Existing Rated Separation Required?

[Nat]

Hello - I'm working on a renovation of an existing 2 story + basement office condo building, not sprinklered, VB, Business, total SF 15,709, in Michigan. The walls between the tenant suites are rated 1 hour, as are the doors from the suites to the common corridor. There's also a drywall ceiling above the ACT ceiling. The rated walls are called out at shear walls and they go from the roof down to the first floor, but not to the basement slab. Original drawing done under 2000 Michigan Building Code. The building code info calls out the height and area limitations as "2 story 9,000 sf/floor". Table 506.2 of the 2021 MBC lists 9,000 sf allowable area for B/VB/NS. With frontage increase it can be up to 15,750 sf.

Part of the new design is to combine 2 suites on the 2nd floor, which eliminates the rated separation between the two suites by creating an opening in the 1 hour rated shear wall. All the existing and new tenants are B use, so there's no rated separation requirement. I'm not sure why the walls between the suites are rated and why the ceilings are rated, so I'm concerned about not maintaining those assemblies. Could it be related to a structural requirement for the shear wall? Any help would be greatly appreciated.

I've attached the 1st and 2nd floor plans. The suites in question are 500 and 700.

 

[Codeguy49]

If the uses are all Group B Occupancy then there would be no required occupancy separation unless the Michigan code has some specific requirement for separation. You indicated the Type of Const is VB so with this, there is no requirement to protect framing elements; pointing at the rated ceiling, which could have been rated if it was Type of Construction that required fire resistance rated construction per Table 601. This is where any protection of structural elements for B occupancy would likely be required. It's always a good idea to check with the AHJ.

 

[Yankee Chronicler]

From the 2000 IBC (applicable if that's what the 2000 Michigan code was based on)"

 

[Tim Mailloux]

Table 506.2 of the 2021 MBC lists 9,000 sf allowable area for B/VB/NS. With frontage increase it can be up to 15,750 sf.

Before my time, but I cannot recall ever seeing a frontage increase that is more than the tabular area. Are you sure that wasn't an area increase for frontage and sprinkler?