An Assessment of the Fire Safety Hazard Associated with External Fire Spread in Tall Buildings with Combustible Façade Material

External fire spread poses a severe threat to the fire safety of tall buildings with the ensuing risk of multiple simultaneous compartment fires and in the worst case, a complete structural failure. However, it is important to understand every aspect of the causes that leads to external fire spread in order to obtain a conclusive assessment of the fire safety hazards associated with combustible facades.
Prescriptive fire safety codes are typically not allowing any type of combustible façade in buildings that are taller than 2-3 stories. However, a performance based approach does not contain height limitations in many countries. The study within external fire spread has shown that the transition from prescriptive to performance based approach can be cryptic and it is important to keep in mind that a performance based design requires that all aspects are taken into account.
Therefore, a method was developed to study the likelihood of fire spread from inside a compartment to the façade in a tall timber building, in order to contribute to the overall understanding of fire safety in tall timber buildings. The method is based on the principles of the analytical Law model, thus the parameters of opening factor, compartment temperature and heat flux were considered.
An accompanying case study was based on one of the most well-known medium-rise timber buildings (Limnologen), which was primarily chosen because it has combustible façades that were only allowed due to the implementation of residential sprinklers. The technical trade-off carried two main concerns, which serve as the base for this discussion. First, the sprinklers are only installed inside the building, thus fire spread that originates from the outer perimeter was neglected. Second, the sprinkler’s effect was not analyzed, thus it was ambiguous whether they were actually needed or not.
The study has its main focus around the second concern and the results show that the residential sprinklers in many cases were found to not improve the external fire spread risk.