height of spiral escape stairs

[dusty]

Gents,

Is there a limit to the height of external spiral escape staircases, I have one proposed for a 5 storey hostel accommodation, there are two means of escape (central stairs and spiral) and numbers are only 6 to 8 a wing so a total of 40 max, so appears OK on numbers, but have a nagging doubt in my mind, all thoughts appreciated.

[ian gough]

Dusty, if the building inspector is happy with the proposal - so be it.

[ian gough]

Ooops - unles you are a BCO Dusty!

[Brian Catton]

Dusty
What is the occupancy, Students, homeless, families, asylum seekers,
Are they all fully abled. Can be severe restrictions on moving less abled persons down a spiral staircase.
Ian I am surprised at your answer having regard to your experience with building inspectors.

[ian gough]

Brian - on the contrary, it's because of my experiences! Seriously though, this is a building regulations matter and whilst fire safety officers may have a view, the ultimate authority here is the BCO. Sometimes we can get bogged down in matters such as this and unless one has firm views, it is sometimes better to let the 'lead' authority do the work/research - and worrying! You make some goods points though Brian re care premises etc.
And let me have your view on my post re 'access for appliances' - I'm sure you have considerable experience on this!

[dusty]

Many thanks to you all, Building control also had doubts and asked me, having looked at several documents it would appear only numbers limit the height by use of tread going width and length, occupation would be students, given that 5588 accepts these for disabled refuges, we will not pursue the issue; While it may be a Building Regulations issue, I  fortunately have a very good working relationship with Building Control so we tend to agree theses items rather than leaving them solely to each other, once again many thanks.

[ian gough]

Dusty, I presume it will be 'weather protected' i.e enclosed? Just been shown (actually today) another new project where a high spiral was not!

[fred]

On a similar thread - query from one of our FSO's - can anyone point us towards guidance on determining occupancy capacities for single staircase premises where the staircase is a spiral - albeit in a protected shaft.  ADB, BS5588-3 and BS5395-2 are not very helpful apparently.

Please don't hit me with risk assessment - or Building Control ......

[kurnal]

Fred
Dont know whether there is a more definitive answer but a  common sense approach may help (though I guess I am trying to teach my Grandad to suck eggs)

Firstly Table 7 in the B doc is calculated based on the flow of people aong an exit  route of given width  (eg exit width 1200mm would allow 240 persons to pass in 2.5 minutes  - derived from post war building studies.) Then add an allowance based on the available floor area within the staircase, ie the area of the number of treads plus an allowance for the half landings at crush density of 0.3 persons per square metre. So the constant for each 3 metre storey height on a normal 1200 mm stair is 45 persons.

Secondly the guess work comes in. It is known that persons are less confident on spiral stairs due to the variation of going on the tread and the open aspect giving the awareness of risk of falling. So in my experience on most spiral (not helical) stairs most people want to hold onto the handrail. So that will often limit the flow to one person per tread unless the spiral is unusually wide.

So one person per tread would equate to a single unit of exit width- in the old days that was worked in inches and a flow of feet per second  nowadays I think expressed as 600mm and a flow rate of 0.8 m/sec on stairs (Check CIBSE gude E) ie 40persons per minute.

In summary my approach would result in a maximum of 100 persons plus  a constant of 13 per floor.
Hope this will at least prompt some discussion.

[wee brian]

If its ahostel then numbers are unlikely to be the issue. Part B limits external stairs to 6m above ground and asks for adequate weather protection.

[fred]

Kurnal - brilliant - many thanks - that's exactly what I was hoping for.