CFPA-E Guidelines

[Tom Sutton]

I have never been able to fully understand the relationship between travel distances and travel time and how they interact. Also how to use ASET RSET, for instances where does the data come from. During some research I came across a document "CFPA E Guideline No 19 2009 Fire Safety Engineering concerning Evacuation from Buildings" which helped a lot.

There are many other guidelines that some of you may find useful check out http://www.cfpa-e.eu/Default.asp?Id=5.

[colin todd]

Thomas, travel distance and travel time do not interract.  Simple.

[kurnal]

Dont understand Colins stand on this. I would suggest it takes you longer to walk a long way than it it does a to walk a short distance.

So if a fire is likely to develop quickly you might not have time to walk very far before you cannot see in the smoke or the temperature or toxic gases get to you.

In summary the RSET takes account of how quickly the fire will be detected, how quickly people will respond, how far they have to travel, how fast they will walk on their way to the place of safety. This is the required safe evacuation time.

Then there is the available safe evacuation time ASET. How long it will take from ignition to conditions in the building becoming untenable.

If the ASET exceeds the RSET - Result Happiness. If RSET exceeds ASET - Result tragedy.

In practice there needs to be a safety margin- ASET must exceed RSET by the safety margin.

Reducing the distance to be travelled is one way of increasing the safety margin, on the other hand if ASET greatly exceeds RSET then travel distances can be extended to the limit determined by the safety margin.

ASET is calculated using computerised systems including CFD modelling.

[Dinnertime Dave]

In practice there needs to be a safety margin- ASET must exceed RSET by the safety margin.

Unfortunately this is the point where things can get contentious. What is a safe margin?  Do we look for 50% more, 100% more?       

[Tom Sutton]

Thomas, travel distance and travel time do not interract.  Simple. 

I agree Colin and yes simple. After reading the first paragraph of Guideline No 19 the penny dropped.

1 Introduction

Saving human life is the most important objective in fire protection processes. Proper evacuation plans are essential to reach this objective. Two methods can be considered:

•   The first is the prescriptive method which deals with the size and number of exits together with the maximum length of the escape routes;
•   The second is the performance method which deals with the maximum time limit for evacuation.

The first method is based on three main points:

•   density of people;
•   flow of people;
•   length and width of evacuation routes.

The second method establishes if the required evacuation time is less than available safety egress time.

My mistake was I thought there was only one method and could not see how travel time could equate with travel distance. Now I realize travel distance and empirical knowledge is the basis for method 1 and travel time and scientific knowledge is the basis for method 2. They are totally separate.

 Kurnal I understood the mechanics of ASET and RSET but I could not see how you would use them in relation to the CLG guides. Of course you don’t the CLG guides are simply a combination of a fire precautions survey and the FPA guides with a large dollop of flexibility.( Method 1) ASET and RSET is all about fire engineering solutions ( Method 2). I am fully conversant with Method 1 and pretend Method 2 doesn’t exist but guideline No 19 does explain it fully.

[nearlythere]

I notice in CFPA Europe guidance in FS in Homes for the Elderly - Oct 2011 - there are three stages of evacuation and time.

Evacuation of the room of origin in 2 - 3 mins.
Evacuation from an adjoining room in the same fire situation in up to 15mins.
Evacuation from neighbouring compartment in up to 30 - 60 mins.


Is this at odds with our thinking with evacuation of a zone within 2.5mins?

[kurnal]

Tom the commentary to clause 16 of PAS 79 expresses the inter relationships quite well too and suggests that there can be an overlap between the two approaches.

[Tom Sutton]

Kurnal I wouldn't fully agree, it says, In assessing the likely consequences of fire, for the purpose of carrying out the fire risk assessment to which this PAS relates, it is not normally necessary, or appropriate, to carry out calculations of the type used in the practice of fire safety engineering. However, the principles of fire safety engineering may be used, in a subjective manner, to assess the likely consequences of fire, using the principle of "timelines" that forms the basis of fire safety engineering.

The way I understand the above is that it not normally necessary, or appropriate, to use ASET/RSET, however using the principles of ASET/RSET in a subjective manner is acceptable. For example if the travel distance is excessive to that prescribed in the relevant code of practice, you could use the time line and introduce compensatory conditions to increase the evacuation time, therefore the travel distance, for instance reducing the time to alarm or by improving fire management.

[kurnal]

Indeed. But buildings are fire engineered at design stage  often by somebody who will actually never see the finished building.
I agree that it is inappropriate to go back to the drawing board and re model the design but it is essential that the fire risk assessor has regard to the design parameters and ensures that the way the building is managed and used is consistent with the design as engineered. And - to pick up on Daves point above- the safety margin and whether it is appropriate and suitable. The fire engineered solution is not a holy grail, somebody must ensure that it is and remains valid. This is the role of the fire risk assessor.
(I was looking at proposals recently for a large warehouse  with 140m travel distance and a declared ASET/RSET safety margin clearly shown on the approved plans as 15 seconds.)

This is where the  overlap occurs. As a fire risk assessor we will review and make a qualitative judgement on whether the variation from the engineered solution is tolerable or on the other hand whether it is so significant as to warrant a complete review of the modelling by the engineer responsible. Apart from the FSO2005 the Building Legislation need also be considered in a new fire engineered building.

[Tom Sutton]

Thanks Kurnal certainly something to consider and so as a competent FR accessor you would need to have a least a working knowledge of ASET/RSET but not necessarily a deep understanding to a point that you could produce a fire engineering solution.

[Fishy]

In concept, the design of acceptable means of escape is all about time – will people be able to commence their evacuation and get to a place where they are reasonably safe from the immediate effects of the fire within a relatively short period of time (i.e. whilst the fire remains ‘small’)?  Can they then get to a place of safety without going back into a higher fire risk area, & before the fire grows to such an extent that they are exposed to unacceptable risk?  In most buildings distance will probably be the prime influence on time only where there are no significant flow restrictions (i.e. where there is nowhere on the escape route where people have to wait for a significant period of time) or where there are dead ends.

Having said this, if you look at the England & Wales Approved Document ‘B’ (for example), it doesn’t directly measure ‘success’ as regards design of means of escape in terms of time – only distance and capacity (width, mostly).  Time will have influenced the distance/capacity criteria in the document, but it is not used as a criterion in itself.   BS 9999 is the same.

For design purposes time is usually, therefore, not used as criterion for means of escape.  The exception would be fire engineering designs, using ASET/RSET, where you would normally model the RSET, using whatever people movement models are validated for the relevant environment, taking into account pre-movement times, etc.  BS 7974, CIBSE Guide E etc give guidance on how to do this.

[colin todd]

Tam There is hope for you old timers yet!  You have hit the nail on the head perfectly. Forget all the gobbldegook that you may have read above.  Your understanding is perfect my friend. Travel distance as used in prescriptive codes is an arbitrary number that was never linked to time. Those who fail to understand that (which I am delighted to note does not include you, Tam) do not understand the basis for prescriptive codes. How worrying is that! No wonder people wanted to chuck them away.

[kurnal]

Well thats as may be but Toms link has given me an idea for a new board which will link to useful websites, archives and sources of data. See the Fire Safety Board.

[nearlythere]

Good move K. Very useful.

[wee brian]

We do need to tread carefully with some of this malarkey.

the ASET RSET approach is quite simplistic.  it doesn't really deal with dead ends very well.

people get trapped by fires when they get a bit too scary, not when it is unsafe to pass. fires can also grow out of sight.

As people have already said the 18m dead end rule has no basis in science, but, if a fire is so far away that you don't see it until its too late to escape past it then the dead end is too long. ASET & RSET calcs don't tend to address this. Maybe the 18m isn't a bad rule of thumb!