Mixing Construction Types

[Noob]

Hey All. IBC 2018 here.
Was thinking about podium buildings and how they mix construction types (typically Type I and Type III or V). I suppose there is some well thought through economics to the podium model, essentially getting the extra floors without having to go fully Type I or II, essentially the most floors for the least cost.

My question is however, have you experienced mixing of other construction types in the same manner? Such as 3 stories of Type VB on top of a Type IIIB first floor? I've got an R-2 building with a dedicated A-2 ground floor. I can only get 3 stories out of Type VB sprinklered, so it would make sense (I think) to make my first floor a Type IIIB "podium". Has anyone seen this strategy done before?

Can you think of any potential downsides? thanks,

 

[RLGA]

Cannot do it. Other than Section 510.2 "Horizontal building separation allowance" and some other sections in Section 510, the only way you can mix various types of construction is through the use of a fire wall, which can only be vertical.

 

[Noob]

Appreciate it Ron. It's interesting that they didn't adopt the logic of allowing this - more restrictive construction types below less restrictive ones, all within the current height, area, story limitations and horizontally separated. Seems like it would meet the overall intent of chapter 5 (I suppose not the letter though). Maybe it will move in that direction and we'll start seeing mass timber podiums instead of concrete, with Type VB on them.

 

[redeyedfly]

Separate the A-2 occupancy from the R-2. Read 508.4 and see Table 508.4.

 

[steveray]

Appreciate it Ron. It's interesting that they didn't adopt the logic of allowing this - more restrictive construction types below less restrictive ones, all within the current height, area, story limitations and horizontally separated. Seems like it would meet the overall intent of chapter 5 (I suppose not the letter though). Maybe it will move in that direction and we'll start seeing mass timber podiums instead of concrete, with Type VB on them.

You CAN build that way....you just don't get the "extra" storie(S)...

 

[RLGA]

You CAN build that way....you just don't get the "extra" storie(S)...

Just to clarify, you can technically build a Type VB building on top of a Type IIIB building. However, if you did, then the entire building is considered a Type VB building.

 

[steveray]

Exactly what I meant...Limited to the worst mixed construction type....Unless special provisions are available....It's the top of the building (or size) that is the concern with construction type....So the better support (unless way better) of IIIB is meaningless on the bottom as it doesn't fit the podium stuff...

 

[Yikes]

Exactly what I meant...Limited to the worst mixed construction type....Unless special provisions are available....It's the top of the building (or size) that is the concern with construction type....So the better support (unless way better) of IIIB is meaningless on the bottom as it doesn't fit the podium stuff...

You and RGLA are referring to IBC 602.1.1, where you can (for example) build a structure of type III materials and features yet call it Type V if you want to.

 

[redeyedfly]

Just use separated occupancies VA and you can build three stories of R-2 above the A-2 first floor. VA does not cost any more to build than VB. All of your walls are 1hr anyway with typical gyp.

 

[RLGA]

Just use separated occupancies VA and you can build three stories of R-2 above the A-2 first floor. VA does not cost any more to build than VB. All of your walls are 1hr anyway with typical gyp.

Yes, the OP can do that, but they were asking us if they could put a 3-story Type VB, Group R-2, on top of a 1-story Type IIIB, Group A-2.

The answer really is no, because the building will be considered Type VB construction since it is the most restrictive construction type, and Type VB construction for Group R-2 is limited to three stories. This means if he did what you proposed (which is a perfectly logical and acceptable approach to include a Group A-2 in a 3-story building of that construction type), the building would be limited to the Group A-2 occupancy on the first story and only two stories of Group R-2 above and not the three stories the OP was looking to have.

 

[redeyedfly]

Yes, the OP can do that, but they were asking us if they could put a 3-story Type VB, Group R-2, on top of a 1-story Type IIIB, Group A-2.

The answer really is no, because the building will be considered Type VB construction since it is the most restrictive construction type, and Type VB construction for Group R-2 is limited to three stories. This means if he did what you proposed (which is a perfectly logical and acceptable approach to include a Group A-2 in a 3-story building of that construction type), the building would be limited to the Group A-2 occupancy on the first story and only two stories of Group R-2 above and not the three stories the OP was looking to have.

Understood. My point is that it would be pointless since in his example he can get what he wants for free by simply calling it VA and listing a few UL #s for the bearing walls. Nothing else changes in the actual construction (maybe a few more screws in the gyp).

 

[mark5211]

TMK, Section 510 Special Provisions is the only place in IBC where mixing of construction types is allowed. So if you don't use a 510 subsection or divide building with firewalls, then you have to use one construction type only. For specific scenario mentioned, code appears to allow 4 stories of R-2 over 1 story A-2 with mixed-use in type 3. Or 3 stories of R-2 over 1 story A-2 with mixed-use in type VA. Note that you cannot use type 3 for lower portion of a section 510.2 podium building. 510.2(2) requires type 1A. Regarding VB, by the time you factor in all of the required 1-hour assemblies for corridors and DU separations [both horizontal and vertical], you basically already have a VA building. Watch out for sprinkler requirements. I do not believe you can use 13R for R-2 if mixed with non-incidental A-2.

 

[RLGA]

I do not believe you can use 13R for R-2 if mixed with non-incidental A-2.

Yes, you can. NFPA 13R, Annex A Explanatory Material, states, "In buildings containing a residential occupancy properly separated from other occupancies, the use of this standard in the residential occupancy and NFPA 13 protection levels in the nonresidential occupancy(s) is appropriate."

 

[Noob]

Excellent dialogue. Looks like Type VA will be the best choice due to the 4-story R-2 limitation. Likely to go with NFPA 13 (due to the increased property protection) - in which case most of the fire resistance ratings between dwelling units and the corridors will drop to 30 minutes.

At the first floor, do you all see any issue with utilizing exposed 1-hour mass timber structure (glulam beams, columns, possibly floor panels). I think it checks out, but I need to verify for "combustibility". If I did steel, I would need to wrap it to get the rating.

 

[Genduct]

Cannot do it. Other than Section 510.2 "Horizontal building separation allowance" and some other sections in Section 510, the only way you can mix various types of construction is through the use of a fire wall, which can only be vertical.

Using the theory oft he Podium Approach mentioned, I am wondering if the concept of RATED ASSEMBLY isn't more applicable, and not the strict use of Fire Wall as the only allowed Rated Assembly too restrictive?

Fire Walls do create 2 seperate Structures, So I can see your point. But we have the Podium exception that implies we can do that horzontally as well

Just thinking out loud, any additional thoughts you can add