Exit Capacity Difference ADB/DCLG

[Richard Stevens]

Hi

Can anyone help with why there is a difference between exit width capacity between ADB & the HM Government Guides.

Table 4 ADB for a 750mm door is 60 persons however DCLG states for 750mm, 80 for high risk - 100 normal & 120 low risk.

I appreciate that ADB is fairly black & white and is for new builds and refurbs whereby DCLG is for existing building so will give a degree of flexibility.

The history of 40 persons per minute via a 525 with door will give you your  2 mins (high) 80 persons - 2.5 mins (normal) 100 persons 3 mins (low) 120 so all adds up but

1050mm  ADB = 220 - DCLG 160 - 200 - 240, so these are double the  750mm figures but 1050mm is not double 750mm?

Probably looked at it too much and missing the obvious but please humour me.

Cheers

Richard

[Tom Sutton]

I wouldn't get to hung up on the figures, most have no scientific foundation, I believe these figures were arrived at by comparing other codes and adding their penny worth. Take for instance to number allowed for an inward opening door, the factories act was 10 , post war studies was 40, the blue guide for the FPA was 50 and now it is generally accepted as 60.

Under part E of Scottish technical standards (1993) version it quotes:

The intention of the requirements for means of escape is that everyone within a building may reach either a place of safety or, in certain circumstances a protected zone within 2.5 minutes of becoming aware of an outbreak of fire. The requirements for the number and width of exits assume a unit width of 530mm per person and a rate of discharge of 40 persons a minute. The allowable travel distance (the distance which it is assumed one can travel along an unprotected escape route within the time specified) varies according to the purpose group and situation.

However in the more up to date tech standards in Scotland (2001) amendment's, it says at the introduction to part E, "The intention of the requirements for means of escape is that everyone within a building may reach either a place of safety or, in certain circumstances, a protected zone within a reasonable travel distance. This also applies to the very latest Scottish Building Standards.

I think you are back to risk assessment, just use the figures as a benchmark, because most of them have stood the test of time. Also it is necessary to have good reasons for you decisions, i.e. high ceilings, the escape route will remain tenable for longer therefore more people can escape, low ceilings the opposite.

[Richard Stevens]

Thank you 

[Tom Sutton]

Richard check out Post War Building Studies 20, 28 and 29 at http://www.mace.manchester.ac.uk/project/research/structures/strucfire/DataBase/References/defaultOther.htm and Human Behaviour in Fires at  http://webarchive.nationalarchives.gov.uk/20070507012938/http://www.communities.gov.uk/index.asp?id=1125555 you may find them interesting.