Compartment sizes ADB

[Firemac]

HI. this may seem like a silly query. In approved document B (Diagram 28) an unsprinkled multistorey retail unit is allowed 2000 metre squared per storey in a compartment up to a volume of 20,000 metre cubed. This would perhaps depending on storey heights allow a three storey retail in a  single compartment. Does this however permit voids between the floors making the floor area all one? Could you now possibly say your floor area now say 5750 metre squared because of large voids even though it is all part of the same compartment. Does the limit on the area in APDB take into account that there may be large voidsbetween the floors?
The requirement for sixty minute rated floors (although not compartment floors) is to protect the occupants escaping on these floors. Is this the correct assumption?

[PhilB]

HI. this may seem like a silly query. In approved document B (Diagram 28) an unsprinkled multistorey retail unit is allowed 2000 metre squared per storey in a compartment up to a volume of 20,000 metre cubed. This would perhaps depending on storey heights allow a three storey retail in a  single compartment. Does this however permit voids between the floors making the floor area all one? Could you now possibly say your floor area now say 5750 metre squared because of large voids even though it is all part of the same compartment. Does the limit on the area in APDB take into account that there may be large voidsbetween the floors?
The requirement for sixty minute rated floors (although not compartment floors) is to protect the occupants escaping on these floors. Is this the correct assumption?

I think you need to look at diagram 28 again, why are you considering cubic area? it is the square metres in retail units that you are interested in not cubic metres. Compartment floors are not required unless the building is taller than 30m or if the floors are provided to divide a building into separate occupancies.

The 60 minute floors are required to keep the building standing for the required period.

[kurnal]

Hi Firemac
Just take another lok at table 12.
Only for buildings in purpose group 7 - storage buildings- is volume of compartments taken into accout.

It is confusing I will grant you. Have another look and if you are still confused post again.

The fire resistance of the elements of construction is there for the protection of occupants, persons in the vicinity so the building doesnt fall on them, and for the safety of firefighters who may have to enter the building to rescue someone. This latter point is particularly clarified in relation to raised storage areas note 2 to para 7.8. If it applies to mezzanines then the principle must hold good for all structural requirements under B3.

[kurnal]

Sorry Phil- I hadnt refreshed for a while!

[PhilB]

Sorry Phil- I hadnt refreshed for a while!

No problem old boy, I think we're saying the same thing.

[Firemac]

Thanks for clarfication, The volume thing was not the main issue, the real query was are large voids permitted between these 60 minute floors. From what you are saying I could have 8 storey retail building all in the same occupancy (top floor less than 30 metres above ground), every floor around 2000 metre squared and fire resisting but each has a large void:- aestheic reasons but NOT a atrium) This would translate to a very large fire in the building and very difficult to put out. Just seems very counter intuitive.

[wee brian]

yes thats it in a nutshell.

You're not the first person to find it a bit confusing but those are the rules.