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Why don’t we build more high-rises from wood?

Building with wood is good from a climate point of view. In addition to being a natural resource where the production of timber from forestry consumes less energy, the growing forest binds our carbon dioxide emissions. But while the number of detached houses and apartment buildings built with timber increases, few high-rise buildings are built from wood. RISE has investigated why this is the case and what it would take to make more more wooden high-rises a reality.

Mjöstornet
Photo: Moelven
The Mjöstornet tower in Brumunddal, Norway is the world's tallest wooden building with its 18 stories and 85 meters.

Questions of sustainability have become an increasingly important issue in the construction industry. A large part of the focus has been on finding smart solutions for the way finished buildings consume energy for heating and cooling. Now interest in materials and energy consumption during the construction phase is also becoming more important. It has long been recognised that wood is a sustainable material. When forests are felled to become planks, beams and other structural timbers, the emissions are stored in the wooden houses that are then built and the plantation owner can plant new trees that absorb new emissions and the virtuous cycle continues.

According to statistics from The Swedish Federation of Wood and Furniture Industry , building with wood has increased in recent years but this is primarily apartment buildings with a maximum of four to five floors. High-rises in wood are still relatively uncommon. 

A problem-solving journey to do something no one’s ever done before

So what is the real reason there are no timber high-rises? The simple answer is that because no one has ever built timber high-rises, there is a dearth of building recommendations and experience to draw on. To be the one who begins construction means breaking new ground. 

Marie Johansson, senior researcher at RISE, wants to remedy this. Recently, a project completed in which RISE, together with several actors from the construction industry, architects, consultants and building systems suppliers, examined what it takes to build high-rises from wood and what the challenges are.

“Because this has never been done before, there are many new issues that have not previously been considered. That's why we started this project,” she says. 

During the work, participants planned for the construction of two 22-storey wooden buildings. The project showed that there are two main problems that need to be solved when building high with wood. 

“One, according to calculations, is that the building becomes so light that it may sway a bit, not least at the top. That’s been one of the technical difficulties, the other one being fire. How much fire protection do you need to have and how do you tally this with the stipulated building regulations we have in Sweden today? she says. 

Several aspects to take into account

Alar Just, senior researcher at RISE who took part in the project and who is an expert on fire and fire resistance in timber structures, points out that the wood should preferably be mixed with other building materials to resist fire better and that the number of naked wooden surfaces should be as few as possible. 

“Architects often like wood to be visible, but this can lead to a rapid spread of fire. Same goes for the frontage, it’s vital that it’s not just wood all over, but a mix between wood and non-flammable materials. And sprinklers are always good to have, even in concrete houses, they save lives,” he says. 

He has also investigated how the most common load-bearing wooden structures, CL timber (cross-laminated timber), and Glulam (glued laminated timber) behave during fires. 

“Glulam is a little safer. It almost always becomes self-locking when it starts to burn but cross-laminated timber burns more powerfully and the design contributes more to the fire,” he says. 

But even though fire protection is a major challenge when building high-rises with wood, it is not something that prevents construction. 

“It’s possible to build fireproofed buildings with timber, but we need to map out safe solutions where we protect against all possible scenarios that can occur,” he says. 

He also thinks it is not just a matter for actors in the construction sector but also something that emergency services and the Swedish Civil Contingencies Agency, MSB, must take responsibility for. 

“If we’re going build more high-rises with wood, firefighters must also be equipped to extinguish fires in tall timber buildings. The extinguishing process may not be the same as for concrete high-rises,” he says. 

Today’s technology opens up for future hybrid constructions

Now the project’s conclusions will be compiled in a handbook for architects and designers. 

“So they know how to dimension timber high-rises," says Marie Johansson. 

But how high is it actually possible to build with wood? 

“With today's technology it’s possible to build 20 storeys, but if you are going to go higher it will require more development," says Marie Johansson. 

She also envisions a future where it is no longer solely about building in wood, concrete or steel but where the materials are mixed more. 

“One possibility is to combine the materials in hybrid constructions, which is more optimal in many ways. The different materials are good for different things; for some parts of a building it’s better to use concrete and for some it’s better to use wood” she says.