It should be noted there is no reference to IBC §508.4 in the exception above, nor is there a reference to IBC CODE ANALYSIS
As indicated, the building has been designed as a non-separated mixed-use structure complying with IBC §508.3. Per IBC §508.3.3:
No separation is required between non-separated occupancies.
Exception 1 is not applicable since there is no Group H (High Hazard) occupancy in the building.
Exception 2: Group I-1, R-1, R-2 and R-3 dwelling units and sleeping units shall be separated from other dwelling units or sleeping units and from other occupancies contigu§508.4 in IBC §420. IBC §420 requires vertical and horizontal fire-resistive separation of Group I-1, R-1, R-2 and R-3 dwelling units and sleeping units from other dwelling units or sleeping and other adjacent occupancies regardless of whether the building follows a separated or non-separated mixed-use. Had the building been required to comply as a separated mixed-use building in order to meet height and area requirements, than additionally, the occupancy separation requirements of IBC §508.4 and Table 508.4 would be applicable and require a 1-hr fire-resistance rated horizontal assembly supported by 1-hr fire-resistance rated supporting construction for the occupancies involved in this project.
Requiring the fire-resistance rating of supporting construction in a non-separated mixed-use occupancy building of unprotected construction unfairly penalizes the building owner/designer who chooses to construct a smaller building as a consequence of not providing the additional occupancy separation in the first place. In essence, there would be no distinction (and no need for the current prescriptive code language) between non-separated mixed-use occupancies and separated mixed-use occupancies. Further, the vast majority of multi-story buildings with R-1 and R-2 occupancies also contain other occupancy types (i.e. fitness rooms, storage rooms, community rooms, etc.); so following the logic of requiring fire-resistance rating of supporting construction would prohibit the use of unprotected construction for buildings containing residential occupancies. It is quite clear that the IBC does permit residential occupancies of unprotected construction.
Koffel Associates has contacted technical staff at the ICC for their interpretation of this issue. ICC technical staff supports our interpretation that no fire-resistance rating of the supporting construction is required in a non-separated mixed-use building of unprotected construction.