A common concern raised in regard to the use of wood as a building material is whether or not there is an increased risk of fire. Engineers and fire researchers today have a significant body of knowledge of how timber constructions perform in fire. The structural stability of timber is well understood and importantly it is predictable, allowing timber constructions to be created that meet the same fire safety codes as steel and concrete buildings.
* Gerard, R., Barber, D. & Wolski, A. . (The Fire Protection Research Foundation, 2013).
Heavy timber constructions have an inherent level of fire resistance. This resistance increases with the thickness of the wooden elements because when timber is exposed to fire the outer layer burns and turns to char. Charring creates a protective layer that acts as insulation and delays the onset of heating for the cold layer below. With continued exposure to fire the char layer grows, increasing the insulation and slowing down the burning rate, providing greater time for escape or intervention.
An additional benefit to heavy timber is the ease of repair after a fire. The charred sections can visually be assessed and evaluated for residual capacity, and the damaged timber can then be cut away and replaced. This is in contrast to steel, which buckles under extreme heat. In light timber frame constructions the walls and floors are typically encased in non-combustible gypsum plasterboard to provide protection from fire. This provides the same level of fire resistance as a completely non-combustible material.
* Buchanan, A. . (National Institute of Standards and Technology, 2014)
Just because steel is a non-combustible material does not mean it is unaffected by fire. The thermal conductivity of steel is significantly greater than wood (200-1000 times more). This creates a thermal bridging effect, allowing heat from a fire in one part of a building to spread rapidly to other parts. Fire can also raise the temperature of steel enough to compromise its strength, with a reduction in its load carrying capacity by one third when heated above 540°C causing beams to buckle and floors to collapse.
* Maxim, P., Plecas, D. & Clare, J. . (University of the Fraser Valley, Centre for Public Safety & Criminal Justice Research, 2013)
A study that examined the rate of injury from hotel, motel and aged care home fires in America and Canada from 1980 to 1998 found that the presence of sprinklers had a greater impact than the combustibility of the building material (Figure 7 below). In fact sprinkler systems have been shown to be one of the primary factors in limiting fatalities and fire damage in structures of all types.
* Richardson, L. R. Fire losses in selected property classifications of nonresidential commerical and residential wood buildings. Part 1: Hotels/ motels and care homes for aged. 31, 97–123 (2007).
* Frangi, A. & Fontana, M. Fire Performance of Timber structures under Natural Fire Conditions. in 8, 279–290 (2005).
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