INTERNATIONAL COOPERATION

Unraveling theMystery of Wind
Damage to Buildings
In our last issue, we looked at the research activities of Japan’s Building Research Institute concerning
earthquakes and buildings. In this issue, we focus on the Institute’s research into the impact onbuildings of wind.

W

ind that damages buildings in Japan can belinked to typhoons andtornadoes. A typhoon isa tropical cyclone generated in thePacific Ocean or the South China Sea
with wind speeds of 17.2 m/s or greater.
Typhoons are huge vortexes that may
exceed 1,000 km in diameter. Everyyear, twenty-seven typhoons are generated on average, eleven of which typically approach Japan, with an averageof three making landfall. In 2004, tentyphoons reached the Japanese coast,of meters to hundreds of meters in diameter (photo 1). The wind at the centerof a tornado is extremely violent, andhouses directly in its path are destroyed
immediately. Approximately 1,000 tornadoes occur in the United States everyyear. Japan itself is not entirely free oftornadoes, with around twenty-seven ina typical year. Per unit of area, the incidence of tornadoes in Japan is actuallyquite significant, about half that of theUnited States. Areas where tornadoes
occur are mostly coastal areas andopen and flat land, which can be foundthroughout the country (figure 1).
As a consequence, many parts ofJapan are subject to strong winds eachyear. It seems, however, that the damage
that wind can cause buildings still remains something of a mystery.

causing record-breaking rainstorms in a
number of areas. According to data fromthe Cabinet Office, these typhoons
caused great damage, killing 214 people,
injuring 2,542, fully destroying or causing more than 50% loss to about 1,200dwellings, half-destroying or causing 20–50% loss to another 9,300, and partially destroying or causing less than 20%
loss to 67,000. Tornadoes, meanwhile,
are violent, rotating columns of air involving an ascending air current, which
may occur under a cumulonimbus cloud,for instance. Tornadoes range from tens

TOYOHASHI CENTRAL FIRE STATION

Damage Characteristics

Photo 1. A tornado on the horizon in Toyohashi, Aichi Prefecture,
September 24, 19996

Earthquake forces are first transmitted
to the foundations of a building, then toits structural framework, such as columns and beams, and finally reach theceiling, internal walls, exterior walls androofing. On the other hand, wind forces
first hit the cladding, such as roofing,
exterior walls and windows, before theirimpact reaches the structural framework.
Thus, the load paths of earthquake andwind within a building are the opposite
to each other. And unlike earthquakes,
strong winds such as typhoons buffetbuildings continuously over an extended
period, and place extreme pressure onridges and the corners of roofs.
Earthquake forces are transmitted

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HOKKAIDO REGIONAL DEVELOPMENT BUREAU, MINISTRY OF LAND,INFRASTRUCTURE, TRANSPORT AND TOURISM

JAPAN METEOROLOGICAL AGENCY

Figure 1. Areas where occurrences of gusts inShanshan made acluding tornadoes were confirmed in 1961–2009
landfall on Nagasaki
Prefecture andcaused damage tobuildings and otherstructures, alongwith high windflooding, mainly on
Kyushu. Major examples of damage
from the typhoon
include the brokenroof of the Iizuka
Cosmoscommon,the culture hall of
Iizuka City, Fukuoka
Prefecture (photo 3).
This building had
a hemispherical roof
made of waterproof
stainless steel sheets.
More than half ofthe sheets werepeeled off by thestrong wind and fellto the surrounding ground and the was not seen as caused by a wind thatroofs of adjacent buildings. Damage to was stronger than had been assumed.
the surrounding buildings was minor,
so it is believed that the stainless steel Wind Resistant Designs
roof had already been damaged, andthat this became obvious when the roof Buildings are designed to be wind resiswas peeled off by the strong wind. tant to prevent damage from strong
Consequently, the damage to the roof winds. Skyscrapers and large-spanstructures are considered to be susceptible to wind. Therefore, structural designers carefully design structural
frameworks of such buildings to be not
only earthquake-resistant but also windresistant. However, structural designers
were not very interested in wind resistant designs for cladding, although themajority of damage from strong wind
involves cladding. In 2007, it became
mandatory for structural designers tosubmit to the local governments structural calculation sheets proving the windresistant designs of building cladding, inaddition to evidence of the wind resistant designs of the structural frameworks, before buildings are constructed.
This is likely to lead to wider recognition and application of wind resistantcladding designs by structural designers.
Creative measures will be needed, suchas indications of the wind-resistant performance of cladding that are easy forstructural designers to recognize.
Photo 2. The many objects that turned into flying debris in a major tornado in
Hokkaido, November 2006
As in the example given in the pre-from the ground to a building, causing
damage to every part of the building. In
contrast, strong winds mostly damage
the cladding such as roofs, exterior walls
and windows, which are directly hit.
Generally, damage to the cladding lets
rainwater into the building, which may
soak furniture, goods stored in warehouses and equipment in factories, forexample. Financial damage from theimmersion of indoor goods may bemuch greater than the damage from theloss of roofing itself.
It is also pointed out that strong
winds may cause serious secondary
damage to adjacent buildings because
they cause roofing to break away, becoming flying debris. Photo 2 shows theflying debris caused by a tornado that
occurred in Hokkaido on November 7,
2006. It was one of the largest tornadoes
ever to occur in Japan, and the debris
around the store includes structural
frameworks that look like lattice beams,
cladding such as roofing, interior materials and fittings, and even an object thatlooks like a refrigerator. Flying debris
like this damages the cladding of otherbuildings, including walls, roofs, andwindows, and this in turn causes otherdamage. This sort of chain reaction has
frequently been cited.
On September 17, 2006, TyphoonINTERNATIONAL COOPERATION
vious section, strong winds also cause
flying debris to hit and damage cladding.
This requires the development and proposals of methods for testing and assessing the crashworthiness of claddingin Japan.

The Building Research Institute conducts onsite investigations of areas hit
by strong winds, as well as buildings
damaged by earthquakes, to determine
the degree of damage. The Instituteconducts extensive surveys and research on measures to minimize damage to buildings from strong winds.
Examples are as follows.
1) The Institute has been advancing
research aimed at proposing a standard
format of structural calculation sheets
proving wind resistant cladding designs
for buildings, which now must be submitted to the local government, as mentioned in the previous section. This format will make it easier to judge thewind-resistant performance of cladding,which will help reduce damage fromstrong winds.
2) As in the case of IizukaCosmoscommon, the roofs of particularbuildings may be damaged even whenIIZUKA COSMOSCOMMON

Measures taken by the Building
Research Institute for Reducing
Damage from Strong Winds

Photo 3. The roof of the Iizuka Cosmoscommon in Fukuoka Prefecture damaged byTyphoon Shanshan in September 2006

the strength of the wind is within thescope assumed by the designers. To
reduce damage to cladding in these instances, the Institute has studied causes
and responses, and the results of the research are reflected in the manuals intended for people who work on construction sites.
3) Methods of understanding thedamage from disasters are specified
in Japan (Criterion for Judging Levelsof Damage to Dwelling Houses
Concerning Disasters). The results of theInstitute’s studies were reflected in therevision of these Standards, which were

Photo 4. The BRI’s tornado simulator
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made in 2009, and methods for determining the degree of damage from strong
winds were added to the Standards.
4) Tornadoes are extremely violent
gusts of wind. It is extremely costly tomake a building that is strong enoughto withstand tornadoes. Moreover, because the probability of damage to abuilding caused by a gust such as a
tornado is extremely low, the current
Building Standard Law of Japan (BSL)
only assumes strong winds such as typhoons and winter storms, with gusts
such as tornadoes outside the scope of
the BSL. However, some buildings(such as disaster-prevention facilities,hospitals, nuclear power plants andchemical plants) are required to be
fully resistant to gusts such as tornadoes, even though the probability ofsuch phenomena is extremely low. For
this reason, recent years have seen
growing research into the wind effectsof gusts like tornadoes on these buildings both in Japan and overseas.
The Building Research Institutehas also begun researching gusts, suchas tornadoes. For instance, it has installed a tornado simulator ( photo 4 ;developed with reference to the tornado
simulator of Iowa State University inthe United States) and has agreed toconduct joint research into damage
from tornadoes with other research institutes in Japan.