Separation of rising and falling stairs

(Geoff Merryweather) #1

What is the basis of the requirement for separating stairs from basements from the stair above with a door at the ground (exit) level in the C/AS docs? It was in the old C/AS1, and is or was in the UK Approved Document B, where a lot of the older C/AS requirements seem to have come from.
Is it to stop people continuing to their doom in the basement, because they didn’t see the exit sign or the big see through glass door to outside in front of them? We rely on exit signs elsewhere to guide people to that point.
Is it to stop smoke filling the entire stairwell as someone leaves the basement?
The latter seems logical, but most basement stairs have a smoke lobby at basement level, a fire door at that level and then one at ground level with only a handful of people and FDS modelling i have done in the past shows that a large sized basement will maintain stair visibility over the entire stair with only a smoke lobby.
Despite being in the C/AS docs for 20 years, architects seem to love a continuous stair and the door swing becomes a problem with buildings being increasingly limited for space.

(Tania Morgun) #2

Hi Geoff, Smoke lobby in the basement only required where escape is via single stair. Where two or more exits provided from the basement, smoke lobby is no longer a requirement. As we know, compliance of fire doors from the basement deteriorates with building use. I have seen many fire doors wedge open, or not a fire door at all, (possibly replaced when damaged). I believe fire separation of basement from upper levels in the stair is the main intent to provide safety for people escaping from fire. Philosophy is similar to the intend of smoke separation in the stair, where stair height exceeds 25m in height. I am not sure why fire separation is required at ground floor, not a smoke separation, possibly to separate vertical and horizontal safe paths? If you are verifying compliance by modeling, you shall not follow C/AS docs at all, as they are based on the most conservative approach due to intend to make it usable in the worst case scenario. Regards Tania

(Frank Micallef) #3

Hi Geoff,
I can’t comment on why C/AS has included the requirement but the reasons the London Fire Authority/UK included fire doors were generally stated as being for the reasons you indicated.
1.To stop occupants from upper levels inadvertently descending to basement levels.
Many stairwells are designed as continuous stairs and it is not always apparent when the exit level has been reached - particularly with internal stairways. In theory it should be possible to resolve this with adequate signage and changes in finishes but the screen and door made it difficult not to notice when the exit level had been reached.
2. Basement fires can be particularly nasty as there is usually limited access and few if any windows to ventilate a fire and associated smoke placing additional stress on weaknesses in stair and duct protection connecting basements to the upper floors. The ground floor fire door and screen was seen as an added safety factor protecting stairs and means of escape from upper levels. The UK code similarly had requirements for basement smoke lobbies though I can’t recall all the details.

(Geoff Merryweather) #4

C/AS 3.5.1 says
`> Except in cases where there are two or more escape routes serving only the basement firecells and each terminates in a safe place, safe paths serving basement firecells shall be preceded by a smoke lobby``

which makes most basement stairs having a smoke lobby, and in particular the situation here where the stair is used by other levels and so would have to have a smoke lobby.
Aside from curiosity, since it is again a case where the basis of a requirement or magic number is lost in the mists of time, my thought was to clearly define the problem you are trying to solve. Removing the door reduces the level of redundancy in the building design so you need to know what it was intended to achieve before you can be sure it is not needed or to provide an equivalent level of risk or other features.
As it is a one size fits all requirement, I can see cases where people would continue down the basement, especially if there was smoke in the stairwell.

(Tania Morgun) #5

Geoff, Interesting topic… Research and real fire events investigations indicate that people will not use stair that are filled with smoke. I would not base my design on assumption that people will travel down to the basement because stair is full of smoke and they cannot see exit sign. Building code, C4.5, “means of escape to a place of safety in buildings must be designed and constructed with regard to the likelihood and consequence of failure of any fire safety systems”. If we make basement fire door redundant, using C/AS we have a margin of safety with ground floor fire separation to protect egress from upper levels. Using SED, we shall consider stair pressurization, or a door. Then we have to make design robust with redundant pressurization… I know what my clients will choose… Regards Tania