Survey of Bridges and Their Damage Mitigation Provision in Recent ...
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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