phased evacuations

[alfi]

Gents,

In tall or complex buildings its expected a phased evacuation is the best method to evacuate the property effectively, however if the building now wanted to adopt an "all out strategy" due to staff problems not understanding the process and hearing bells in the distance etc. My question is would occupancy levels be affected by this change in strategy, would staircases have been designed for a phased not an all out? , the building has 20 floors!!, or is it purely a management policy put in place under the old fire certificate

[kurnal]

Tony
If the building was designed for phased evacuation its staircases would have been much narrower than those incorporated into a building with an all out strategy.

If you simply switched to an all out strategy there is a serious risk that persons on the floor where the fire is may not be able to evacuate the risk area due to congestion on the stairs.

It may be possible to change depending on the actual number of persons actually using the building, if this figure is stable and managed. The original design capacity of the staircase would have been calculated on floor area. If actual use is much less than this - and always will be - then you could look at all out if the staircases are wide enough to evacuate all floors. See table 7 in the ADB for further guidance - or BS9999.

[alfi]

thanks Kurnal

[CivvyFSO]

Tony, just put some figures to Kurnals comment:

Adopting phased evacuation in a 20 storey building with about 200 persons per storey would require 2 x 1100 stairs. (Looking at ADB this might not seem quite correct, but assuming that on the fire floor you lose an exit, the alternative route needs to take everyone from that floor without narrowing, so we need a 1050mm exit leading onto a 1100 stair.) You generally don't have to discount a stair if it is designed with phased in mind as there should be lobbies or a pressurised stair.

The same building with simultaneous evac in mind would require 4 x 1200 stairs. This is because a 1200 stair will accomodate 1095 persons for simultaneous evac. (There are other ways to meet the requirements, this is just an example) Also, I am again assuming lobbies or a pressurisation system is in place so we still don't need to discount a stair.

So basically, if the building above was built to accomodate phased, and then you switch to simultaneous, you are 2600mm short on staircase width. This will more than double the evacuation time, which is not good when you are in the fire-affected compartment waiting to enter the stair.

But, also in line with kurnals comments, if the building only has something like 50-100 people per floor then there is a chance that it might suffice.

Aside from this: If it is a pressurised stair you are even more stuck as that is designed to only have a certain number of doors open.

[jokar]

Tony, as Kurnal has suggested you can use BS 9999 for existing buildings and this may alleviate some of the problems that Civvy has quite rightly pointed out with regard to number of staircases and their widths.

[alfi]

Gents,

great feedback thanks hugely