Separated Occupancy & Fire Rating

[StructMH]

I'm looking at a multi-story building that was originally designed with a B-occupancy (Category II). A study is currently being done to assess the impact of upgrading the entire first floor (on-grade) of the structure to an I-2 occupancy (Category IV). From a structural perspective, Chapter 9 in the IEBC would seem to require a full-building lateral analysis using the current code. My understanding is that the entire structure would need to satisfy lateral design criteria based in Occupancy Category IV. Gravity loads aren't of major concern at this point if the area under consideration is slab on grade. The architects have said a horizontal fire wall/barrier may be possible by fireproofing the underside of the composite deck and steel beams at Level 2. The confusing part is this: Does any building code explicitly state that a lesser-rated structure may be constructed above a higher-rater structure? Theoretically, I feel the 1-hour rated structure above could collapse after 1.5 hours of a fire and then the 2-hour rated structure would then be tasked with supporting the crumbling structure above. Logic tells me that this shouldn't be permissible, but I am yet to find any US code language (IEBC or IBC 2009 or ASCE 7-05) that might impose structural limitations as it relates to separated occupancies. Has anyone else come across a similar situation?

 

[wannabeSE]

Use the higher occupancy category. See 2009 IBC 1604.5.1. ASCE 7-06 1.5.1 has similar requirements but is not as clearly stated.

1604.5.1 Multiple occupancies. Where a building or structure is occupied by two or more occupancies not included in the same occupancy category, it shall be assigned the classification of the highest occupancy category corresponding to the various occupancies. Where buildings or structures have two or more portions that are structurally separated, each portion shall be separately classified. Where a separated portion of a building or structure provides required access to, required egress from or shares life safety components with another portion having a higher occupancy category, both portions shall be assigned to the higher occupancy category.