TECHNOLOGY

Just How Good Is Wood?
Wooden buildings constructed with traditional techniques show remarkable seismic resistance. Kawai
Naohito of the Department of Structural Engineering at the Building Research Institute explains what wemight learn from these buildings based on the results of tests being conducted in Japan.

Jen buildings have gained recognition
from the environmental perspective inrecent years as the perception is thatcarbon dioxide emissions at the time ofconstruction are low, and that they areeffective in terms of energy conservation and carbon dioxide reduction. The
traditional wooden buildings were sustainable structures, part of a cycle offorest cultivation and building production that made effective use of locallyharvested wood, soil and other materials.
From the viewpoint of the environment,
one might say that they provide one
model.
Another attractive point isthat building a frame using timberand logs for posts and beams creates spaces with few walls thatfeel both approachable and expansive. In addition, in contrast to themodern industrialized production
techniques, the value of manual
production methods and the face
to face connection between thebuilder and the one living in thehouse seem to be undergoing areevaluation as something that is
The five-story pagoda at Itsukushima-jinja
desirable for modern society withshrine on the island of Miyajima, Hiroshima
its economic priorities.
Prefecture
apan has a long history ofconstructing buildings withwood. Many wooden buildings such as Buddhist temples
and Shinto shrines are still standing, including the Golden Hall of the Horyu-ji
temple, the oldest extant wooden building in the world. Five-storied pagodas ofaround 30 meters in height, sometimes
exceeding 50 meters, are also counted
among them. Farmhouses have also
been built in wood since time immemorial with the Hakogike house, allegedly
built in the Middle Ages, still standing.

The History of Seismic
DamageIn ancient times, earthquakes
were natural calamities beyond
human understanding, and little
thought was given to constructing
quake-resistant buildings. Before
the emergence of engineering
concepts, buildings seem to havebeen extremely fragile in the faceof natural disasters. In Japan,
where every year brings a risk ofHakogike house in Kobe, Hyogo Prefecturestrong winds such as typhoons,
Even today, temples and shrine there was some consideration of thebuildings are often constructed in wood, wind in itself. Major earthquakes, on theand about 85% of detached houses are other hand, occur with a frequency ofbuilt in wood. Compared to buildings once in several hundred years, and it isconstructed with other materials, wood- believed that the accumulation andtransmission of techniques to resist
earthquakes has gone beyond the potential time interval.
If we exclude the five-story pagodas, there are countless examples of thedestruction of temples, shrines and private homes by severe earthquake damage. As an example of a five-story pagoda toppling over in the wind, there is thedestruction of the five-story pagoda at
Shitenno-ji by typhoon Muroto in 1934.
With regard to seismic resistance intraditional wooden buildings, the first scientific survey of earthquake damage wasprobably carried out after the Nobi earthquake in 1891. The seismic survey of theseismic resistance of residential dwellings
was undertaken by Josiah Conder, aBritish architect working and living inJapan. He identified issues such as the lackof diagonal bracing in the walls, the lackof foundations, and heavy roofs.
Since then, wooden homes havesustained damage through repeatedearthquake damage, and there have been
advances in researching the seismic
resistance of wooden buildings. At thetime of the change from so-called traditional construction to new wood construction backed up by engineering, theattention focused on the weight of thewalls. The Building Standard Law formulated in 1950 required the installationof diagonal braces in the walls, and thatthe sum of the value obtained by multiplying the factors (quantifying the seismic effect) of the length of the walls inall directions on every floor to be morethan the reference value for the floor
space. Today, the regulations on earthquake resistance for wooden buildings
have changed in terms of numbers, but
the way of thinking remains.
Seismic Resistance in Traditional
Wooden Homes

In recent years, the scientific view has
again come to focus on seismic resistance in traditional wooden buildings.
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MAINICHI SHIMBUNTECHNOLOGY

Damage to wooden buildings in the Nobi earthquake of 1891, centeredon Gifu and Aichi prefectures

posts are simply placed on top of thestone without fixing in place, the plinths
slide on top of the stone resulting in little
damage to the frame.
Among the research projects fundedby the Government, there are also thosethat carry out vibration testing on fullsize traditional wooden houses. The
photographs show the results of applying huge seismic motion to observe thestate of destruction after confirming adequate performance at the standard ofassumed seismic motion. In this regard,
the graph shows an example of the analytical model and the results of analysis
undertaken at the Building ResearchInstitute using the Distinct ElementMethod. Using a 3-D model based on
testing of components such as the clay
wall and the connecting parts, it is possible to predict seismic response withconsiderable accuracy.
Through research projects that include empirical testing, we are discovering the potential for seismic designin traditional wooden buildings basedon the flexing resistance in clay walls
and posts.

For the past ten years or so, a lot of re- Institute on open frames featuring clay
search has evaluated the seismic perfor- walls only in the upper section. The situmance of traditional wooden structures ation varies depending on the thickness
in practice, attempting to leverage the of the logs or whether the plinths are
fixed or not. If the diameter of the posts
results in modern design.
Originally, there were no engineering is a fairly thick 150 mm, the clay wall isdesigns for traditional wooden buildings, destroyed, but if the diameter is a fairly
but they were built based on traditional thin 135 mm, the posts snap. Also, if thetechniques handed down andbacked up by the experience ofthe carpenter. Extremely advanced engineering judgment
and analytical techniques are required to turn this into engineering models for analysis. Since
this has been difficult in the past,
there are aspects that have beenleft out of the standards, but
thanks to modern advances inseismic engineering, the engineering approach has now become feasible.
Seismic testing of a frame consisting of
Test situation: When the logs are thick, theConceivable quake-resistantposts and a partition wall. (Buildingwall in the upper section is destroyed.
elements in traditional wooden
Research Institute, 2008, 2009.)
buildings include the effectiveness of clay walls as shear
walls, resistant posts that bend
because partition walls are attached to the posts (a construction technique whereby theposts are simply placed on topof a stone) that reduces seismic
input, and the self-reliance oflarge posts such as the ones inancient temple construction.
The photographs show aspects of vibration tests conductWhen the circumference of the post is a size thinner, the post snaps. If the plinth
slides, however, there is little damage.
ed by the Building Research20Vibration testing of a two-story full-size wooden
home. (Research project by the Ministry of Land,Infrastructure, Transport and Tourism, 2008.)

Seismic Resistance of the Five-Story
PagodaAt first glance, the five-story pagoda
looks like an unstable structure, butthere is no historical record of a pagoda
having been destroyed by an earthquake.
There are many examples of pagodas
escaping destruction even in major
earthquakes with other buildings collapsing and sustaining major damage.
The illustration is a record of damage toa five-story pagoda in the Great Ansei
earthquake of 1855. The top is bent, but
the pagoda escaped destruction, inmarked contrast to the collapsed buildings in the surroundings.
About a hundred years ago, theseismic resistance of the five-story pagodas was treated as a seismic engineering riddle with a variety of theories put
forward. They included emerging engineering concepts that have actually been

Example of analytical model and analytical results. The colors show
degree of damage, from red (most damaged) through orange toyellow (least damaged). The gray areas suffered no damage. The redcircles show where the supporting pillars are damaged. (Nakagawa
Takafumi, Building Research Institute.)

put to use today, such as the potential
for high-rise buildings to withstandearthquakes, or the mass damper technology for controlling vibrations byplacing a weight at the top of a building
and inserting a damper.
Research such as vibration measurements, manpower excitation and strong
motion observation has been carried out
on pagodas to find an engineering explanation for the seismic resistance of thefive-story pagoda. So far, these measurements have been done on the twenty-twoextant five-story pagodas built before theEdo period (1603–1867). In recent years,
we have also carried out vibration testing, not on full-size pagodas,
but on mockups scaled down
to one fifth the size of a fivestory pagoda (6.4 meters). In
terms of normal seismic motion, it is clear that replicating
the type of oscillation where
all stories incline in the same
direction is difficult, and that
there is no major variation even
if the shape of the central pillarof the pagoda is changed.
On the basis of this research, the reason five-story
pagodas seldom collapse inearthquakes is that they sway
slowly because of the heightof the pagoda itself, which is

Seismic damage to a five-storypagoda. (From the archives
Ansei kenmonshi, 1856)
21about 30 meters. As a result, the foremost
reason is that in normal seismic motion
where many components sway minimally, resonance is not easily induced.

Vibration testing of a mockup of afive-story pagoda. (Forum for Wood
Architecture and the National ResearchInstitute for Earth Science and Disaster
Prevention, 2004.)

There is something about traditional wooden buildings in Japan that attracts people and makes them feel safe,
an appeal that is not found in contemporary buildings. However, while it
may be true that they are attractive, wecannot close our eyes to problems withseismic resistance. It is the role of us
who live in the present day to accumulate objective data and to identify what
is good about traditional wooden buildings through engineering appraisals,
and to transmit the ancient knowledge
to the future.
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