External stairway

[Meady]

Sorry - silly question time...

This is a specific case; but is also a general question as to whether internal stairs must be protected if there is also a second means of escape being an external stair.

An existing building which is being converted from a residential accommodation (small block of flats) in to an office/admin block. There is ample room for an external stair to be fitted.

Built 2008.

Three storeys (ground, first and second) plus basement/cellar. 60-min fire separation between basement/cellar and ground floor.

Footprint of each floor is approximately 10m x 12m. 

L3 alarm system / emergency escape lighting being installed.

Risers will be enclosed within 30-min protection.

Standard construction (brick, breeze block, concrete floors, flat roof).

Occupancy levels will be approximately 10 per floor (none in basement/cellar which is used for plant room / electrics).

The client wants a complete open plan design with an open internal staircase. Building Control will be involved - client is at drawings/plans stage at present.

The glazing along the external stair with 30-minute glass which is sealed closed. 

The fire door from first floor leading to external stair will also be FD30 standard/

Question: I just want to double check; is the open plan internal stair acceptable as long as;

A) External escape stair is fully protected along route
B) Travel distances from any point in the first & second floor levels are acceptable (less than 18m in single direction to only protected means of escape)
C) Occupancy levels from each floor are less than 60

I've been reading BS9999 & ADB which I think is relevant as this is a complete conversion of a building. Item 18.7 of BS9999 in particular.

Or am I misunderstanding this and does the internal stair also have to be protected??

In view of low occupancy levels; I think that only 1 means of escape from each floor is acceptable as long as the travel distances are within 18m?

In your experience(s), are there any other compensatory features the client could consider which may make an open internal stair acceptable?? Higher levels of AFD perhaps? Sprinklers??

Any help or guidance in right direction much appreciated!! Sorry - new query for me!

[AnthonyB]

Already answered this on Firenet. It can be done in some circumstances.

[Meady]

Thanks Anthony.