Tenability within Means of Escape

[GB]

In relation to tenability limits of means of escape, I recall that 7974-6:2004 stated that the minimum clear layer height of 2.5m above the floor level and a maximum upper layer temperature of 200C was cited for a zero exposure.

I don't recall any other height given in 7974-6 for means of escape analysis.

I have came across BCO's accepting 2m as the analysis height for visibility.

Given that 2m is 6ft 5in in old money (which is well above the average person height in the UK) and most domestic common escapes are 2.4m in height - has anyone on the forum come across a lower height being used for analysis and if so any reference documents supporting say 1.8m which is the UK average height for men?

[John Webb]

BS7974-6:2004 is after my time in fire research. I assume the 2.5m height minimum clear height and max 200degC temperature is solely the criteria for escape under a hot layer, and not related to visibility. Work I did late 1980s for the Home Office and for other subsequent work used a visibility criteria of 0.1 OD/m (approx. 10m visibility) at 2m height. But from where the HO and I picked and agreed on this criteria I have no record or recollection! The 2m height gives some allowance for extra-tall people, and a margin of allowance for odd movements of smoke which might impede escape.

[ahmedh]

The other height mentioned is 3m. You haven't said what this building is, existing or new etc.

Quite correct the height doesn't relate to visibility so if BCO has accepted it on that basis it is flawed.

This criteria is to do with radiant heat of smoke layer and also human behaviour i.e. people wont want to travel with smoke close to their heads.

Don't know of such a document.

I would suggest that the calculation are checked with specific scrutiny of design fire size, layer temp and thickness.

[GB]

The design fire is a medium t-squared fire and the building type is a refurbishment of an industrial unit to a gym with occupants awake.
We have an L1 AFD system with 2m3/s extract and a 1.5m2 AOV as an inlet.
Occupants will be awake and familiar with the 1st floor mezzanine being the area of analysis.
The fire is within a double height area which the mezzanine overlooks.
99th percentile premovement time is 120s

[lyledunn]

Am I right in saying that you are trying to support compliance through an engineered approach as a result of poor design in the first place? Buildings like this should be designed to ADB or perhaps 9999. Reliance on AOVs or smoke vent systems in smaller buildings that are not subject to appropriate oversight should, in my opinion, not be permitted.
Just come this morning from testing a fire alarm in a sports club. Batteries dead and mains off, deliberately switched off a couple of months before hand because panel was bleeping and annoying the girl in the office. At least MOE was straightforward and fully compliant.

[Fishy]

Never seen 1.8 m used. 

I would stick to what the relevant parts of PD 7974 say, unless agreed otherwise in a BS 7974 QDR. 

Don't forget: a) you need to be a reasonable distance away from a 200^oC smoke layer in order for radiant heat flux not to be an issue; b) the layer boundary will be mixed and turbulent (to a degree), so some conservatism is warranted.

[ahmedh]

b) the layer boundary will be mixed and turbulent (to a degree), so some conservatism is warranted.

fuzzy layer 

Fire engineered approaches invariably require higher levels of management.