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Fire Engineering Occupant Evacuations

Sub-system E – Occupant Evacuation and Control is the main section of IFEG pertinent to assessing occupant evacuations and for consideration of the effectiveness of emergency exit signs.[1] The section includes consideration for the following:

  • Evacuation plans
  • Occupant training
  • Emergency communications
  • Egress signage

Sub-system E (SS-E) is used to analyse the evacuation of the occupants of a building. By doing so it enables estimates to be calculated during an evacuation to determine the time from fire initiation required for occupants to reach a place of safety. This required amount of time is referred to as the ‘Required Safe Evacuation Time’ or RSET.

RSET comprises a number of components that vary according to the number and locations of occupants with respect to the fire. This timeline includes the following in order of occurrence:[2]

  • Fire initiation (t0) is time zero for the analysis of the fire, evacuation and determination of RSET
  • Occurrence of cue (tc) is the time of a cue that indicates the occurrence of a fire. The cue may be from an automatic alarm device, aspects of the fire itself or people warning others
  • Recognition of cue (tr) is the time at which occupants, having received a cue, recognize it as an indication of a fire
  • Initiation of movement (td) is the time at which occupants begin the evacuation movement. This may occur after a delay during which occupants carry out other actions (including ‘no action’)
  • Completion of movement ™ is the time when occupants reach an (internal or external) safe area.

All these events (or points in time) are separated by time periods that comprise the components of RSET. These event are used to define the components of RSET, including:

  • Cue period (Pc)
  • Response period (Pr)
  • Delay period (Pd)
  • Movement period (Pm).

Various phases may be identified to represent one or more of the above periods during the stages of the RSET, including:

  • Detection phase = Pc
  • Pre-movement phase = Pr + Pd
  • Movement phase = Pm
  • Evacuation phase = Pr + Pd + Pm
  • RSET = Pc + Pr + Pd + Pm

The IFEG points out that when considering the RSET in any fire occurrence there could be differing evacuation management plans in place that may necessitate any one of the following evacuation strategies:

  • Staged evacuation: “In high-rise buildings with an emergency warning and intercommunication, the evacuation maybe managed by trained personnel, with occupants on floors furthest from the fire placed initially on alert and evacuated progressively only if the fire continues to develop.
  • Assembly in refuge areas: “In particular types of buildings, the concept of a fire safe refuge, where occupants go to a special fire compartment to await rescue by the fire service rather than evacuate, is sometimes used.
  • Protect in Place: “A further and more recent development is the ‘protect in place’ concept. Occupants are encouraged to remain where they are, rather than try to evacuate through potentially smoke-filled corridors and/or stairs.

The IFEG states that in all “buildings the consideration should be given to the question of providing safety for persons with disabilities. Use of refuges and use of elevators for evacuation of persons with disabilities, are some of the options that may be considered.

Considering the ability to provide an accessible means of egress from all accessible parts of a building in the Fire Engineering Brief will ensure that the needs of all occupants are quantified. An occupant analysis can be undertaken to determine the likely occupant groups, the abilities and estimated evacuation speeds can them form part of the fire engineering process.

This analysis should consider the likelihood that an evacuation by people with activity limitations, or family group members with such limitations, will be more successful when they have suitable signage in place to help make informed decisions on which route to take. On this basis, it would be advantageous to provide suitable signage to identify all components of the accessible means of egress (see Part 6 of this paper) to help reduce the evacuation phase (being Pr + Pd + Pm) and in turn the RSET.

[1] International Fire Engineering Guidelines, p.1.8-1

[2] International Fire Engineering Guidelines, p.1.8-2

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Buildings need exit and emergency signs to identify parts of the accessible means of egress. An Accessible Exit Sign Project Initiative.

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