Is the evacuation strategy based on total evacuation or do occupiers other than the unit involved stay put in their flat?
Are all walls and floors compartment walls to a one hour standard?
Are all flat entrance doors FD30S?
Is the longest dead end situation no more than 7.5m?
Theres a few indicators. For a stay put policy, - if the answers to the other questions are all yes- a common solution is to provide 1.4sq m natural AOVs to the lobbies or corridors adjoining the staircse and a 1 sq m OV in the staircase. If all automatic the lobby AOV should open before or atthe same time as the stair OV.
There are other ways of doing it with mechanical powered smoke extract systems rather than relying on natural ventilation, the shaft size can then come down to about 600mm.
The whole idea is to protect the staircase against the ingress of smoke to ensure that it is safe to use even if later in the incident it is necessary to evacuate more people. The AOV in the lobby adjacent to the staircase creates a small negative pressure differential between lobby relative to the staircase and therfore protects the staircase against the ingress of smoke. Well thats the theory.
