SCIENCE

Fire Safety Measures
Enabling Construction ofLarge Wooden Buildings(Part I)To create a large-scale wooden structure, it is important not only to ensure the safe evacuation of occupantsin the event of a fire, but also to take fire safety measures to prevent the fire from spreading and minimize
damage to neighboring structures due to collapse, scattering of embers and other hazards. Through activities
such as full-scale fire testing on three-story wooden school buildings, the Building Research Institute has
been advancing research into ways to improve fire safety in large wooden buildings.
Promoting the Use of Wooden
Construction

In recent years, there has been heightened interest in the use of wooden materials and wooden buildings. Until now,wooden building materials have primarily been used for the construction of detached homes in Japan. The BuildingStandards Law has imposed strict restrictions on large wooden buildings
from the viewpoint of fire safety.
During the Edo period (1603–1867),
Edo (now Tokyo) suffered several major
f i res, i nclud i ng t he G reat Fi re ofMeireki in 1657. While countermeasures
such as the widening of roads and thecreation of fire barrier zones were taken
in response, major fires broke out repeatedly, in excess of a hundred times.
Large fires continued to cause major
damage throughout Japan during theMeiji (1868–1912) and Taisho (1912–1926) periods. During the Great Kanto
Earthquake of 1923 in particular, some
76,000 people died and around 430,000
homes burned down in the major fires
that followed. For this reason, restricting
wooden buildings and the fireproofingof cities came to be regarded as the best
fire safety measures. When a fire breaks
out after an earthquake, fire-fighting
operations ca n not be relied upon.
Accordingly, in terms of fire-resistive
performance in Japan, large buildings
are required to remain standing evenafter being subject to the heat of a fire,based on the assumption that no fire30

The Japan Journal MARCH 2014

fightingo p er at ion s
would befor t hcoming in suchan event.
Many
major cities
overseas
have a lso
experienced
large fires.
Notableamong themis the GreatFi re ofL o n d o n(16 6 6 ) , i nwhich 85%of the city,Photo 1: An eight-story wooden apartment building in Sweden
which comprised tightly packed
wooden buildings between three and field of building construction.
Beginning in the 1990s, progress
five stories high, burned down. The fireis also famous for the subsequent ban on was made in the development of techwooden buildings that was issued and nologies related to wooden structuresand fire safety in Europe. At the samethe use of brick masonry for rebuilding.
In this way, while large wooden time, fire safety standards underwent
buildings have traditionally been restrict- revision, and today the construction ofed, the building of medium-rise and medium-rise wooden buildings is possilarge-scale structures such as apartment ble. One such example is the construcbuildings, schools and office buildings tion of the wooden apartment building
for which the use of wooden materials depicted in Photo 1. Even in Japan, regand members has posed difficulties until ulations governing performance were
now is an effective way to increase the established in fire safety standards inuse of wooden building materials in the r esp on se to t he r ev ise d Bu i ld i ng

resistive buildings andmade from a reinforced concrete struct u r e o r si m i la r. A s
pointed out above, theestablishment of performance-based regulations in fire safety
standards has enabledwooden fire-resistive
structures, but as will
be discussed below,
using wooden building
materials in a visible
way is difficult. Given
this, research is underway to utilize woodenbuilding materials andPhoto 2: A three-story wooden school building
constructed for full-scale fire testing purposes
members in a visible
way while ensuring adequate safety against
Standards Law in 2000, and this has fires even for buildings made of a
expanded the scope of use for wooden wooden construction (Photo 2).
building materials by enabling theadoption of fire-resistive wooden con- Ensuring the Fire-Resistive
structions. Even so, the technical hur- Performance of Wooden Buildings
dles are still high, and fire-resistive
wooden construction has not yet been In terms of the fire safety performancerequired of buildings, generally speakwidely adopted.
Amid these circumstances, the Act ing the larger the building, the higherfor P romot ion of Use of Wood i n the fire-resistive performance demandP ubl ic Bu i ld i ngs wa s e n a c t e d i n e d i n memb ers such as posts a ndOctober 2010, and the Ministry of beams. As wooden building materials
Land, Infrastructure, Transport and are regarded as being susceptible toTourism indicated its intention to re- fire, it has traditionally only been posvise regulations after conducting the sible to construct small buildings usingnecessa r y resea rch rega rd ing f i re wood. Measures generally used to ensafety of three-story wooden school hance fire-resistive performance evenbuildings. At present, the Building when wooden building materials areStandards Law requires that three- used include the fire protection memstory school buildings should be fire- brane covering technique, where theBurning
marginal layer
designNecessarycrosssection

Fire protection
membranecovering

Figure 1: Concepts of Burning Marginal Layer Design and Fire Protection
Membrane Covering

wood is covered with gypsum board,and a burning marginal layer design(Figure 1).
By covering the wood with a noncombustible material, the fire protection membrane covering technique canmoderate temperature rises in woodenbuilding materials, making it harder forthem to burn. However, as the wood isobscured with a covering material, thesurface of the wood is not visible.
On the other hand, the burning
marginal layer design utilizes properties to slow the speed at which burning
progresses to the inside of woodenbuilding materials by burning andcha r r ing the sur face of the wood.
Therefore, only members with a crosssection equal to this thickness that willburn (burning marginal layer) are required. The surface of the wood can beshown, allowing the sense of wooden
construction to be expressed.
Overseas, the use of fire protection
membrane coverings in a way that does
not show the wooden building materials
is common, but in Japan, there is astrong interest in showing the woodenbuilding materials and displaying that abuilding is of wooden construction if
one is to use wooden building materials
in the first place.
Fire-Resistive Performance of New
Wooden Members

Until around 1990, even wooden buildings in the West were limited in size toabout two stories high, but today it ispossible to construct buildings morethan five stories high. These kinds ofwooden buildings make judicious use of
a new kind of wooden member known
as Cross-Laminated Timber Panels, orCLTs (Figure 2). These are panels witha relatively thick cross section bonded atright angles to the grain.
While constructing a building using

Figure 2: CLT Panel Configuration
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SCIENCE
100

Burning Marginal Layer
燃えしろ試験体

Depth from Heating Surface (mm)Charring Rate Approx. 0.7mm / min.

Test Specimen

90

Burning Marginal Layer
防火被覆試験体Charring Rate Approx. 0.4mm / min.

Test Specimen

8070Fire-resistive
Gypsum Board
15mm

6050403020

CLT90mm

10Fire-resistive
Gypsum Board
15mm

0Photo 3: CLT panels immediately after aheating test

CLT panels still poses difficulties inJapan, there is a demand for CLT panels
to be made easy to use. Given this, likethe burning marginal layer design usedin buildings that utilize large-cross section laminated materials (heavy timber)for posts and beams, we are advancing
consideration to enable the applicationof burning marginal layer designs inwall and floor CLT panels.
We created wall test specimens thatcombined CLT panels with different
properties such as covering method andtree species, and conducted heating tests
to ascertain its fire-resistive performance.
The results of the tests showed that forcedar-based CLT wall panels (without
any fire protection covering), the charring rate was approximately 0.7 mm per
minute, which represents a similar charring rate to heavy timber. In addition, forcedar-based CLT wall panels with a
15-mm fire-resistive gypsum board asfire protection covering, our results
showed that not only was the charring
start time of the wood components delayed for between twenty and thirty minutes, but also that if the fire protection
covering did not drop off, the charring
rate was reduced to approximately 0.4
mm per minute (Photo 3, Figure 3).
Based on these experimental results,
CLT panels are assumed to exhibit asimilar charring rate to that applied toheavy timber, and the application of aburning marginal layer design is thought
to be possible. In addition, if the fire
protection covering not falling off is established as a condition, the dimensions
32

The Japan Journal MARCH 2014

0102030405060708090 100 110 120

Time to Reach 260°C (min.)Figure 3: Charring Rates of Various CLT Panelsof the burning marginal layer could also
be reduced and a more rational fireresistive performance could be ensured.
Charring Properties of Combined
Members

With a burning marginal layer designfor posts and beams, first and foremost
members with a large cross section areused, but it is often difficult to securelarge-cross section members when it
comes to structural large-section gluedlaminated timber. If a burning marginallayer design can be applied in a similar
fashion by combined members with aabcd

Photo 4: Charring properties of combined members(Gaps a: 0 mm, b: 3 mm, c: 5 mm, d: 20 mm)

m e d iu mcross section
which areavailable on
the market,
there would
be greaterf re e dom i nterms of design andconstruction.
Given t h is,
we a sc ertained thecharring
Photo 5: Fire behavior in wooden interior finish spaces (left, when heated at 100 kW; right, at the end of
properties ofthe test)
combinedmembers
made up of medium-cross section mem- materials, but this prevents the creation fire safety of spaces that use woodenof interiors that show the wooden materi- materials for interior finishing.
bers by means of heating tests.
We have also conducted other fireWith the combined sections, heat als. Given this, we conducted tests to recpenetrates the gaps between members reate the fire behavior during the early experiments designed to ascertain theand charring advances. As test speci- stages of a fire using a space utilizing time taken to reach flashover and therisks of a fire spreading to upper floors
mens we combined two laminated posts wooden materials for the interior.
due to flames emerging from below
Photo 5 depicts such a space withof cedar (120 mm x 240 mm) and conducted heating tests by varying the gap a quasi-noncombustible material used when creating classroom-sized compartbetween members by between 0 mm in the ceilingand 20 mm. The results, as shown in a n d wo o d e n
Photo 4, clearly indicate that when thematerials used
gap between members is greater than 5 i n t he wa l ls.
mm, charring of the gap portion ad- W h e n t h evances all the way in. Since burning up s p a c e w a s
is limited when the gaps produced due heate d usi ng
to the drying of the wood, construction a ga s bu r ner
work errors or other factors is up to at 100 kW foraround 5 mm, this is not regarded as t e n m i nut es,
likely to cause significant problems in the tip of theterms of fire safety.
f la mes reachThrough these tests, it became ev- t h e c e i l i n g ,ident that by combining medium-cross but since thesection members in this way, a similar c e i l i ng d o e sbu r n i ng ma rg i na l layer desig n to n o t b u r n , i tlarge-cross section laminated wood barely spreads
Photo 6: Fire test using classroom-sized compartment
can be applied.
l a t e r a l ly.
T herea f t er,
when the space was heated with a burner ment and finishing interiors with wood
Fire Behavior in Wooden Interior
at 300 kW for a further ten minutes, the or other materials (Photo 6).
Finish Spaces
If we are able to properly evaluate
wooden wall materials burned to the exWhen wooden materials are used for in- tent that the fire spread laterally under- the fire safety of wooden interior finish
teriors, as interior wooden materials burn neath the ceiling, but the fire died down spaces by pursuing further research in
together with combustibles stored inside without spreading further.
the future, we expect the scope withina room, the heat release rate is greater
In this way, we found that even which wooden materials can safely be
and the time until flashover is reached is when wooden materials are used in used will widen.
shorter, increasing the fire risks. The walls, provided the ceiling finishing is
Building Standards Law restricts the wall made non-combustible, burning during This report was compiled by Hagiwara Ichiro
and ceiling finishing in the habitable the initial stages of a fire is controlled to (director), Kagiya Koji and Suzuki Junichi (chiefrooms of special buildings to flame- some extent. We were able to gain valu- research engineers) at the Department of Fireretardant wood and other flame retardant able insight in terms of evaluating the Engineering, Building Research Institute.
The Japan Journal MARCH 201433