The LWF Blog

ASET vs RSET Part One| Available Safe and Required Safe Egress time comparison

November 15, 2013 3:55 pm

Spaciousness and flexibility are key elements of modern architecture. Open plan living, open plan working and large un-divided spaces offer the possibility for tailor made interior designs and flexible use for the entire lifetime of a building. 

With regards to fire safety, this design philosophy often conflicts with prescriptive regulations, as prescriptive guidance documents are intended to provide guidance for some of the more common building situations. Traditionally, prescriptive design is based on passive fire protection which includes the sub-division of spaces into compartments, to contain a fire in the location where it occurred, and on the selection of building materials and methods to reduce the size and spread of a fire.

The main objective of passive fire protection is to enable people to reach a place of safety before conditions for evacuations become untenable.

Fire Safety Engineering is recognized within the Building Regulations as a potential means of providing an alternative approach to prescriptive fire safety solutions. Fire Engineering uses engineering tools to assess if the proposed specific building design will ensure a sufficient level of life safety or whether additional fire safety precautions are required. 

When considering means of escape, the prescriptive approach focuses on a building’s structure – ignoring the fact that both fire hazard and safe evacuation are time-dependent. Using performance based (fire safety engineering) design allows more accuracy, while still observing general guidance in a benchmark sense.

To implement fire safe designs that are not compliant with current legislation, the fire engineer must verify that the level of fire safety in the proposed design is equivalent to the level of safety outlined in current Building Regulations.

A common fire engineering tool to analyse the conditions in a building in the event of a fire is the ASET vs. RSET comparison.

The basic aim of this approach is to show that the calculated time before conditions become untenable (Available Safe Egress Time- ASET) always exceeds the required time to safely evacuate the building (Required Safe Egress Time – RSET). As such, safe evacuation of a building is always possible.


The ASET is generally defined by acceptance criteria based on temperature, visibility and toxicity of smoke within the fire escape routes. Guidance documents give advice about the tenability limits and give maximum/minimum figures for these criteria.

The ASET can be modified and extended based on many factors that are not necessarily considered and specifically addressed within the prescriptive approach. For example:

 Type and amount of combustible material/fire load

 Ceiling heights

 Smoke ventilation systems

 Physical barriers like downstands/smoke curtains

 Geometry of the room of fire origin

A fire engineering approach utilizes these factors to calculate the specific ASET for the building or space, always considering adequate fire safety factors, to maintain a conservative approach. 

While hand calculations remain a possibility to carry out such assessments, more visually user-friendly methods, such as computer modelling, are available to assess defined assessment criteria.  

Two types of computer models are available for use when designing fire scenarios: the zone model and the CFD (Computational Fluid Dynamics) model.   Zone models are used to predict the height of smoke layers and tenability conditions in buildings or rooms with simple geometries. 

If assessment areas are more complex, the fire engineer usually uses a CFD model that is considered to be the most appropriate tool to assess conditions in the event of a fire. 

The basic capabilities of CFD models have been continuously validated, considering issues such as fire growth, flame spread, suppression, sprinkler/detector activation, and other fire specific phenomena.

So, now that the ASET is theoretically calculated, the RSET is the essential remaining part of the equation. 

Next week’s LWF blog article will look at how to calculate the RSET and will conclude this short series of two.

If you have any queries about fire engineering, or would like to speak to someone about ASET vs RSET, please call Peter Gyere at Lawrence Webster Forrest on 020 8668 8663.

LWF was established more than twenty years ago to provide fire engineering and fire risk management consultancy, develop fire engineered technology and to apply fire safety standards including fire engineered techniques.

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