Pressure management in compartment fires

Fire-induced pressure has not been considered a threat for structural or occupant safety in apartment fires. The situation may be changing as the building envelopes are becoming much more air-tight due to the energy efficiency requirements and the construction of high-rise buildings. In this project, we investigated the effects of the building's air-tightness, ventilation configuration and the fire growth rate on the peak overpressures inside the fire compartment and smoke spread within the mechanical ventilation system.

A series of fire tests was first carried out in a real apartment building to investigate the phenomena and to collect data for model validation. The experimental observations showed that the fire-induced pressure can prevent the use of the inwards-opening door during the early stages of the fire, thus delaying escape from the burning apartment significantly. Another observation was that the pressure can become sufficiently high to break structures: A complete window frame was blown out of the light-weight wall of the apartment when pressure exceeded 1600 Pa.

Numerical simulations using the Fire Dynamics Simulator -code were then carried out to extend the analysis for different fire growth rates and to investigate the role of different buildings and ventilation configurations. The modelling of the envelope leakages and the ventilation system was first validated, and the uncertainties of the pressure predictions were quantified. The simulations were then used for two case studies. The first case was a prison cell fire, representing a specialized building type with small compartments. The second case study was an apartment building with centralized mechanical ventilation system. The results indicate that the pressure of the compartment containing a fast-developing fire will become sufficiently high to break structures if the envelope leakages are at the level of near-zero energy buildings, and if the ventilation ducts are closed by fire or smoke dampers. Keeping the exhaust side ventilation duct open was found sufficient to maintain the pressures at safe level. Smoke spread to the other fire cells or apartments was avoided in this configuration if the ventilation fan was kept running during the fire. The risks of inwards-opening doors could not be avoided in modern buildings.

Based on the research results, we recommend that a requirement to consider the fire-induced pressure in building design will be added to the Finnish building codes. Specifically, the evacuation risk of the inwards-opening doors in apartment buildings should be taken into account.