the scope of bridge engineering are:

Open vents on the superstructure (girders and parapets) to alleviate buoyancy

effects by reducing the vertical projected area of bridge deck to prevent uplift.

Defining aerodynamic geometries to mitigate damaging effects of structures

under flow and U-type and longer wing walls to reduce scouring.

Soil stabilization to prevent erosion.

Seismic isolation bearing with proper uplift or tensile restraints.

Locating bridges away from the shore. This involves planning of use of land to

avoid new developments in tsunami run-up areas.

Uplift Restrainers for Isolated Structures

Uplift- or tensile-resistance mechanisms of isolation systems can be the key at

preventing damaging effects due buoyant and hydrodynamic forces acting on

bridges equipped with isolators. However, these mechanisms may increase the

tsunami induced forces in the substructures as they provide the reaction force to

counteract the uplift and/or tensile effects.

Detailed analysis are required to ensure that the failure mode does not change

from the deck uplift to piers, abutments and/or foundations damage, since such as

failure mechanism implies costly and cumbersome repairing.

The XY-FP Bearing Uplift Restrainer for Elastomeric Bearings

Counterweight to prevent uplift

Recommendations and Conclusions

It is critical to prevent bridge failures under extreme events,

preventive strategies that should be secured by engineering multi-

hazard resilient bridges. In this project, the need for tsunami

resilience of bridges is emphasized.

Recent tsunamis have shown that bridges are particularly sensitive

to both earthquake and tsunamis effects. Factors influencing this

sensitivity includes the lack of dual-hazard resilience of bridges, of

knowledge for adequate dual-hazard assessment, and that of specific

provision that combine multi-hazard measures on current codes,

guidelines and regulations.

There is limited information and specific guidelines for local coastal

areas to estimate tsunami load parameters, and basic research to

understand the relationship among multiple variables controlling

tsunami effects on bridges is scare. Testing, experimentation, and

rigorous analysis are at initial stages. This void in knowledge in turn

may lead to high vulnerability of bridges.

The recent earthquake and tsunami in Japan in March 11, 2011,

illustrated the need for reinforcing dual-hazard (tsunami/earthquake)

resilience of bridges. A thorough evaluation and development of

protective solutions for bridges will ensure the continued and safe

operation of critical bridges during and following an extreme event

and to support recovery operations.

Future Directions

Quantitative analysis and estimation of tensile and uplift effects of

tsunamis on bridge components.

Numerical analysis to estimate the effects uplift- or tensile-

resistance mechanisms preventing damaging effects due buoyant

and hydrodynamic forces acting on bridges equipped with isolators.

Acknowledgment

This research was partially supported by The National Science

Foundation (Award number CMMI-0927178), Dwight D. Eisenhower

HBCU Transportation Fellowship and by the TRB Minority Student

Fellows Program.

Restrainer for FP Uplift Bearings FS Bearing and Uplift Restraint