1.5 Fire modelling to determine ASET
For particular design scenarios, the designer must demonstrate that the occupants have sufficient time to evacuate the building before being overcome by the effects of fire.
In fire safety engineering terms, the ASET must be greater than the RSET.
ASET is defined as the time between ignition of the design fire and the time when the first tenability criterion is exceeded in a specified room within the building. The tenability parameters measured at a height of 2 m above floor level, are—
- a FED of thermal effects greater than 0.3; or
- conditions where, due to smoke obscuration, visibility is less than 10 m except in rooms of less than 100 m2 or where the distance to an exit is 5 m or less, where visibility may fall to 5 m.
Visibility is generally the first tenability criterion exceeded in calculations unless any exception is applied.
Calculate the ASET by modelling the fire using the design fire as specified. In most cases there will be a number of locations for the fire that could produce the lowest ASET for a given escape route. Check a number of rooms to determine the limiting case.
It must be demonstrated that the proposed level of safety is at least equivalent to that provided by a building compliant with the Deemed-to-Satisfy Provisions.