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BS9999 occupancy calculations

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If lobby doors leading to a protected stairway are 1200mm, but the stairway is 1000mm, which figure do I use for calculating max. occupancy capacity when using BS9999?

Risk profile of building is A2.

Also, a strange one, but the clear width of the stairs reduce further to 900mm at the very base!  Building is a couple of hundred years old...

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From what you say it is unlikely the buildings was built to BS 9999 so why are you using it for your calculations, it looks like cherry picking. Either way common sense would dictate the narrowest part of an escape route should be used to calculate maximum numbers in that escape route.

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As Tom says BS9999 cautions against using it in FRAs in the first place (unless the property was designed and built to it) and against cherry picking which is not accepted.

If the premises can meet the whole document in terms of it's design, build and management it's OK to use.

You have to go by the narrowest point as if you over occupy based on the higher widths elsewhere you will get a potentially fatal pinch point (as was the case in the layout of the Station Nightclub where many of the 100 dead got stuck in a crush)

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As Tom says. If the Government Guides lead to an untenable situation then Fire Engineering is sometimes an option as is the correct use of BS9999 (in whole not cherry picking sections) - but the later does't always work out. I used BS9999 to try and design out installing a third stair in a conversion of a mill into offices due to excess travel distances, but the upgrading in floor fire resistance, shafts and provision of fire service rising mains associated with a BS9999 design ended up making the traditional benchmark approach easier and cheaper!

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