Back to blog | June 17 2021 | Valérie Levée | Science, architecture and occupational health and safety journalist

Wood resistance to fire and earthquakes

 

 

 

 

 

All over the world, wooden buildings reach for the sky. In the city of Québec, the Origine condominiums reach 41 metres, the Treet in Norway peaks at 49 metres, and the HoHo in Austria dominates at 84.5 metres. Even the Brock Commons in Vancouver, built in a seismic zone, reaches 54 metres. Like any building, they are designed to provide the level of safety required by the construction codes, including fire safety and earthquake resistance.

Whether the building is made of wood, concrete or steel, a fire originates in furniture and interior finishes, and, if not smothered, moves into the structure of the building. No matter the material, the Québec Construction Code requires that the building remains standing for one or two hours, depending on the number of floors, which is the time needed for people to evacuate and for firefighters to take action without fear that the building will collapse on them.

Yes, but wood burns. So how can the wooden building stand while on fire?

“This is because, when it burns, a charred layer forms on the surface of the wood and we know that this layer slowly progresses at the rate of 0.65 millimetres per minute towards the interior of the wood,” explains Christian Dagenais, chief scientist at FPInnovations and visiting professor in the Department of Wood and Forest Sciences at Université Laval. Therefore, it is possible to size the structural elements accordingly so that even after one or two hours of charring, there is enough left to stand.

Photo FPInnovations – Fire resistance test

Steel and concrete do not burn, but that does not mean they are more resistant during a fire. When heated, steel bends quickly and concrete bursts.

So wood structures can be resistant to fires, but what about earthquakes? Steel and concrete are stronger than wood, you might say.

“Except that what enables a building to withstand an earthquake is its ability to deform to dissipate its energy,” explains Marc Oudjene, a professor in the Department of Civil and Water Engineering at Université Laval. As such, a wood structure has two advantages.

When the earth shakes, the ground that was stationary starts moving at an increasing speed, causing buildings to move with force proportional to their weight. Because wood is lighter than steel or concrete for equal resistance, the wooden building is subjected to a force that is inferior to the one inflicted on a steel or concrete building.

This force must then dissipate in the structure so that the building does not collapse. In a wooden building, it is the fasteners, the nails and screws, that give the structure its flexibility to deform and cushion the force of the earthquake.

There are also calculations and rules for nailing and screwing in the right place, at the right angle, at the right depth. In the end, whether made of wood, steel or concrete, buildings meet the same requirements of the Construction Code, and the calculation of structural elements must follow the standards prescribed in the Code. All materials come with risks that have to be managed differently.

 ©Photo : Mathieu Létourneau-Gagnon
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