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RE: Fire Rating

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>We would like to construct a concrete floor using
>UL D920 with 4 1/2" min. of normal weight concrete
>above an approved composite metal deck that has 2
>hour fire rating with no fireproofing. If the floor
>is poured with 5" over the deck, then where we have
>grout set tile (1 1/2" depression)  we would
>have only 3 1/2" of concrete over the metal deck
>plus 1" of grout as a setting bed for quarry tile.
>Would this meet the 2 hour fire rating? We would not
>count the quarry tile as contributing to the fire rating.

I asked my fire engineering expert colleague Farid Alfawakhiri to answer
your question. Below is what he replied.


The Design Information Section in the beginning of the UL Directory,
subsection "III. FLOORS-CEILINGS AND ROOF-CEILINGS", item "1. Concrete"
contains the following statement

"When lightweight concrete is specified, the use of normal weight concrete
is not recommended because its lower insulating properties could cause
higher unexposed surface temperatures."

Therefore, it is quite questionable whether you can use normal weight
concrete within UL D920 where lightweight concrete is specified.

Personally, I do not see a problem with your D920 design modification (the
way you describe it), but since it is a matter of judgement, you need to
convince your AHJ (and get its formal approval) for that. 

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