Timber tower blocks to shoot up in towns and cities
High sustainability levels, short construction times
and low weight mean that timber construction
systems are becoming increasingly sought after. This
also applies to timber tower blocks which, with
suitable fire protection, can be made equally as safe
as other forms of tower block.
Timber has great potential as the new sustainable
building material in cities and, in future, we will see
many more timber structures. Not least timber tower
blocks.
This was the clear impression left following the theme
day 'Gentænk træ' ('Rethink Timber'), which took place
at the School of Architecture in Copenhagen earlier this
year. Here, DBI and CINARK assembled a number of
experts from Denmark and abroad to present the
opportunities for using timber instead of steel and
concrete or in hybrid solutions whereby timber is used
along with steel and concrete.
In general, timber has two major advantages. One of
these should be viewed in the context of the global
megatrend that sees people flocking to the big cities,
which in turn creates a need for more housing. With
light timber structures, building on top of existing
buildings is a realistic option.
The other advantage is related to timber's
sustainability. That is to say, unlike steel and concrete,
timber is a renewable and CO2-neutral resource. In
addition, timber is a light material that requires less
energy for transportation and handling. And finally,
worn-out timber can be burned, thus saving on coal or
oil.
- There is an ongoing discussion as to what should be
included in the equation. But, no matter how you
calculate it, in a life cycle analysis, timber as a building
material comes out extremely positively in relation to,
for example, concrete. Timber is much more
sustainable than other materials, affirmed Michael
Koch from Træinformation, the Timber Information
Association, at the theme day.
Global race to build tower blocks
The interest in timber as a building material is global
and, all over the world, the race is on to build higher
and higher tower blocks. The record holder is a 14storey building in Bergen in Norway while a 22-storey,
75 metre residential tower block is due for completion
in Stockholm in 2023, when Sweden's biggest housing
association, HSB, will be celebrating its 100th
anniversary.
- As innovative architects, we must also take
responsibility for the environment. Therefore, in
future, we need to use timber more for building, said
architect Ola Jonsson from C.F. Møller's studio in
Stockholm, who designed the new record-breaking
timber tower block.
In Sweden, they are somewhat ahead of Denmark
when it comes to exploiting the advantages of timber.
- Multi-storey timber construction is well established in
Sweden and eight storeys is nothing unusual. At the
moment, a lot of effort is going into pushing the
boundaries by expanding knowledge and refining
construction systems. The construction of tower blocks
could be an important business area for the
construction industry, said Pierre Landes, project
manager at the SP Technical Research Institute of
Sweden.
Modern timber-based construction systems are quite
technologically advanced. The systems can include flat
timber elements or volume structures, both of which
are manufactured in a factory. These can be based on
C24 strips, CLT (Cross Laminated Timber) or laminated
wood. These forms of precast construction help
DBI – Danish Institute of Fire and Security Technology
Jernholmen 12, DK-2650 Hvidovre, Denmark ۰Tel.: +45 36 34 90 00 ۰E-mail: [email protected] ۰ en.dbi-net.dk
Timber tower blocks to shoot up in towns and cities
shorten the construction time while the fact that they
are factory-manufactured provides high quality
components. Often, tower blocks are constructed as
hybrid constructions where the timber is combined
with individual components in concrete or steel.
Denmark behind with timber construction
In Denmark, there is a common misconception that it is
forbidden to build higher than four storeys with loadbearing timber structures. The height limit originates
from the List of Examples of the Fireproofing of
Buildings. However, these are only for guidance for
compliance with the fire safety requirements in the
Danish Building Code, Bygningsreglementet.
The Danish fire requirements are function-based so
there are no formal barriers if it can otherwise be
documented that a timber tower block meets the fire
requirements in the Danish Building Code.
- As timber construction is relatively rare in Denmark, it
is rather an expression of tradition than of engineering
problems, explained fire safety technology adviser
Anders B. Vestergaard from DBI at the theme day.
Automatic sprinklers provide safety
Naturally, the fire safety of timber buildings was also
up for debate. Here, Ville Bexander from the Swedish
Fire Protection Association talked about safety when
using timber for the construction of tower blocks.
As a rule, timber structures are not as fireproof as, for
example, concrete structures. This is because timber is
flammable so load-bearing timber structures lose their
load-bearing capacity more quickly than concrete as
the fire progresses. Moreover, the fire can get bigger
simply because there is more flammable material
present. Flammable facades and the greater risk of the
fire spreading to other buildings have a further
negative impact on fire safety.
On top of this, there is the possibility that the fire can
spread throughout the timber structure and be difficult
to extinguish. However, timber constructions can be
fireproofed by means of, for example, gypsum boards
or by using fireproofed timber which can delay the
spread of the fire.
According to Ville Bexander, however, the most
effective fire protection is automatic sprinkler systems.
The Swedish experience shows that sprinkling
extinguishes up to 99% of fires, results in 50-80% fewer
fatalities and reduces damage costs by 50-70%.
- A timber tower block with automatic sprinkling
actually has a higher level of fire safety than a concrete
house without sprinkling, explained Ville Bexander.
The need for sprinkler systems in high timber
structures received the backing of fire safety engineer
Frederik Nystedt from WUZ Risk Consultancy AB who
pointed out that a combination of sprinkling and focus
on critical details in the construction can provide high
levels of fire safety. In this regard, a critical detail is the
design of the base of the roof to prevent a fire
spreading. On top of this, timber elements with cavities
must be fitted with inbuilt fire barriers.
- It is possible to obtain the fire classifications R120 and
EI120 for timber construction elements. However, you
should be aware that the elements continue burning
after the two hours so there is the risk of the building
collapsing later, said Frederik Nystedt.
Timber normally burns at a rate of 0.5 to 1.0 millimetre
a minute. This makes it easy to calculate timber when
dimensioning for fire safety purposes because, in
principle, timber loses its load-bearing capacity as time
passes.
DBI – Danish Institute of Fire and Security Technology
Jernholmen 12, DK-2650 Hvidovre, Denmark ۰Tel.: +45 36 34 90 00 ۰E-mail: [email protected] ۰ en.dbi-net.dk