BB100 Schools

[Ken Taylor]

Under the old BB7, the occupant capacity for dining rooms and gymnasia was 0.9m2 and that for assembly halls was 0.45m2. Presumably they have used BB7 in producing the new guide and listened to those of us who generally lamented it's departure in favour of the 'Constructional Standards' - which were subsequently incorporated in ADB - and then removed.

[Pete M]

Stu,

Now I have the original formula rather than that currently in ADB (with no explanation) I understand where the figures come from and in fact they correspond quite closely to thos in the PWBS when you convert imp to met.

I would still like to know however why BB100 assumes that the stair calcs require adjustment for the discounting of a storey exit when ADB says the exact same formula provides an adequate width (accounting for the loss of a storey exit).

Apologies for sounding like a pain but I am involved in a number of projects where BC and LFRS have differing views on the subject and short of looking to DCLG for a determination I could do with a little background/advice here.

Cheers

Pete

[slubberdegullion]

Pete,

The previous few versions of ADB allowed you more freedom in interpreting them in this respect and it was a logical step for everyone who applied tables 7 or 8 to take into account the loss of a storey exit when there was no requirement to lose a whole staircase (e.g. lobby approach).  That tied the vertical assessment in nicely with the horizontal assessment.

But the calculations were always a bit fiddly and people often didn't get them right or forgot to discount a storey exit anyway.  

People also argued that the discounting of a single storey exit throughout the whole building didn't have a lot of effect on the final outcome anyway.  Ok, by not discounting a storey exit the stair widths would tend to be marginally smaller but it was felt that they would often default up to the same result anyway.

Also, it was recognised that tables 7 and 8 are based on a lot of assumptions that aren't always applicable and yet time has demonstrated that stair widths derived from these tables usually provide satisfactory means of escape.  Therefore, the conclusion was that the tables probably err far on the side of safety anyway so there was some leeway to be had.

And how could that leeway be had?  Well, lose the fiddly method for discounting a storey exit when no staircase need be lost.  In other words, lose a storey exit for assessing exit widths for horizontal means of escape but don't bother to lose the storey exit for sizing the staircases.

My pal Andy Kelly enquired a few times of the authors of ADB how we should reconcile the discrepancy but, to my knowledge, the reply never came.  I guess it languished a while on the too difficult pile then one day quietly slipped into the bin.

So staircase calcs are much easier now than they used to be.  Except when it comes to schools, it appears.  I didn't know that BB100 had resurrected the lose a storey exit method - I bet they didn't mean to!
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Well, just had a look at it and there it is.  Put in, apparently, quite deliberately.  So, use that method for schools - it errs on the side of safety (which is what we want for the little darlings, isn't it?) - and use the simpler ADB method for all other buildings.....I guess.

The worked example in BB100, if done the ADB method, would have a figure of 315 per stair instead of the 350 and the result would be:

w = (315 + (15x3) - 15)/(150 +(50 x 3)) = 1.15 m             a bit smaller than the BB100 figure of 1.267 m

In the circumstances you find yourself in, I wouldn't be surprised if FRS people are still using the old lose-a-SE method while BCOs have more quickly moved on to accept that you no longer have to.

If you compare the methods on some very short buildings, by the way, you can get some very large differences between the results.

Incidentally, BS 5588 part 11 has always had worked examples that do not discount a storey exit for assessing staircase widths and that has always been out of line with the horizontal means of escape.  ADB does have a strong tendency to follow the 5588s.

Stu

[Tom Sutton]

Pete could it be when calculating the capacity of a staircase you ignore the ground floor (n-1) as it is only the upper floors who are going to use it?

Another way to look at it is because the staircase is discharging at the base then more people could enter the staircase increasing the numbers. Take a 1000mm width staircase with four storeys, capacity 40 person per storey n-1 = 120. This could be achieved in one minute you still have one and a half minute left which will allow another 60 people to use that staircase = 180, ignore the n-1 and you have a small safety factor?

One group considers the staircase a static component the other a dynamic one?