Help needed with calculating occupancy

[Fairway123]

Hi All,

We've received a fire strategy report for a building that has us tied up in knots. It is littered with errors (including using wrong risk profile for calculations). I'm hoping for some guidance, although I appreciate that it is hard to offer a steer without seeing the building but here goes.

The building is a well managed office and has a very good alarm system, although it has been converted from a large/grand house. The building is of heritage importance, and means that some alterations are prohibited.

The building has ground, and 3 upper floors which are each connected by 2 staircases. They are encased in fire protection, but without lobbies means that we need to discount 1.

The width of the storey exits are 1200mm and the stairs are 1200mm+. However the final exit from the escape staircase to the place of ultimate safety is 650mm. The fire strategy has used the horizontal escape figure for this door (650mm) as the limiting factor for the occupancy levels of the upper floors.

Is this correct? I thought that the horizontal escape capacity was only used in the initial stage of egress to reach a place of relative safety? Once occupants are in a protected staircase the width of stairs for vertical escape are used?

I cant find anything that specifies the width of final escape doors, although it is possibly implied I suppose?

Can anyone offer a steer?

Many thanks

[wee brian]

The pinch point at the bottom of the stair is the narrowest point so its going to be a factor. Normal practice is that an escape route should never narrow. 

[Phoenix]

Is this correct? I thought that the horizontal escape capacity was only used in the initial stage of egress to reach a place of relative safety? Once occupants are in a protected staircase the width of stairs for vertical escape are used?

Hi,

There is no guidance on what the correct approach is in such a case so you have to work it out for yourself.  Staircase size guidance incorporates factors for the holding capacity of the stairs as well as for the flow rate into, down and out of the staircase.  With such small final exits any significant number of people is bound to produce a backlog in the staircase which will feed back into the risk rooms, possibly including the one where the fire is located. 

The problem with the method used is that it gives no allowance for the holding capacity of the staircase.  This is not a problem for life safety as it errs well on the side of safety but it may be a problem for the building users if the occupancy numbers produced are too small for them to be able to function as they wish.  If they are happy with the resulting occupancy numbers then I would leave it at that.  If not then you could be justified in giving some allowance over and above the figures produced by the method you describe.

Precisely what that allowance should be is difficult to say.  You could determine the allowable occupancy on the (false) basis of there being a 1200mm final exit from the stairs.  Doing this would give you an absolute upper limit for an acceptable figure.  The true safe figure will lie somewhere between this upper limit and the figure found by taking the 650mm exit width as the overall limiting factor.

Whereabout in between is hard to say.  If the stairs are narrow with small landings then I would give very little allowance over the already calculated figures using the 650mm as the exit width.  If the stairs are generously wide with large landings then I might venture as high as half way between the two limits.  I don't think I would go any higher than halfway.

I wouldn't advise doing evacuation modelling using software to examine the possible outcomes of an evacuation as it is unlikely to produce results that are beneficial.

[Fairway123]

Thanks for this. Much appreciated.