Lawrence Webster Forrest (LWF), Fire Engineering and Fire Risk Management Consultants
Lawrence Webster Forrest (LWF), Fire Engineering and Fire Risk Management Consultants



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E-mail: fire@lwf.co.uk

Arc28 Evacuation - The Human Factor

Right click to download this fileArc28.pdf

Passive fire protection has traditionally been the first line of defence for life safety in buildings. Such measures typically include compartmentation, as well as selecting building materials and methods that will reduce the size and spread of the fire. Buildings are also designed to allow people to escape in the event of an emergency.

“Standards for fire protection in new buildings have been applied through Bylaws or Regulations for over a century. Regulatory control has mainly been achieved through a framework of prescriptive rules, which depend heavily on simple standard fire tests and Codes of Practise. Levels of performance have been arrived at and are modified largely on the basis of experience. It has been argued that prescriptive rules are highly
empirical and enforced more or less rigidly and could lead to costly over-designs.” associated with a broad range of hazards. In large or complex buildings, the design philosophy often conflicts with prescriptive regulations, so an alternative solution is required. Using performance-based (fire safety engineering) design allows us to be specific, while still observing general guidance in a benchmark sense.

A building’s means of escape should be designed so that the calculated time available before conditions become untenable (Available Safe Escape Time, ASET) exceeds the Required Safe Escape Time (RSET) needed to evacuate the building with a reasonable margin of safety.

When quantifying escape and evacuation times, we assume that time will elapse before the fire is detected (Δ tdet) and each occupant:

1. becomes aware of the fire (Δ ta)
2. recognises and reacts to the alarm ( Δ tpre)
3. travels to a place of safety (Δ ttrav).

These stages are illustrated in figure 1, comprise the total design escape time for the occupants to reach a place of safety.

The evacuation time is the interval between the time at which a warning of a fire is transmitted to the occupants of a space and the time at which those occupants are able to reach a place of safety3.

Δtevac = Δtpre + Δ ttrav

Where:

Δ tevac is the evacuation time
Δ tpre is the pre-movement time
Δ ttrav is the travel time

Pre-movement time (Δ tpre)

Pre-movement time begins at an alarm or cue and ends when travel towards an exit has begun. It has two behavioural elements for each individual occupant – recognition and response times. These determine the pre-movement time for each individual person.

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