IIIB and IIB

[Sifu]

Proposed building addition, 2021 IBC/IEBC. Single story, all sprinklered, E with B offices. Addition is the offices, about 1100sf², existing building is 43,000sf². Proposing mixed use/separated per 508.4. They do not list accessory occupancies for the B offices, however the aggregate for everything in the building would be below 10%.

The existing building is shown as IIIB, the proposed addition is IIB. They are not proposing a fire wall between the construction types, which is what I would generally look for. Since it all fits in IIIB, why not go that way? What am I missing here? Even if future additions were proposed, since both occupancies have the same area, story and height limitations does it matter? I can't see how the more restrictive FSD for IIIB could be it in this case, so I am just wondering if they just saw IIB as less restrictive because of that and chose it.

Is this just a poor code analysis? Seems like with proper wording and use of the code they could do what they want.

 

[steveray]

Technically....IIB is higher so screw the 2 hour walls...

602.1.1 Minimum Requirements​

A building or portion thereof shall not be required to conform to the details of a type of construction higher than that type which meets the minimum requirements based on occupancy even though certain features of such a building actually conform to a higher type of construction.

I want to see where this goes....

 

[classicT]

I've had this happen to me. Typically is was poor research on behalf of the design firm. Usually they make an assumption based upon wood/steel stud and general appearance. Sure would be nice, and more accurate, for them to just pick up the phone and ask the AHJ for a copy of the CO with the construction type.

 

[steveray]

Technically....IIB is higher so screw the 2 hour walls...

602.1.1 Minimum Requirements​

A building or portion thereof shall not be required to conform to the details of a type of construction higher than that type which meets the minimum requirements based on occupancy even though certain features of such a building actually conform to a higher type of construction.

I want to see where this goes....

Just realized my sawhorse ran out......Screw the 2 hour walls and call it all IIIB.....

 

[Sifu]

I agree, it may not end up being an issue, since the entire building could be IIIB, but the entire building may not be a IIB. Without a fire wall, the entire structure would need to be IIB.....non-com, which is uncertain, but I can only assume at this point that there may be combustibles since they went with IIIB. That can be confirmed. OR, as T said, poor research...and maybe understanding. I can call it a IIIB all day, but it is the DP that needs to do that.

 

[Mech]

Did they try calling it Type IIB to avoid fire rating any load bearing exterior walls?

 

[Yankee Chronicler]

I agree, it may not end up being an issue, since the entire building could be IIIB, but the entire building may not be a IIB. Without a fire wall, the entire structure would need to be IIB.....non-com, which is uncertain, but I can only assume at this point that there may be combustibles since they went with IIIB. That can be confirmed. OR, as T said, poor research...and maybe understanding. I can call it a IIIB all day, but it is the DP that needs to do that.

A building cannot be two construction types. A firewall creates two buildings. Without the firewall, they have to classify the entire structure as either II-B or III-B. If the existing portion was originally III-B, to classify it as II-B they will have to provide a thorough analysis justifying reclassification is II-B. The easier solution is to call it all III-B.