I’m trying to assist a colleague and cant put my finger on the guidance (not the commentary), can anyone recall where it was stated/agreed that we could allow visibility to briefly drop below 10m (to no less than 5m for a period no greater than 60 secs) eg ceiling jet curling around at the perimeter wall of a warehouse (not the 100sq.m room rule in C4.3c)
I have print outs of the ever changing MBIE ‘clarifications’ and "interpretations’ from their website and I cannot locate any reference to an allowance for a drop in visibility. Regardless I believe these interpretations had and have no legal standing, and are not Section 175 Guidance.
60 seconds is a very long time! Visibility is a Building Code requirement so non negotiable. This would be a clear Building Code fail. There were previous discussion on the allowance for a ‘blip’ , but I cannot recall where these initiated. Perhaps one of the draft documents published. From what I recall it was decided to not allow this concession. The drop in visibility discussed was closer to the < 5 second mark. This would allow someone to take a step or two to the side and regain visibility, and would allow for typical scenario where an occupant opens a door and jet of smoke escapes which temporarily reduces visibility.
I generally find that if you use FDS and put in enough measuring devices a detailed analysis will result in visibility being demonstrated.
My bad. I should have stated <5m vis ‘in a 60 second time period’ not for 60 secs, ie the 'blip you refer to.
I believe the proposal to allow a ‘blip’ was thrown out, but if its only a couple of seconds it can usually be demonstrated using FDS that that 10 m Building Code criteria is still met.