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Robust fire protection of wooden structures

When building with combustible materials such as wood, extra care must be taken in both design and construction to achieve robust fire protection, details must be properly executed and fire protection systems must work over time. Robust fire protection is an important component for increasing the use of wood in a sustainable society. Fire classification of construction products is important for fire safety but is not sufficient to achieve robust fire protection.

Four key classifications

There are four basic fire safety classifications for structures and one classification specifically developed for timber structures. The first deals with ignition and fire spread on the surface of the structure, sometimes referred to as 'reaction to fire'. This classification can be used to select appropriate finishes in escape routes, for example, to minimise the risk of rapid fire spread that could impede evacuation. The extent to which a finish or building product contributes to the development of a fire when installed on both the walls and ceiling of a standardised room is the starting point for the classification. Exposed wood in escape routes must be treated to ensure a sufficiently low risk of ignition and fire spread.

The second classification concerns the fire resistance of the structure. It classifies the ability of a structure or building component to maintain its function when subjected to a standardised fire exposure. The standardised fire exposure is called the standard fire curve which can be seen as a kind of simplified representation of a fully developed fire in a room. The standard fire curve is simulated using fire test furnaces. After testing, the structure receives a classification where the most common classes are R the load-bearing capacity, E the integrity and I the insulating capacity. The different classes are always followed by a time indication, for example, when a wall that does not carry any load has the class EI90, it means that it encloses and isolates the standard fire for 90 minutes.

The third classification, the so-called K-class originally developed for timber structures, relates to fire resistance. The classification indicates how long a protective layer protects the structure from ignition at the standardised fire exposure.

The fourth classification refers to the resistance of the façade structure, including any air gaps, to vertical fire spread. This property is tested in a facade rig where a 6 metre high and 4 metre wide facade is exposed to flames emanating from a window during a fully developed fire in a room. The classifications of finishes, structures and fire resistance can be determined using standardised fire tests. The fire resistance class can also be determined by theoretical calculations according to the instructions in the common European construction rules, the Eurocodes. We therefore have a common European system for both classifications and theoretical calculations, but each country decides which requirement level should apply nationally. Only the fire classification of facade structures is done according to national standards.

Achieving robust fire protection - more than just classifications

A common approach to meeting fire resistance requirements is to use different types of protective layers such as plasterboard to form a protective membrane that delays the spread of fire, protects the supporting structure and prevents fire from entering hidden spaces in the structure. All installations, penetrations through the membrane and all connection details between elements must be fire rated, designed and installed correctly as they could otherwise jeopardise the performance of the protective layer. A German study (from 2006 for different types of houses) showed that 50 % of the installations inspected in the study compromised the fire protection membrane. During the lifetime of a building, there is a risk of damage to the fire protection membrane, for example through holes from removed or new installations. If there are combustible materials behind the protective membrane, the risk of a possible fire continuing and spreading in the timber structure increases.Post-extinguishing in structures where the fire has spread through the protective membrane to the timber frame requires more effort than in completely non-combustible frames.

Experience from previous high damage fire incidents shows that improvements to a few strategic details would have significantly reduced the spread of fire A robust and well thought-out fire protection strategy that stands the test of time is an important step in the sustainability and insurability of timber multi-storey buildings. A strategy to keep track of all important details should be the first step for the whole industry.

This is an article from our magazine Trävärden, view it here! (Link)

Daniel Brandon

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Daniel Brandon

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Alar Just

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