SFPE (NZ) TECHNICAL PAPER 95 - 5
SPECIFIC EVACUATION DESIGN TO NZBC
Dick Gillespie, Fire Risk Consultants Limited
Introduction
Most evacuation studies to the NZBC are being carried out to the Acceptable Solutions, but there are a number of designs being done by specific calculation. The problem is that no two designers seem to be using the same basic method for determining if a building can be evacuated safely, and some of those designer's design parameters are not stated, which makes the review process difficult if not impossible in some cases.
Since, in most cases, the reviewer is being paid, albeit indirectly, by the designer’s client, it is the designer’s duty to the client to facilitate review of the design.
This paper offers a method for specific evacuation design, within the established component methods available, identifying, without setting, the design parameters, and suggests minimum requirements to facilitate review.
Basic Method
A series of potential fires is considered to establish, in terms of evacuation, what is the worst case.
Each fire should be modelled as being a reasonable worst case for that space.
Only one fire is considered for each component scenario. Although it may be convenient to include second fires in other scenarios to eliminate them as potential worst cases.
For each of these potential fires the time for which the fire space remains tenable should be established, also it needs to be established if the selected fire threatens any part of an exitway, either by breaching a fire wall (after its fire rating time) or by radiation through an opening/window.
For each fire space its evacuation time should be calculated, assuming that evacuees will start to move immediately on hearing the alarm. A fire space safety factor is derived by dividing the tenable time limit by this evacuation time.
For each other occupied space an evacuation time should also be calculated, as above except that a start up delay should be included after the alarm has sounded and before movement starts towards the exits.
In both of these evacuation times a suitable way finding bias needs to be applied to allow for the occupants to choose between familiar and unfamiliar exits. This bias should be applied to the theoretical capacity of the exits from the space. Until some credible research establishes typical figures for way finding bias, an upper and lower bias should be estimated and both calculated for.
Movement through the exitway system is modelled using a ‘ball bearing’ type of model. Model loadings being taken from the exitway throughput as established by the space evacuation. A building safety factor is derived as the ratio of the exitway firecell rating to the time it takes to complete the whole evacuation.
8 Exitway Tenability
If the hot layer is hot enough to cause concern, or exitways are exposed, a cumulative fraction of radiation injury threshold is calculated for those parts of all exit routes with exposure to radiation from the potential fire being considered and its hot layer.
To aid design review the following is a suggested as a minimum amount of information to be included in the fire safety report :
a) A full calculation should be provided for the worst case.
c) The calculation parameters assumed :
ii Start up delay(s)
iii The basic egress velocity used.
iv Tenability limits in terms of minimum layer height, illuminescence and radiation threshold.
v Egress velocity reduction for low layer height and/or low lighting levels, if relevant
vi Exitway movement model used
Source should be provided where parameters are taken from the literature.
This completes 'the method', what follows is a more detailed and personal look at the parts of the method.
Discussion
In the discussion below there are some suggestions for parameter values. The values considered acceptable for these parameters varies according to viewpoint and experience. However one would hope that this variance will reduce as a consensus develops, which will only happen if the various views are brought into the open and discussed.
Safety Factor Assessment
This should be determined for each potential fire scenario and the worst of these is the fire space evacuation safety factor. What range of safety factor is acceptable to the Authority Having Jurisdiction depends on the nature of the occupancy being considered and the effect this has on the various factors either assumed or not considered such as :
The awareness of the occupants
The mobility of the occupants
The way finding of the occupants
The size and growth rate of the model fire(s)
The location of the model fire(s)
The nature and reliability of smoke control equipment
and other factors I havn't thought of......
Thus a particular design can be allocated an egress safety factor which can be compared with what is acceptable to society (as determined by the AHJ). A minimum safety factor of 2 is suggested as appropriate.
Tenability
The NZBC C2 requires that a building's occupants are able to escape without suffering injury. Injury has not been legally defined (yet) and until it is It is suggested that the injury threshold be defined as injury sufficient to cause permanent damage to human tissue. For example 2nd degree (blistering) burns which can cause scarring would be above the threshold but 1st degree burns (skin reddening) would be below it. The same conditions that limit tenability also can reduce egress velocity, and therefore increase an evacuee's exposure to the condition. There are three main conditions that limit tenability and/or egress velocity :
lighting level, layer height/toxicity and exposure to radiant heat.
The ability of a normally sighted evacuee to find and get to an exit is severely prejudiced by a lack of sufficient light to see his or her way. Tests on blindfolded people1 have shown that egress velocities are reduced to 1/3 of their normally sighted velocity. As the hot layer descends high level lighting is reduced in intensity. The depth of layer at which this occurs depends on the optical density of the hot layer, this varies with the fire characteristics and can either be estimated intuitively from evidence such as live fire videos of can be obtained from fire models such as fire simulator. Illuminated exit signs and emergency lighting should be placed as low as possible, since those at high level will become less effective as a hot layer descends.
Layer Height
The height of the hot layer in the space is obviously critical to continuing evacuation, whilst the layer is well above head height evacuees should be able to travel at their best speed.
In order to offer prudently conservative design one should assume that the air inside the hot layer is sufficiently toxic to cause 'injury' therefore as the layer descends below head height evacuees will have to bend over and their travel speed will slow. This has not been quantified by research and it would seem best to avoid designs where the hot layer is below head height, but if used the velocity reduction estimate needs to be stated in order for the reviewer to be able to assess the calculation properly. Crawling on hands and knees (velocity = 1/10 x basic ?) may be possible for short distances under a layer 1m or so above the floor but crawling on one's stomach would seem to be part of an unsuccessful design. Velocity reduction factors due to layer height and poor light levels should be multiplied by each other before use.
What is head height ? Firstly one must remember that the underside of a hot layer is not flat and that modelling delivers a mean height. Some basketball players might be 2.2 or 2.3 m tall but designing a child care centre for layer heights safe for these exceptional people would seem overly conservative, on the other hand designing for a high layer height at a sports complex including basketball courts would not. Current consensus would seem to favour 1.9 - 2 m minimum layer heights for normal egress velocities but no lower than 1.5m (FEDG) with reduced egress velocity. A velocity reduction factor needs to be applied to and stated for egress velocities under hot layers below head height when walking upright.
Exposure to radiant heat
If the layer is hot enough radiant heat from it could injure evacuees beneath it. The effect of radiation is cumulative above a threshold level which will not cause 'injury', above this threshold the length of exposure and temperature both have to be considered2. Normally this only becomes significant in large un-sprinklered spaces, but a calculation should be done where the layer temperature rises above 143ºC before evacuation is complete.
Lower Layer Tenability
It is possible that the lower layer becomes hot enough or is sufficiently toxic to form a limit on tenability, but this is unlikely unless there is an artificial mixing of upper layer gasses into the lower layer. Therefore the lower layer should be considered tenable unless the ventilation is arranged so that upper layer gasses are entrained in the lower layer either by the air supply to the fire, air conditioning or the air supply induced by a mechanical smoke extraction system through high level inlets.
Evacuation Parameters
Typically the 5 phase evacuation (FEDG) can be reduced to 3 phases : 1 the time to alarm, 2 the time it takes for evacuees to start moving and 3 the time it takes for them to reach a safe place.
Alarm delay
Thermal fire detector response can be modelled successfully using commonly available computer models, if the detector RTI is known) and smoke detector response can also be modelled by some of those programs if the characteristics of the detectors and alarm system is known (obscuration / m and electronic delays etc.)
The 'equivalent to thermal' approach suggested in FIRE SIMULATOR3 and some other programs offers an approximate time at best and does not allow the delays inherent in the smoke travel into the detector and the electronic delays associated with the detector scan rate or if part of an addressable detection system the delays inherent in the loop address and processing.
Any design based on smoke detector initiated alarm in less than 30 seconds should be considered suspect.
Start Up delay
For occupants in the room of fire origin this may even be negative (if they see the fire before the alarm sounds) but zero is suggested as normally appropriate. For people in rooms where the fire may not be so obvious start up delays should be taken from appropriate research such as that by Jonathan Simes4.
Travel distance
For each occupied fire cell determine the shortest escape route from the furthest occupiable point in the fire cell to a place of safety, as defined by a place isolated from the effect causing the tenability limit, and calculate the egress time for the space using the longest escape route.
If furniture layouts (and partitioning layouts) are known and fixed then the route should avoid the furniture and walls. For designs where the layout is not known or is able to be changed without a further Building Consent the longest possible straight line route should be multiplied by 1.4 to provide a reasonable allowance for obstructions.
Egress speed
The basic egress velocity should be based on occupant density and selected from appropriate research on normal able bodied people such as that by Pauls5.
The occupant density of fire cells is simply the number of occupants divided by the walkable floor area. This can be used to establish the flow rate of people arriving at and, as an initial estimate, passing through an exit into the exitway system. Note that walkable floor space can be changed without requiring a building consent, so designs should use prudent estimates of the walkable area if there is a possibility that future layout changes could increase the area free of obstructions.
If it takes longer for all the occupants to pass the exit door(s) than the travel time from the most distant point in the firecell then the travel time can be ignored, and the time taken to clear the space is the time to pass the door(s). If not, then the last arriving occupant can expect not to be delayed in passing into the exitway and the rate that the firecell's occupants start into the exitway system is dependant on their arrival rate at the exit (usually assumed to be a uniform rate).
Way finding
Deciding which exit to use, like start up delay, is subject to psychological factors with evacuees show a preference for leaving by the way they entered the space. The literature 4, 6 discusses this in a general way but makes few, if any, specific recommendations. An exit preference factor is obviously needed, which in the absence of properly researched data must be estimated.
To promote discussion towards consensus on this point it is suggested that an exit preference factor of 1/2 to 1/3 be applied to the effective width of unfamiliar exits, when allocating evacuees to the exits available.
Other (non-fire) occupied spaces
Should be treated as for the fire space clearance except a start up delay needs to be allowed for.
Exitway system
Once into the exitway system evacuees will have little choice of direction and their flow can be modelled mathematically using a 'ball bearing' model. For systems with any significant complexity a computer model such as Evacnet7 can be used. For simpler systems the flow can be calculated according to a recognised method from the literature. Models such as Exitt8 which have in-built human behaviour factors are only valid where the real occupants match the profiles in the model. Exitt was written for domestic residential occupancies and is not easy to use for commercial occupancies. The next version of Exitt is expected to include a range of evacuee profiles easier to use in commercial occupancies.
References
1 T Jin : report of FRI Japan Vol 2 No 33
2 Clements & Gillespie, ICFRE 1995 Proceedings page 209
3 Fire Simulator is a module of the FPETOOL software suite available from NIST
4 J Simes, Chapter 5 of An introduction to fire safety engineering design : Chapman & Hall 1993
5 J Pauls SFPE Handbook 2nd Ed Ch 3-13
6 Various authors SFPE Handbook 2nd Ed Ch 3-12, 3-13 & 3-14
7 Evacnet software by TM Kisko & RL Francis, University of Florida : 1986
8 Exitt is a module of the Hazard I software suite available from NIST