FIRE ENGINEERING (FIRE SAFETY - ISI)

Smoke extraction engineering ensures that in the event of a fire, the occupants of the facility can evacuate into a breathable atmosphere, with a bearable temperature and a sufficient visibility.

This approach makes it possible to better understand the smoke propagation phenomena, according to different situations:

• Complex volumes
• Modification of the locations of vents
• Modification of extraction rates
• Small extraction vents’ areas,
• Long or bulky containment blocks,
• Insufficient air supply,
• Overpressure,
• High ceiling,
• Non-compliant car parks,
• Multi-level hoppers,
• Non-isolated related buildings requiring a demonstration of independence.

Smoke extraction engineering aims to assess the effectiveness of a smoke extraction solution by evaluating in a global, concrete and appropriate manner, the evacuation conditions of the facility in a realistic fire situation.

Smoke extraction studies are based on several numerical modeling and simulation tools. The most widely used ones are the zone fire models (CFAST) and CFD models (Fire Dynamics Simulator - FDS) developed by the NIST:

• CFAST: Two-zone fire model which predicts the temperatures of the hot and cold layers in each room, the smoke layer height, the concentration of toxic components and the opacity.
• FDS: widely recognized and used computational fluid dynamics’ software for fire modelling in the context of fire safety engineering studies (ISI). This software models three-dimensional physical phenomena associated to both the development of a fire and the transport of its effluents (combustion gases).

A smoke extraction engineering study ultimately aims to:

• Assess the performance of different solutions to support decision-making.
• Justify the effectiveness of a smoke extraction solution that does not meet the regulations’ prescriptive requirements.

In terms of fire safety, it is also essential to be concerned with the evacuation of people. 

Evacuation engineering aims to help operators in the implementation of safety procedures for their facilities and the definition of evacuation strategies.

This approach is essentially based on estimating the evacuation time which corresponds to the time interval that elapses between the start of the fire and the moment when the last occupant enters a safety zone.

The evacuation time estimation depends on several parameters: reaction time, walking speed, location and number of emergency exits, clearances of escape ways, stairs and doors, presence of obstacles in the facility, tenability conditions (toxicity, opacity, ambient temperature and radiation), etc.

The estimation of this time is done according to one of these two models:

• A simplified model which is essentially based on the calculation of the required evacuation time according to the speed of movement of a human during an evacuation scenario, as well as the flow of people at doors and on stairs.
• Numerical simulation models. The most widely used tool in evacuation modeling is the EVAC software developed by the Technical Research Center of Finland (VTT). It is fully embedded in the Fire Dynamics Simulator (FDS) software and thus, benefits from its predictions concerning the tenability conditions in buildings. The evacuation model used by EVAC requires, for each floor of the building, the creation of a "directions field" calculated using a 2D mesh, which is completely independent of the mesh used by FDS.

Evacuation engineering ultimately aims to:

• Quantify the required safe escape time (RSET) which is to be compared to the available safe escape time (ASET).
• Quantify different evacuation scenarios.
•  Optimize the building evacuation strategy.

Any type of establishment is subject to regulatory requirements in terms of fire safety, which require the structures to have a fire resistance degree in order to preserve the stability of the building and to prevent the rapid spread of fire for the minimum required time to alarm and evacuate the occupants. 

These requirements impose generic degrees of fire resistance which are determined under standardized fire tests. Hence, these are based on a prescriptive approach of the fire safety (Eurocodes).

The effects of a fire can affect the load-bearing capacity of a facility structures, so it might compromise its resistance and result in the building collapse. This being, it is judicious to apply the fire safety engineering principles in a performance-based approach of the fire behavior of structures, in parallel to the prescriptive one. This approach, mainly aims to characterize the thermal effects (flow, temperature) on the load-bearing elements of the structures under real fire conditions, using a 3D fire calculation (modeling and simulation).

The different calculation steps require the use of specific calculation software:

• Fire development tools (simplified models, fire zone models, fluid mechanics models like FDS),
• Finite element codes for heat transfer calculations (SAFIR, ANSYS),
• Finite element codes for the fire behavior of structures (SAFIR, ANSYS, LENAS).

The structural fire behavior engineering allows to optimize the measures proposed to ensure the mechanical resistance of structures, while still guaranteeing an optimum level of safety in a fire situation.

Thanks to its engineers’ skills in fire safety, Safengy can intervene on complex projects such as tunnels, stations and other Public Receiving Facilities (ERP) or even in nuclear power plants. 

Our expertise allows us to offer: 

• Studies in smoke extraction engineering
• Fire modeling and simulation 
• Calculations via FDS, CFD and CFAST

In addition to these skills, Safengy's expertise in related disciplines is complementary, since we are able to further develop fire protection and fire and gas detection studies for the same project.