Occupancy capacity of Spiral Staircases.

[StuartH]

I undertake occupancy assessment of buildings for a customer where they are wanting to occupy buildings upto their maximum capacity having regards for floor space, storey exit, and staircase capacity. I have recently assessed a premises which is provided with a spiral staircase used as a escape stair. Common sense suggests that the occupancy capacity of a spiral staircase should be reduced however I am unable to find any supporting documents.

I would welcome the views of anyone who knows of any guidance confirming how to calculated the occupancy capacity of a spiral staircase.

[kurnal]

Its a mixture of guidance and common sense. Generally most spirals will only allow a single file of people and they will move a little more slowly than the normal figures given for travel on staircases (7974 if I recall). Top flow rate will be about 30 persons/minute in my opinion. It may not be appropriate to allow for the stacking reservoir effect unless the the staircase is a protected route.
The other issue is that spirals are often accommodation staircases anyway and so not counted for this reason.
Theres a few points made in 9999 about the use of spirals as a means of escape.

[StuartH]

Thanks Alan.

The staircase in question is located in a protected route. Thinking about it these are also fairly frequently found as external escape staircases.

[kurnal]

Yes thats right, but I think the single file  generally applies unless its a helical rather than a spiral. Nobody likes using one without hanging on to the rail or rope.

[Phoenix]

Hi,

Just to add slightly to kurnal's comments, it's not generally possible to apply table 7 (table 15 in 9999) to a spiral staircase.  They should be looked upon as governors of flow and not as possessing holding capacity for occupants (table 7 uses this latter principle).  It has been possible, in some cases, to apply table 7 (or table 15) to helical, but not spiral, stairs where those stairs have a large radius and the inner edge of each step has a suitably long tread length.

Stu

[jokar]

There is a BS for Spirals , it is mentioned in BS 9999 and the wording in the original document is that 2/3rds of the width of the stairs is to be used when assessing moe.  Hope that helps.