Heavy-timber addition to non-combustible building?

[PatrickGSR94]

I have a church building, Group A-3, Type II-B construction, 2 stories, fully sprinkled, open space all around. It's had several previous additions, and is currently near the limit of of mixed-use nonseparated, at ~31,000 SF per floor (35,625 allowed with multi-story sprinkler and frontage increases). Addition will add about 7,500 SF.

I ran the numbers on the different occupancies and areas, and if the new addition is done as mixed-use Separated (with no separation required between E and A, 1-hour between E/A and B), the weighted average comes out to 0.99 with the addition, IF it's all done as Type II-B to match the existing building.

However we're looking at doing the addition in heavy timber as Type IV. Even though Type II is a "higher" building type, IV actually allows more area. But if the entire building's weighted averages comes out to less than 1.0 as Type II, could we do the addition as Type IV without needing a 3-hour fire wall?

 

[RLGA]

Maybe. Different construction types need to be separated by a fire wall. Type IV is a combustible construction type, whereas Type IIB is a noncombustible construction type. However, combustible materials in Type II construction are limited to those listed in Section 603, and item 19 covers heavy timber. If you can stay within the limitations established by this item, heavy timber can be used in Type II construction.

 

[PatrickGSR94]

don't think that will work, as those sections mainly cover 1-hour or lower rated walls, or exterior walls. We're wanting to do HT posts and beams, HT floor planks etc.

So it sounds like a 3-hour self-supporting wall is going to be required to make it work.