Definition of a compartment

[SidM]

Why is a protected corridor not a compartment for the purpose of stopping travel distance?

[jokar]

Presumably because it is not totally sterile.

[Phoenix]

You stop measuring travel distances when a point is reached beyond which the route is virtually guaranteed to be safe. 

A final exit works because from that point on people can disperse safely away from the building (note that you probably would not stop measuring travel distance at a final exit that leads to a passageway down the side of a building where the route is unprotected from a fire within the building).

A storey exit into a protected staircase works because from that point on people are separated from the fire by fire resisting structure and can make their way to the final exit that discharges from the staircase to ultimate safety, and because doors onto staircases tend, in general, to be kept shut by the occupants.  I know you can think of a dozen examples where the occupants use wedges but we're meant to police and stop that.

An exit that leads to a horizontal protected route does not work because it might have the fire resisting structure but the general tendency is for occupants to keep doors into corridors open.  Also, the odd photocopier creeps out into the corridor and display boards tend to get put on walls.  Simply, a protected corridor is just not reliable enough to end a travel distance.   

Now that you know that, you also know what you have to do if you want to argue the case to stop a travel distance at an exit to a protected corridor.  Not usually an easy argument to win.

Stu

[Phoenix]

Incidentally, an exit that leads to another compartment is ok for stopping travel distance because beyond that exit people are separated from the fire.  However, it is no good for stopping travel distance if it is the only route out of the compartment under inspection (unless it leads to a protected staircase).  This is because the fire might be in that other compartment and might be affecting the means of escape beyond the door that leads to that compartment.  The guides simply say that travel distances can stop at an exit to a separate compartment but the proviso I mention is important to remember.  I have seen people get this wrong.

Stu

[SidM]

A risk assessor for a school has argued that in a 35m dead end you come out of the classroom, through some corridor doors and stop TD in a protected corridor with classrooms in it; the argument being that there is AFD in the classrooms and you would get early warning and be able to get out before the 60 children are affected by fire.  So, could someone tell me exactly why you can't stop TD in a protected corridor?

[nearlythere]

A risk assessor for a school has argued that in a 35m dead end you come out of the classroom, through some corridor doors and stop TD in a protected corridor with classrooms in it; the argument being that there is AFD in the classrooms and you would get early warning and be able to get out before the 60 children are affected by fire.  So, could someone tell me exactly why you can't stop TD in a protected corridor?

Think Sid there has to be a cut off point for horizontal travel distances otherwise you could have a protected corrider 200M long and spend ages getting to the storey exit.
Some clever person has come up with the benchmark distances that we all know, love and question so well and it has been incorporated in the books.Your RAer has made an assessment of the situation and considered that the measures in place can allow such a horizontal travel distance.
I certainly don't agree with this and would not accept it as a valid compensatory factor.

[Tom Sutton]

So, could someone tell me exactly why you can't stop TD in a protected corridor?

The code hugging answer is the guidance says so, (Educational Premises)

The distance should be measured from all parts of the premises to the nearest place of reasonable safety which is:

• a protected stairway enclosure (a storey exit);
• a separate fire compartment from which there is a final exit to a place of total safety; or
• the nearest available final exit.

The risk assessment answer is you can provided you can justify it.

In previous guidance horizontal protected routes were always accepted as a place of comparative safety.

[CivvyFSO]

So, could someone tell me exactly why you can't stop TD in a protected corridor?

I would agree with a mix of what NT said and what Stu said.

Mentally turn the corridor into a vertical escape route. You are relying on one route with potentially many doors opening up onto it, many of which are quite likely to be open at any one time. In a single stair condition we generally limit the number of floors, unless extra provision is made, i.e. Lobbies, pressurised stair etc. As soon as we have 2 stairs we know we should have a safe alternative route. People walking into the corridor do not have the luxury of an alternative corridor.

That being said, with 2 directional travel in the corridor, some smoke-stop doors, it would not be too hard to justify extended travel distances. i.e. Don't 'stop' the travel distance, simply justify the actual distance by being able to turn your back on it and walk away). Dead ends are another problem entirely.

That risk assessor should also be considering that schools have a serious problem with arson, and that problem is surfacing more and more in the daytime while schools are occupied now.

[Phoenix]

So, could someone tell me exactly why you can't stop TD in a protected corridor?

Hi Sid,

I think I did that when I said that they are not considered reliable enough.  And others have added their useful comments to give a pretty complete picture. 

I am slightly surprised that this school ever got built like this in the first place.  I cannot imagine it being a new building because it is so far off the recommendations that have been around for many years, so it must be an old building. 

If it is then I can understand the approach of the fire risk assessor you talk of.  He's gone in and been faced with a very long dead end that is way above the recommended limit.  What's he to do?  Recommend an additional exit?  Well, I think most would agree that that would not be a bad idea.  But there might be practical problems with that solution.  If there are, what then?  Think of some reason why it might be acceptable?

Now, of course, none of us (who have pretty universally condemned this situation) know anything of the building or its management.  So we cannot really give an accurate judgement on this particular scenario.  But maybe there do exist good reasons for considering that the means of escape are acceptable.  I not suggesting that there are, I'm just saying that there might be.

If I had been that fire risk assessor and I had wanted to demonstrate that it was a satisfactory layout, I would not have mentioned ending the travel distance at the corridor doors because that isn't really what it's about.  The problem is not about complying with some fundamentally arbitrary distance limitation, it is about demonstrating that everyone who resorts to that part of the building is safe.  That means leaving the world of travel distances and entering the world of available safe escape times and required safe escape times.  I'm not talking of going over to quantitaive fire engineering techniques but of making a qualitative assessment of whether everyone can get out safely under any foreseeable fire conditions.

If there is good fire separation to this protected corridor and there is good automatic fire detection protecting the dead end and there is evidence of good response times to fire alarms (and don't forget to consider night schools and parent evenings, etc) and there is evidence of good corporate fire policy making and enforcement (particularly with regard to maintenance) within the organisation then I might consider that the dead end is not too significant a problem.  If any of these were in question then I would probably judge the dead end to be excessive and recommend that an alternative escape route is provided.

I could go on, but it's saturday night...

Stu



 

[Tom Sutton]

That means leaving the world of travel distances and entering the world of available safe escape times and required safe escape times.

I agree with you Stu, other than, are you leaving the world of travel distances once you have calculated the safe escape times you must convert them to travel distances to apply it to the situation. The arbitrary travel distance to a place of comparative safety is the bench mark and if you exceed it you must justify it, preferably in writing.

[CivvyFSO]

TW, I see where you are coming from, but you don't convert anything back into travel distances. You calculate the ASET, you calculate the RSET, so long as you have a good margin of safety between them it shows that the escape route remains safe through evacuation. The travel distance is just a variable that is fed into the RSET calc, but the result is time-based, not distance based.

[Tom Sutton]

Thanks Civvy I afraid BS 7974 is a complete mystery to me but I do know about ASET through Pas 79. I think can see how ASET/RSET may provided the answer in an engineered solution still do not understand how you can apply time to a non engineered situation. Do you walk the means of escape to an area of relative safety with a stop watch? I also need to see how the calculations for RSET are done without attending a course at Greenwich or buying the appropriate part of BS 7974 or am I urinating against the elements.

[CivvyFSO]

It is the ASET calc that is the hardest, as this involves calculating the output of the fire over time and the heat/CO levels/visibility levels. Eventually one of the limits will be hit, and that is your ASET time.

For RSET you need the time to raise an alarm, (equations or software required) the time for a person to react to the alarm (Read from a table), then the time taken to walk to safety. (Distance to safety/assumed walking speed, not distance x speed as quoted in PD7974-6)

It would be hard to do with no teaching at all, but in a similar vein to the 'smoke control' thread, some time spent looking through the guidance, and possibly at some examples, would go a long way towards it. You would need the tables from certain documents regarding fire size, growth rates, the equations, walking speeds, pre-movement time etc etc. It is not rocket science, but it is not simple multiplication either. I reckon a 1 day course with the appropriate literature/tables supplied could cover it to a reasonable level.

Back to relate this to the thread, you would look at a fire it the worst possible place, see how the smoke etc from that fire would affect the escape route, when an alarm would be raised, how long before people start moving, and how long it will take to reach a safe area.

In the example given earlier, it seems that the dead end corridor joins another corridor. In that case the worst case fire would be in the corridor it joins, which is the perfect reason for NOT stopping TD once you reach that corridor.

[Tom Sutton]

Thanks civvy that’s enough to be going on with. 

I still don't understand how you apply all this to an existing building when you are conducting a Fire Risk Assessment unless you go for an engineering solution.

[Phoenix]

What you could do, TW, and I do this sometimes to give myself a reality check, is go to the area in question.  Stand in the most remote location then imagine that the space is full to its maximum capacity with other occupants.  Then ask yourself, where might a fire realistically start on the escape route?  Where would be the worst place for it to start?  If one started there now, could you and the guy in the wheelchair next to you get out safely with plenty of safety margin?

That's fire engineering.

Stu