Fire behavior: from perception to technical criteria

By Marcelo González Retamal, director of the Master's in Construction Management and Sustainability USS, School of Architecture USS Concepción.

When discussing wood in construction, one of the most frequent objections is that "wood burns." And it's true. But from a design perspective, the relevant question is not whether a material is combustible, but how it behaves structurally during a fire.

When exposed to fire, wood begins a thermal degradation process known as pyrolysis. At temperatures close to 120°C, it starts to decompose, and above 250°C, ignition can occur. However, the most important thing is that, as it burns, it forms a surface layer of charcoal that acts as a thermal insulator. This layer protects the interior of the element, slowing down heat penetration and allowing the core to maintain its structural capacity for a significant period.

This behavior allows the loss of resistant cross-section to occur gradually and predictably. The charring rate of wood is relatively constant, on the order of 0.6 to 0.9 mm per minute, which enables design considering this progressive reduction.

Unlike other materials that may fail suddenly upon reaching certain temperatures, wood allows its performance to be anticipated and structural elements to be sized to meet the fire resistance times required by regulations.

In mass timber systems (glulam or CLT), the greater thickness of the elements allows surface charring to function as a natural protection mechanism, extending the time during which the structure can remain standing during a fire.

This does not mean that wood has no challenges regarding fire. Its combustible nature is an aspect that must be managed from the design stage: when done correctly, its structural behavior under fire conditions is well understood.

The discussion about wood and fire often remains at the perception level. However, what is relevant is understanding that it is a material whose performance can be designed and controlled. And that is where the difference between an opinion and a technical criterion lies.

The column in theAcoforag Magazine