Tall wooden buildings provide new and unique challenges

Designing fire protection in a building of more than 16 floors imposes special requirements on us, fire protection designers. These buildings are classified under the building class BR0 and are considered to have a very high protection need. Here, the National Housing Board requires analytical dimensioning to be used to ensure that fire safety complies with the regulations given in BBR. It is not possible to use simplified dimensioning where the design of fire protection follows the proposals for solutions that are stated in the general guidelines, in the building regulations. Instead, the designer needs to perform a careful overview of the building and its unique requirements with respect to design and operation.

Lack of materials

When all the conditions are known, a careful risk identification is performed, forming the basis of the fire safety design. There is a considerable lack of guidance materials within the area, and in a recently concluded development project, financed by the Swedish Construction Industry Development Fund, SBUF, we at Briab, along with the fire safety team and other industry players, developed a guide for fire technical dimensioning in tall wooden buildings..

Four aspects of risk identification

In an apartment building, fire safety is designed at a minimum level which mainly intends to protect the individual’s property and enable people to remain in place, in the event of fire. Often fire safety measures that require high levels of operation and maintenance are avoided, as these usually require access to the apartments of the residents, which is avoided with regard to personal integrity. Even though most of the deadly fires occur in apartments, few measures are taken to prevent them, except for fire detectors that provide early warning and fire cell division that avoids the spread of fire. However, in a tall building, further measures are required for the fire safety to meet the society’s requirements. What these are cannot be determined until after the specific building’s unique risks have been identified. If the tall building is designed using a wooden construction, the risk identification may be more demanding. The National Housing Board provides some guidance and takes up four specific aspects that make up the basis for risk identification. These are whether: the external extinguishing effort cannot be carried out, the internal rescue operation is complicated, the feared consequence is very large, and the evacuation process may be associated with great difficulties.

Three problem areas for tall buildings

For tall buildings, regardless of the construction material, there are three main problem areas that must be carefully highlighted. They concern the risk of fire and fire development within the building, long response time for the rescue service and long evacuation time. Increased risk of the fire spreading is linked to the rescue service not being able to implement external extinguishing efforts, due to which fire on the facade becomes particularly risky and places great requirements on the material and design. Delayed efforts caused by long transfer routes, orientation difficulties and communication problems with the rescue service mean, in themselves, a risk of the fire spreading even more.

Wood increases the challenge

In addition to the difficulties facing tall buildings, a tall building of wood has even more challenges when designing fire protection. The fire load is a design factor for fire protection, and it increases when the construction is made of a combustible material. Determining protected and unprotected fire energy is necessary and here the building system plays a major role. For CLT boards (see more in article 1), it is about choosing a temperature-resistant glue or protecting with non-combustible cladding. When the load-bearing structure consists of combustible material, there is a risk of the fire spreading to and within the construction. The risk of cavity fires increases if protection is not particularly considered. In a building made of wood, there is no clear end time for the complete course of processes, which affects the design of the construction. There are also uncertainties concerning installation and steel mounts that can lose their function at an increased temperature. The outer coating of untreated wood can intensify the fire development, increase burning outside the fire room and a fire can spread to quite an extent in just a short time compared to a building made of traditional non-combustible material. The fact that the fire does not have a clear end increases the reliability on the efforts of the rescue services.

Client’s responsibility and fire protection engineer’s duty

The client is responsible for determining how fire protection is to be designed with regard to these specific risks linked to the height of the building and the construction material, and the fire protection engineers are responsible for ensuring that fire protection meets the society’s requirements. A tall wooden building presents a number of challenges that must be dealt with, and not much guidance is provided by the National Housing Board. The fire rescue services have little experience of high buildings, and knowledge about the impact of wood construction on fire development is limited. I believe that these considerable uncertainties place requirements of accuracy and prudence, to create a robust fire protection solution.

In the following article, I will give my view on how we can design robust fire protection in a tall wooden building.

Here you can download the report.