Discounting exit routes

[BHCC]

Morning all

I am after some of your expert advice.

I have been asked to calculate the maximum numbers for a school hall. The hall has 3 double exit doors. 2 on 1 elevation and 1 on the opposite elevation. When discounting the widest door for my calculations do I discount both of the doors on the same elevation and base my calculation on the 1 remaining set of doors?

I believe that I should as there is no protected wall between them. If a fire started on that side of the hall both sets of doors could be taken out of action.

The square meterage of the hall is more than enough to satisfy the amount of people required.

Any help would be fantastic

Mark

[Tom Sutton]

Do the 2 doors on 1 elevation meet the 45 degree rule, check out page 73 of Guide 5 Educational Premises, if it dose then you have three exits.

If it doesn’t then you have two, the two on the one elevation counting as one and you would discount both of those doors on the same elevation and base your calculation on the 1 remaining set of doors.

The numbers of persons the doors can accommodate see page 68.

[slubberdegullion]

If a fire started on that side of the hall both sets of doors could be taken out of action.

That statement, above, is the significant one.  The 45 degree rule can be used as a pretty loose rule of thumb for assessing whether two exits should be treated as one but the question you should always ask is, can they both be taken out by one fire?

For example, in a school hall two of the exits might be quite close together and not satisfy the 45 degree rule, but if there is a particularly high ceiling then any fire would be unlikely to spread sideways very quickly and it might be apparent to you that there would be no way that a single fire could affect both exits within the time required for evacuation.

You may or may not recall the, now removed, recommendation of BS5588 pt 6 that said that if a hall had three or more final exits, then you didn't have to discount any!  The commentary on this is still in part 6 if you want to see their reasoning.  There is some validity in it, it assumes that all fires start small and that they are unlikely to be right in the exit, so some people will always be able to use the exit nearest the fire.  It's gone now as a recommendation, but lingers on as a reasoned argument.

Stu

[nearlythere]

If a fire started on that side of the hall both sets of doors could be taken out of action.

That statement, above, is the significant one.  The 45 degree rule can be used as a pretty loose rule of thumb for assessing whether two exits should be treated as one but the question you should always ask is, can they both be taken out by one fire?

For example, in a school hall two of the exits might be quite close together and not satisfy the 45 degree rule, but if there is a particularly high ceiling then any fire would be unlikely to spread sideways very quickly and it might be apparent to you that there would be no way that a single fire could affect both exits within the time required for evacuation.

You may or may not recall the, now removed, recommendation of BS5588 pt 6 that said that if a hall had three or more final exits, then you didn't have to discount any!  The commentary on this is still in part 6 if you want to see their reasoning.  There is some validity in it, it assumes that all fires start small and that they are unlikely to be right in the exit, so some people will always be able to use the exit nearest the fire.  It's gone now as a recommendation, but lingers on as a reasoned argument.



Stu

Can see the logic where with certain conditions there is no need to discard an exit  but from my interpretation of Pt6 it does not apply to the ground floor.

Don't forget that with the 45 degree rule comes the angle of divergance rule.

If two exits from the hall discharge into a common area then that is one means of escape.

[kurnal]

I agree with the comment that if the two exits discharge to the same area then they must be discounted as a single exit. But other than that it sounds like you have three exits to me and should discount the widest one and calculate capacity of the hall by the sum of the remaining two other exits.

[BHCC]

Thanks for your help. I am checking the 45 degree rule as we speak. Another spanner to throw in is that I am basing my calculations on the ones recommeded in BB100. These are different from other guidance (200 people for 1050mm with an additional 10 per 50mm).

If the hall is being used for a school production then more adults ould be present than when there is a school assembly. I am going to use both factors and use the lowest one I think.

[kurnal]

For a mixed function you could safely use the RRO guidance or ADB. Dont get too hung up on the numbers game - they all originate from the same source data of 40 persons per minute passing theough a 20.5 inch unit of exit width much interpolated and rounded over the years.

[CivvyFSO]

Don't forget that with the 45 degree rule comes the angle of divergance rule.

The angle of divergence has been removed from ADB and there's no mention of it in the new guides either. Basically now you can travel 18m towards the exits, and THEN if the exits meet the 45 degree rule at that point you are ok.

[nearlythere]

Don't forget that with the 45 degree rule comes the angle of divergance rule.

The angle of divergence has been removed from ADB and there's no mention of it in the new guides either. Basically now you can travel 18m towards the exits, and THEN if the exits meet the 45 degree rule at that point you are ok.

Is that not the Divergance Rule Civvy?

[jokar]

The only thing that has been removed is the adding on of 2 1/2 degrees for each metre travelled in a dead end condition which meant in a number of cases the angle of divergence was greater than 45 degrees by 25 or 30 degrees.

[Tom Sutton]

The 45 degree rule can be used as a pretty loose rule of thumb for assessing whether two exits should be treated as one but the question you should always ask is, can they both be taken out by one fire?

For example, in a school hall two of the exits might be quite close together and not satisfy the 45 degree rule, but if there is a particularly high ceiling then any fire would be unlikely to spread sideways very quickly and it might be apparent to you that there would be no way that a single fire could affect both exits within the time required for evacuation.

Stu If you consider the 45 degree rule as a pretty loose rule of thumb what other criteria or calculations in addition to your second paragraph would use the decide if the doors are far enough apart to be considered as two exits?

I know this is straying off the thread and is for my benefit. On your second paragraph I assume you would use smoke calculations or maybe some other method but how would you record them on the risk assessment so third parties would know how you arrived at your decisions?

[slubberdegullion]

That's a good question.

Hmmm....

I'll list a few things that I would consider.  

The first thing would be to keep in mind at all times how the situation may change in the future.

Next, combustibles in the vicinity of the doors, layout of those combustibles, height of the ceiling, wall linings/displays by the doors, distance apart of the doors of course, routes to those doors in the room, travel distances within the room, whether the doors lead to a single route on the other side (as has been stated), numbers of people, density of people, mobility of people.  I probably wouldn't do any calcs though it's a possibility that ASET/RSET calcs could, if done properly, give a good insight.  It's a qualitative thing more than a quantitative thing.  Judgement.  

But I'll come back to what I said - "that's a good question."  You're right - this "judgement" has to be written down in a FRA and made apparent to others that follow.  That's not easy if you're taking into account all the variables I've mentioned plus a bunch of others that you can add.  So, in steps the 45 degree rule.  Simplicity itself.  Everyone can use it.  

But it's not the whole picture.....

Stu

[slubberdegullion]

I'll just add to that last post.

It has to be a qualitative judgement because we're talking about the very early stages of development of the fire, and because of the huge variation in times for fires to reach what we would term "established burning" (i.e. the time it takes for the fire to get to noticable proportions) this period cannot be reliably modelled by quantitative methods.  Reliable models tend to stick to the period after burning becomes established.

And, finally, the reason there is such a huge variation in times to established burning is because the variables I mentioned above and the others that you can add to the list are so numerous and so wide ranging in their influence on the early development of fire.  Established burning may take 10 seconds or 10 minutes and it's not always reasonable to assume the very worst.

Stu

[CivvyFSO]

Is that not the Divergance Rule Civvy?

The "2.5 degrees extra per metre travelled in a single direction" was what I always considered to be the angle of divergence rule.

[Mike Buckley]

Just to stick my sixpenny worth in. Looking at the guides they only show the 45 degree rule applying to rooms with two exits and it looks as if this applies to travel distance. Hence if there are only two doors and they are less than 45 degrees apart then they are counted as escape in one direction only. As there are three doors this shouldn't be a problem.

As far as discounting an exit for occupancy then surely unless there is an obvious hazard ie the two doors opening into a common area, or sited next to each other there should not be a problem.