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8 November, 2019

Wood: a fire resistant building material

Fire is one of the great enemies of building materials and can have different reactions. Wood can gradually shrink, steel loses its rigidity and strength, concrete can break when exposed to high temperatures. Therefore, wood is not the only building material that can be harmed by exposure to fire.
Wood has good strength when subjected to fire conditions. Although this information was public knowledge, there was no evidence. In the 1950s, steel, concrete, wood, among others, became the target of research for better development when exposed to fire. After this investigation, wood performed better compared to other building materials, thanks to its different behavior compared to others and its physical and mechanical properties.
Wood is a different combustible material than steel and concrete. Wood, under normal conditions, does not burn directly, first it decomposes into gases that, when exposed to heat, turn into flames (figure 1) and which, in turn, heat up the wood that was not reached, promoting the release of gases. flammable, fueling combustion, becoming a vicious cycle.

Figure 1: If we look closely, we can see that the flames of fire burn at a certain distance from the surface of the wood.

However, some pieces of more robust wood, when exposed to fire, create a surface layer of coal that acts as an insulator, preventing the rapid exit of flammable gases and the propagation of heat to the interior of the section, resulting in a degradation. and heating the material at a lower speed, contributing favorably to the capacity of sustaining the building loads, this is due to the conservation of the physical properties of the wood even after being exposed to high temperatures.

Figure 2: We can see two types of beam (wood and steel) after a fire.

In the image, we can see that there are two types of beams and note that the wooden beam still supports the weight of the steel beam that was destroyed due to high temperatures.

Figure 3: Section of a glued laminated wood beam exposed to fire for 30 minutes.

We can see that each layer behaved differently in different parts of the wooden beam, an important study on the strength of wooden structures. It is also important to note that this beam, even after burning for 30 minutes, is still in good working order after recovery.
Thermal properties and properties related to strength and rigidity are the ones that most influence its performance. Most of these properties are related to the wood's own factors, such as density, moisture content, grain orientation (general arrangement of wood cells in relation to the axis of the structural piece), chemical composition, permeability , thermal conductivity and external factors (fire exposure temperatures, duration of exposure and ventilation in the environment)
Therefore, through this knowledge, we were able to assess the safety of structures affected by a fire and predict the resistance time of the structures of a building.

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