Designing robust fire protection in tall wooden buildings - Briab - Brand, Risk & Säkerhet


Designing robust fire protection in tall wooden buildings

The rate of change in the building industry is high, with increased focus on sustainable construction using renewable materials such as wood. In my first article in this series, I described that fire development can be unpredictable and more intensive when wood is used and in my second article, I gave some examples of the important challenges we must deal with when we build tall wooden buildings. In this article, I present my thoughts on how to design robust fire protection in a tall apartment building made of wood.

crowd management

The parts of fire protection that usually require most attention in tall wooden buildings are linked to evacuation, spread of fire, rescue services and load-bearing capacity. Apart from these specific aspects of fire protection, we also need to assess the overall fire safety, by studying the robustness of the fire protection. In a recent development project, financed by the Development Fund of the Swedish Construction Industry, SBUF, we at Briab along with Brandskyddslaget and other industry players have highlighted the principles for designing robust fire protection in tall wooden buildings. It mainly concerns assessing how sensitive the building is to the different types of faults, and whether there are incidents that can cause simultaneous faults in numerous protection systems. When fire protection is designed, it must be done on the basis of keywords such as the deep defense principle and single error criteria.

Multiple defense lines are necessary

The deep defense principle is a term borrowed from the nuclear power industry, that can be used to explain how fire protection can be built up in a robust manner. A fire-safe building is designed by transferring several defense lines (barriers), see the figure below. The first barrier includes protective measures that have been adopted for protecting the onset of fire and for ensuring that fire safety in the building works as it should. The other barriers are made up of systems that are used for detecting a fire and ensuring that it does not spread. Examples of such systems are fire alarm, non-inflammable material, extinguishing tools and training. The subsequent barriers consist of additional systems that restrict the scope of consequence, in order to avoid catastrophes. The keywords for barriers are “interrupt”, “envelope” and “limit”, and examples of measures are extinguishing systems, evacuation routes, fire cell limits, rescue elevators, etc.

crowd management

Increase the focus on prevention

A traditional design of fire protection in a residential building stresses the importance of having barriers that “envelope” and “limit” as compared to the previous keywords such as “prevent” and “detect”. Our building regulations comprise few provisions that are preventive, compared to those that are damage-controlling. In a building made of combustible material, early measures are of great significance as the expected damage on the construction is much more in these buildings. Working with organizational measures, as well as technical solutions to prevent the emergence of fire, should receive increased focus here. Furthermore, there may be reason to consider whether measures should be taken to enable an early extinguishing effort. Here, access to a hand fire extinguishers along with increased training efforts play a crucial role.

Independent systems

The deep defense principle is about creating several independent and redundant layers of protective measures, to avoid the potentially large damage that can be incurred. The purpose is that no individual system, regardless of how robustly it is designed, should solely provide for the required protection. The keywords in this context are redundancy, independence and diversification. Redundancy means that more than one element (system) has the same function, for instance, having two pumps instead of one. Redundancy also means that a single error criterion can be fulfilled when a component can fail without the system as a whole losing functionality. System independence can be achieved by the redundant elements having separate functions. For instance, the pumps in the above example have separate connections to the water source and are placed in different rooms. Finally, diversification means that components that have the same function are built on different design principles, for instance, one is a diesel-driven pump and the other is electric.

Barriers in collaboration

Normally, the rescue services are an effective barrier in preventing the spread of fire to another fire cell. The recent years’ rescue effort statistics show that a huge 97% of apartment fires do not spread outside the start fire cell. In tall buildings, the rescue service cannot achieve the same high efficiency and fire protection must therefore be complemented with additional barriers. An automatic water sprinkler system can be used to compensate for the emergency services’ reduced opportunities for a quick and efficient rescue operation. A sprinkler system usually has a reliability of about 95%. Therefore a sprinkler cannot solely extinguish the fire with the same or higher possibility as what the rescue services are capable of. The role of the rescue services therefore is still significant to extinguish a fire in the event of sprinkler failure, or if unexpected events occur that fall outside the design conditions. A quick rescue service with small tactic units that can be guaranteed a safe access route can be a suitable strategy for tall buildings.

In the next article, the fourth in the series, I will summarize what we currently know about tall wooden buildings, and provide recommendations on how we can ensure good fire protection in these.

Download the SBUF report here

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