BUILDING RESEARCH INSTITUTE

The Tornado Damage
Mitigation Projectby providing the public with warning information on tornadoes (since 2008) andup-to-the-minute tornado “nowcasts”(since 2010). The Cabinet Office Central
Disaster Management Council meanwhile has included tornadoes as an established weather phenomenon in its
Basic Disaster Management Plan, calling for measures to alleviate physicaldamage and improvements in technicalexpertise to help companies implement
damage mitigation measures.

Japan’s Building Research Institute (BRI) conducts activities ranging fromresearch and development on housing, building and urban planningtechnology, to international training on seismology and earthquake
engineering. Against the background of significant tornado damage inJapan earlier this year, this article introduces some of the BRI’s initiatives
in tornado research and the launch of a frontline tornado research
project to help reduce damage to buildings.

How Tornadoes Damage Buildings

the Fujita Scale (estimated wind speed:70–92m/s) near Tsukuba in Ibaraki
Prefecture, F1–F2 (33–69m/s) in Mokain Tochigi Prefecture and Hitachiomiya
in Ibaraki prefecture, and F1 (33–49m/s)
near Chikusei in Ibaraki Prefecture.
These transient winds and the accompanying weather conditions left one person
dead and fifty-eight injured. In addition
to the human toll, 89 homes were also
destroyed, with a further 197 partiallydestroyed and 978 damaged (as of June13). The trail of destruction had a serious social impact too.
This is not an isolated case. As
Table 1 shows, serious damage caused
by tornadoes has become increasingly
common in recent years. The Japan
Meteorological Agency has responded

In order to get a picture of the damage
caused to buildings by the tornado that
hit Tsukuba, Ibaraki Prefecture, on May
6, 2012, the Building Research Institutejoined forces with the National Institutefor Land and Infrastructure Managementand began conducting a field survey fromthat same day. In addition to typical damage recorded in the aftermath of othertornadoes (scattering of exterior materials, collapse of wooden roof trusses, impact from flying debris, etc., see Photos
2 and 3), the survey found new types ofdamage that had not been observed previously. Photo 4 shows a two-story wooden house whose foundations became detached from the ground, causing theentire structure to overturn. Photo 5
meanwhile shows a five-story reinforced

Photo 2: Destruction caused to a wooden
house

Photo 3: Impact from flying debris(roof)

On May 6, 2012, a mass ofcold air entered the skies
above Japan. At the sametime, warm, moist air was
moving towards a low-pressure area over
the Sea of Japan. Compounded by elevated ground temperatures due to solar radiation, this caused major atmospheric instability from the central Tokai area all
the way up to Tohoku in the north, resulting in cumulonimbus clouds accompanied by lightning, transient winds andhailstorms. These weather conditions led
to multiple tornado outbreaks in Ibaraki,Tochigi and Fukushima Prefectures,causing casualties and substantial damage
to countless homes and other buildings(see Photo 1).
Tornadoes were confirmed at F3 on

Photo 1: A tornado in Tsukuba, Ibaraki
Prefecture, on May 6, 201230

The Japan Journal DECEMBER 2012

Photo 4: A wooden house overturned
with its foundations still attached

Photo 5: An apartment building with extensive damage to non-structural elements

Table 1: Summary of major damage caused by tornadoes in Japan since 2002
Human toll(no. of people)

Scale
Date
2002.07.10
2004.06.27
2005.12.25
2006.09.17
2006.11.07
2009.07.19
2009.07.27
2009.10.08
2011.11.18

2012.05.06

Location

Fujita Width of affected Length of affectedarea (m)area (km)scale

Fukaya City, Saitama Pref.
F2
Saga City, Saga Pref.
F2
Sakata City, Yamagata Pref.
F1
Nobeoka City, Miyazaki Pref.
F2
Saroma Town, Hokkaido Pref.
F3
Mimasaka City, Okayama Pref.
F2
Tatebayashi City, Gunma Pref. F1 〜 F2
Tsuchiura City, Ibaraki Pref.
F1Tone Town, Ibaraki Pref.
F1
Kujukuri Town, Chiba Pref.
F1
Tokunoshima Town,
F2
Kagoshima Pref.
Tsukuba City and other area,
F3
Ibaraki Pref.
Chikusei City and other area,
F1
Ibaraki Pref.
Moka City and other area,
F1 〜 F2Tochigi Pref.
Aizumisato Town, Fukushima
Pref.

F0

Physical damage(no. of homes)

Dead

InjuredCompletelydestroyed

Partially destroyed/
damaged

715
079
7214
10
187
330
41,101
3276
310
105
121
36

100 〜 150
200 〜 400
100150 〜 300
100 〜 300
200
50
200 〜 300100 〜 20020 〜 30

4.5
8.0
9.0
7.5
1.4
6.0
6.5
2.86.0
1.6 〜 1.7

damaged0539000001115
3314331221240100

0.6

30500

17137600

2103650

32012300

200

Source: The Japan Meteorological Agency

concrete apartment building, which sustained extensive damage to non-structuralelements such as windows, window
frames and aluminum balcony handrails.
The buildings in these two photographs
are thought to have been in or around thepath of the tornado. Nonetheless, the reasons for such extensive damage and themagnitude of pressure caused by transient
winds remain unclear. From the standpoint of wind engineering, throwing lighton phenomena such as these is proving
extremely challenging.
To analyze these atypical examplesof damage in greater detail, it is necessary to run tests that recreate the gustload of a tornado and the effects it has on
a building, so as to identify damagecausing mechanisms under the relevantload. Figure 1 is a graphical illustrationof what happens when a tornado passes
close to a building. When the tornado
passes, its horizontal swirling flow andvertical upward flow create lots of flying
debris. This means that the effects on thebuilding can be broadly divided into (1)wind pressures caused by the direct action of the air flow, (2) pressures associated with the variation of the atmosphericpressure field and (3) impact forces
caused by tornado-borne flying debris.
The characteristics of (1) and (2) differ
considerably from the wind force generated by strong winds during a tropical
cyclone. As a result, it is impossible torealistically recreate the same conditionsin tests using a conventional turbulent
boundary layer wind tunnel. In order to
identify the effects and damage-causing
mechanisms of a tornado, we have toleave behind existing testing and assessment methods and adopt a new approach.
Outline of a Tornado Simulator andExperimental Researchsimilar to an
actual tornado,
whilst moving
sideways at thesame time.
Photo 6shows the exterior of thesimulator, with
Figure 3 providing crosssection views.
The simulator
12
consists of a158 partiallymain body76
destroyedequipped with0
1 partially destroyed
a fan, a self35 partiallypropelled cradle
13
destroyedthat moves
00 partially destroyed
from side toside, a stage
that moves up
and down, and a control panel. Standing
at a full height of around 2.3 meters, thebody has an external diameter of 1.5
meters. The cradle meanwhile has a rangeof 1.4 meters from the center, and a maximum translation speed of 0.4 meters per
second. When the fan inside the bodyspins round, it creates an upward airflow,which then blows downwards from theoutside. There are eighteen guide vanes
fitted to the upper part of the body, which
are then angled (0–55 degrees) to force thedownward flow into a swirling motion.
It is possible to vary the maximum
speed and diameter of the swirling flow byadjusting the speed of the fan, the angle of
the guide vanes and the height of the stage.
The airflow conditions created by the simulator have been verified against a tornado

In 2010, the Building Research Institutedesigned and built a tornado simulator toexperimentally recreate the airflow of a
tornado, in order to ascertain the windforce characteristics of transient winds andthe risk of impact from flying debris (seeFigure 2). With additional funding courtesy of the Grants-in-Aid for Scientific
Research program, experiments werethen carried out inconjunction with re- Figure 1: The effects of forces caused by a tornado
searchers from theFlying debris
National Institute forLand and InfrastructureManagement, theUniversity of Tokyo andthe Disaster Prevention
Movement
Vertical upward flow
Research Institute atof tornado
Kyoto University. The
Horizontal
swirling flow
airflow mechanism usedin the simulator is based
Upward force
on a machine developedImpact fromat Iowa State University
flying debris
in the United States andWeight andis able to recreate an unload-bearing
Lateral force
force of building
steady vortex structure
The Japan Journal DECEMBER 201231

BUILDING RESEARCH INSTITUTE

Photo 6: The tornado simulator at theBuilding Research Instituteengineering model (the Rankine vortexmodel) based on tangential wind speed
and distribution of low air pressure.
Wind Pressure Tests Using theTornado Simulator

Using the tornado simulator, researchers
conducted wind pressure tests based on
the premise of a tornado passing directlyover a building. The building was basedon a 12-meter high low-rise structure
measuring 24 meters across by 38 metersin length. The walls of the model building
were made complete with realistic, uniform distributed leakage and a rectangulardominant opening on the right-hand wall,
if viewed from the direction of the tornado(see Photo 7). The dominant opening was
designed to simulate a large hole causedby the impact of tornado-borne flying
debris. With wind speeds reduced in scale
to 1/10 and distances to 1/350, tests recreated a tornado with a maximum tangential wind speed of 98 meters per secondand a core radius of 42 meters.
The following results for wind force
characteristics are expressed as “windforce coefficients,” calculated by dividing measured wind force data by reference velocity pressure.
Figure 4 shows wind force characteristics acting vertically on the roof of thebuilding. The horizontal axis shows themoving position of the simulator in relation to the center of the model (xs) divided
by the core radius (Rm). The findings indicate that, irrespective of dominant opening, wind force is at its maximum when
xs/Rm is -1 or 1. This means that the center of the building would be subjected tothe maximum tangential wind speed when
the tornado is approaching the building
and immediately after the tornado has
passed. In terms of dominant opening, thewind force is approximately twice asstrong when a dominant opening is present. This suggests that preventing large
openings from forming in the wall as thetornado approaches would effectively reduce the force of the wind on the roof.
Figure 5 shows results for horizontal
wind force characteristics, divided into
Direction X, facing the direction of thetornado head-on, and Direction Y, atright angles to the tornado. As with thevertical results, the wind force is at its
strongest when xs/Rm is -1 or 1. In termsof the direction of the tornado meanwhile, the results interestingly show agreater force diagonally backward towards the left as the tornado approaches,and diagonally forward towards the right
immediately after the tornado has passedby. Figure 6 compares these findings
against actual damage caused by the tornado in Tsukuba. Figure 6 (a) is an aerial
photo of a wooden house that was over-Figure 2: Assessment of risks from transient wind due toa passing tornado

Photo 7: The modelbuilding in placeturned with its foundations still attached,with most of the upper structure of thehouse scattered diagonally forward towards the right, based on the direction ofthe tornado. The direction of scattering
more or less matches the direction foundin the tests (Figure 6 (b)). With that inmind, one possible explanation for thisdamage is that the wooden house wasoverturned and scattered immediately
after the tornado passed close by.
As these results were obtained underrestricted experimental conditions, there
are plans to conduct more systematic
wind pressure tests in the future, taking
into account other parameters that need
to be examined. It is hoped that theplanned series of experiments will help
assess the transient wind load of approaching tornadoes under different circumstances. The results should also provide evidence to support the damagecausing mechanisms of tornadoes.

Figure 3: Cross sections of the tornado
Diameter: 1.5m
simulatorTornado airflow simulator

55 degrees(maximum)

Vane angle

Recreates unsteady vortex
structure similar to an
actual tornado

Movement

0 degrees

Guide vanes
Fan

Flying debris

Transient wind

Building
Body

Risk of impact fromflying debris
32

The Japan Journal DECEMBER 2012

Wind force characteristicsof transient wind

Convergence layer height

Movableup and down

Upwardflow

Downward
flow

Stage

Figure 4: Vertical force from a passing tornado
With dominant
opening

Wind force coefficient

Withoutdominant opening

Direction of movement−Figure 5: Horizontal force from a passing tornado
Direction YWind force coefficient

Direction X

Direction of movement−for reasons such as these that load andexternal force due to tornadoes are notcovered under the Building Standard
Generally speaking, the likelihood of in- Law of Japan. Given that serious tornadividual buildings being caught in the do damage has become increasingly
path of transient winds from a tornado is common in recent years however, we areextremely low. Incorporating measures going to have to improve the design ofinto regular structural design would critical facilities that would be crucial totherefore be economically unfeasible. saving lives, protecting property andTornadoes are also isolated events that maintaining social functions in theare rarely picked up by normal meteoro- wake of a disaster (emergency facilities,logical observation networks. As it large-scale commercial facilities, multistands, we have insufficient engineering functional facilities, hospitals, schools,expertise for tornadoes to be reflected in etc.), particularly parts and components
design and verification procedures. It is that would be vulnerable to a tornado.
In light of reFigure 6: The tornado simulator at the Building
cent damage caused
Research Instituteby tornadoes, theEstimated path of the tornado
Building ResearchInstitute has started
work on a tornado
research project tohelp reduce damage to buildings.
The project will
use unique research
Overturned
Direction of scattering
mat foundations
tools such as thetornado simulator(a) Actual damage(b) Test results
Source: Geospatial Information Authority of Japanand numerical simTornado Damage Mitigation Projectat the Building Research Instituteulation techniques to explain the mechanisms through which tornadoes causedamage. The Institute is also planningto set out a new approach to tornadoresistant design for the aforementioned
critical facilities, based on the following
underlying policy.
First and foremost is the belief that,as an extension of conventional windresistant design, we should make buildings safer and improve their functionality
by using building materials with superiorwind and impact resistant performances
for parts and components that are mostvulnerable to tornadoes (particularlyroofs and openings). Disaster-resistantroof tiles are a prime example that has
already been developed. Tests haveshown that such tiles are roughly threetimes more resistant to tensile force. In
terms of openings meanwhile, it has beenreported that laminated glass offers greater impact resistance than regular glass.
As test results from the tornado simulator
show, taking steps to make openings
more resistant to impact from flying debris, to prevent larger openings fromforming wherever possible, is crucial interms of disaster prevention during a tornado, because it helps to reduce the forceof the wind against roofs.
In the United States, researchers
have come up with structural calculation
methods for critical facilities and evacuation shelters at risk from tornadoes.
These could also provide a valuable
source of reference. There is already aframework related to design criteria inplace for different scales of tornado,plotted against the Enhanced Fujita
Scale. As we put that framework into
practice in the future, the key point will
be how to reflect findings from damage
surveys in Japan and experiments using
the tornado simulator on the framework.
Design guidelines for critical facilities in Japan still need to be improved toprotect against tornadoes. That’s whatmakes the Building Research Institute’s
new project such a forward-thinking
frontline research project. If we can actively share research results with thestructural engineers who design ourhomes and critical facilities, and with thepublic, in the near future, it will undoubtedly help to raise awareness of tornado
damage and promote measures to protect
against transient winds and tornadoes.
The Japan Journal DECEMBER 201233