Research Trends and Results

Analysis of damage to road bridges by the tsunami
caused by the Great East Japan Earthquake
TAMAKOSHI Takashi, HeadSHIRATO Masahiro, Senior Researcher (Dr. Eng.)YOKOI Yoshiteru, Researcher
Bridge and Structures Division, Road Department(Key words) Road bridge, tsunami, design
Earthquake.
Among the bridges struck by tsunami, 85 bridges
that are typical bridge types and whose specifications
have been clarified were selected for analysis. The
acting force was obtained using the tsunami waterlevel and flow velocity t the locations where eachbridge was constructed and which were calculated byhydraulic simulations, to calculate the water pressureand hydrodynamic force acting on the bridge girders
based on hydraulic formulae. And resistance force wasassumed to be the self-weight and the shear strength of
the bearing anchor bolts. The figure plots the tsunami
acting force/resistance force (equivalent to thereciprocal of the safety factor) with the lateral axis
representing the horizontal acting force and thevertical axis representing the vertical acting force. The
plot distinguished cases where the Great East Japan
Earthquake washed away the superstructures of actual
bridges and cases where this did not occur. The
predicted damage to the bridges that were washedaway conforms generally to the actual damage, so it ispossible to judge tsunami damage using the tsunami
acting force calculated using the hydraulic formulae.
Among the concrete T-girder bridges and concreteslab bridges, even if the tsunami acting
force/resistance force was less than 1.0, the actual
damage included some that were washed away,
although only a few. In the future, various
specifications such as the state of damage caused by
the Great East Japan Earthquake or the girder shape

1. Introduction
The Great East Japan Earthquake damaged manyroad bridges, as the tsunami washed away theirsuperstructures or washed away the soil behind thebridge abutments. Earthquakes accompanied by hugetsunami could occur again in the future, so to ensure
the functions of road networks in the event of anearthquake disaster, establishing technology capableof appropriately evaluating the impact of tsunami
when constructing or managing road bridges is an
important challenge.
2. Study of design methods considering tsunami
For present day seismic design of road bridges,
methods of verifying the impact of vibrationconsidering restorability have been established, but
specific required performance such as the allowed
form of damage by giant tsunamis and methods ofverifying this performance have not been established.
So methods of considering tsunami acting force, formsof damage considering restorability, and verification
standards that can control the progress of damage arebeing studied.

3 orhigher3

3 orhigher3

3 orhigher 3●： Superstructure washed

Vertical acting force/verticalresistance force

Vertical acting force/verticalresistance force

Vertical acting force/verticalresistance force

3. Study of tsunami acting force
It is necessary to correctly evaluate the acting force
that a tsunami will inflict on a road bridge to perform
design considering tsunami. Therefore, the NILIM hasstudied a method of evaluating tsunami acting force
according to actual damage by the Great East Japan○： Superstructure not22110000112233or
higherHorizontal acting force/horizontal
resistance force(a) Steel plate girder bridge

2211000011223 3orhigherHorizontal acting force/horizontal
resistance force(b) Concrete T-girder bridge

away
washed away

22
Damage prediction and actual
damage conform・ Case of ○: both vertical andhorizontal axes are less than 1.0
・ Case of ●: either the vertical axis
or horizontal axis is 1.0 or higher

11000
0112233or
higherHorizontal acting force/horizontal
resistance force(c) Concrete slab bridge

Figure. Evaluation of tsunami damage to a bridge

Research Trends and Results
will be analyzed to abstract major factors determining
the occurrence/non-occurrence of tsunami damage andto reflect the findings in the calculation of actingforce.
4. Conclusions
The NILM will continue to conduct research tosystematize required performance to resist tsunami
according to the disaster prevention position of eachroad bridge or the emergency restoration period after
the damage, and to reflect the findings in road bridge
design standards.
[Sources]
Web page of the Bridge and Structures Division
http://www.nilim.go.jp/lab/gcg/index.htm