Fire protection for timber structures

One of the most frequently asked questions concerns the structural load behaviour of our timber hybrid towers in the event of a fire. Since wood is known as a fuel, it is assumed that fires in a timber structure such as this would flare up easily and spread uncontrollably. However, does this assumption have any basis in reality?

Let’s begin by answering the question: “Ask any firefighter which type of burning building he is more willing to walk into and he will always choose a timber building over a steel one.” Why? Steel quickly loses its strength and collapses on itself without warning. Firefighters know that the inner core of a timber structure remains stable, protecting the building from collapsing.

But why is that? To answer this, one must understand the properties of wood as a material. Provided the timber elements are properly designed, wood can fully exploit its fire protection strengths, meaning that timber structures always resist fire better than comparable steel or reinforced steel superstructures.

How does a timber structure have to be designed to optimally exploit its fire protection properties? These optimum material properties only take effect when solid wooden beams or laminated composite materials with an adequate diameter are used.

In the event of a fire, the fact that the timber hybrid tower’s supports and diagonals have a massive diameter of approx. 1×1 m prevents oxygen delivery from three sides, meaning that only the side exposed to the fire would slightly char. Another special feature of its fire behaviour makes sure that wood retains its stability during and after a fire: Exposed to fire, this material builds a protective carbon layer, meaning the fire can only attack the material’s outer layer. This air seal caused by the outer carbon layer as well as the material’s density and stable core temperature prevent the structural elements from burning through.

In summary, this means: Thanks to the high material density of the solid wood construction (glued laminated timber) and the associated protective carbon layer, the comparatively low heat conductivity of wood (cool core) and its limited oxygen supply, only a thin layer (several mm) of the surface would char before the fire would be go out automatically. The timber structure’s structural integrity is therefore not substantially threatened by a fire and does not have to be protected from fires by means of additional materials or even chemicals.

Of course – like all other buildings – timber structures have to meet all fire-related statutory safety regulations. The associated fire expert opinions have to be written anew for every individual certification or adapted to the respective location.

In summary, the initial question can be answered as follows: In the event of a fire, the fire behaviour of wood can be calculated and controlled well compared to other building materials. Adequately designed timber composite elements burn much more slowly.

Even if the structure would theoretically burn on all sides at the same time, at a proposed burn rate of 0.7 mm/min (acc. to ÖNORM EN 1995-1-2) it would take more than 50 minutes until the structurally load-bearing cross-section of the timber frame would be reached and the weathering layer would be completely burned off. Under the charred layer, the glued laminated timber remains undamaged and its load-bearing capacity remains intact for a long time. One could say that (glued laminated) timber burns safely, until it goes out by itself.