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Tall wooden buildings could be safer than steel

Increasingly architects and engineers around the world are turning back to wood for construction – even for multi-storey buildings. Wood’s advantages of weight and convenience, as well as resilience, also make it a wise choice for earthquake-prone areas.

But the obvious question is how would a multi-storey building perform in a fire – wouldn’t it be a towering inferno just waiting to happen?

Somewhat surprisingly, the answer is no. A tall wooden building could actually be safer than steel in a fire.

Average building fires reach temperatures of 700 to 1000 C. Steel framing can distort or fail at even comparatively low temperatures, retaining only 10 percent of its strength at 750 C.

Large wooden structural members in comparison can retain their integrity for several hours in an intense fire.

Professor of Civil Engineering at Canterbury University, Andy Buchanan, says modern, well designed multi-storey timber buildings have more than enough fire resistance.

“Fire performance is only a disadvantage for wood if the buildings are poorly constructed with inadequate plaster-board protection of light timber framing and a large number of buildings very close to each other.”

In a fire, heavy timber burns on the surface. This forms a ‘char’ which can be seen when a large piece of wood is burnt on an open fire. Char has a very low density, is porous and a good insulator – just think of all the tall tree trunks left standing after a devastating forest fire.

During a fire this char layer progressively disintegrates on the outside and moves deeper into the wood. The charring rate is relatively constant and for radiata is about 0.7 mm per minute in a fully developed fire.

This means that after an hour the member would have lost 42 mm from each unprotected surface. If an isolated column was, for instance, 400 mm by 200 mm, it would retain a 316 mm by 116 mm cross section of good structural timber.

The size of these members can be specified to ensure that the reduced section size is still adequate to be structurally sound and continue to support the building after this period of fire.

If you avoid collapse you increase the chances for people to escape.
“Mostly what burns in a building fire is not the wood or the wood structure, it’s the solid petrol in all the foam and plastic in the finishings.
“This is a problem irrespective of whether the building is made from wood, steel or concrete,” Prof Buchanan says.

– Ends

Read these.pdfs, also found on the NZ Wood Fire Performance page.

Fire development in buildings – .pdf

Behaviour of timber in fire – .pdf