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 14-storey 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 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 load-bearing 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.