1 hr wood floor with concrete as membrane protection?

[Yikes]

Proposed Type V-A apartment building over Type 1A subterranean parking garage. There are 2 dwelling units over the ramp leading into the garage, thus requiring a 3 hour lid over the ramp. For structural reasons too long to explain here, we are making a 3-hour concrete "tunnel" over the ramp to keep the fire separation; and then the 1 hour wood framed floor for the apartment above this bridges over it with a 2" seismic gap.

My problem is: how should be construct this one hour rated wood floor? Normally, I would put layers of typxe X membrane protection on the underside of the joists, but in this case be only have a couple of inches for installing a membrane. On a practical basis, the concrete "tunnel" will provide a 3 hour separation, so I'm not really worried about burn-through. But to leave off a ceiling membrane and rely on the concrete deck 2" below the assembly is a non-prescriptive solution. Is there a code rationale that supports this approach?

 

[RLGA]

Why would you need to add a 1-hour floor over a 3-hour horizontal assembly? All you need is the 3-hour horizontal assembly for the separation and it becomes the floor for the Type VA construction above.

 

[e hilton]

Do you need the 1 hr floor because of the gap over the tunnel,and this gap creates a “basement” space that extends across both units? Like a common attic, but on the bottom.

 

[steveray]

And then sprinklering that concealed combustible space.....

 

[Yikes]

Why would you need to add a 1-hour floor over a 3-hour horizontal assembly? All you need is the 3-hour horizontal assembly for the separation and it becomes the floor for the Type VA construction above.

In an earthquake, the wood-framed portion of the building shakes at a different rate that the concrete tunnel. If I tie them together they will damage each other, so I need a seismic break. I can build the wood floor as sleepers resting on the concrete. Yes, the 3 hour tunnel achieves the occupancy separation (508.4)and horizontal building separation (510.2). But because the wood building is still type V-A construction, doesn't my wood floor assembly need its own 1 hor construction. I think ehilton described it correctly: the seismic gap creates a 2" deep "basement".

 

[ICE]

Three layers of building paper between the concrete tunnel and the joist could allow a slip.

 

[RLGA]

In an earthquake, the wood-framed portion of the building shakes at a different rate that the concrete tunnel. If I tie them together they will damage each other, so I need a seismic break. I can build the wood floor as sleepers resting on the concrete. Yes, the 3 hour tunnel achieves the occupancy separation (508.4)and horizontal building separation (510.2). But because the wood building is still type V-A construction, doesn't my wood floor assembly need its own 1 hor construction. I think ehilton described it correctly: the seismic gap creates a 2" deep "basement".

If that's the case, then the 3-hour assembly still provides the protection for the floor assembly (it is like installing a suspended 1-hour ceiling system for a floor or roof structure above). As previously mentioned, if the sprinkler system used is NFPA 13, then the concealed space will need to be sprinkled; however, if using an NFPA 13R system, the concealed space does not require sprinkler protection.

 

[Enri Code]

If that's the case, then the 3-hour assembly still provides the protection for the floor assembly (it is like installing a suspended 1-hour ceiling system for a floor or roof structure above). As previously mentioned, if the sprinkler system used is NFPA 13, then the concealed space will need to be sprinkled; however, if using an NFPA 13R system, the concealed space does not require sprinkler protection.

I agree with this. The 3 hour should be all that is needed for reasons stated with regard to NFPA 13 and NFPA 13R if applicable.

I also look at what is being called a "seismic gap" as being more accurately an expansion joint rather than a basement. We may have to sprinkler basements because they can be occupied hence the risk and a need to mitigate the risk. We wouldn't need to sprinkler a 2" expansion joint as it won't have the same risk.

Based on how it is constructed though, there are other risks that you may want to look at more so because you have a cavity and moisture and/or vapor drive or even insects and vermin may become a concern.

 

[ICE]

At most you will have a furred space.

 

[ADAguy]

I agree with this. The 3 hour should be all that is needed for reasons stated with regard to NFPA 13 and NFPA 13R if applicable.

I also look at what is being called a "seismic gap" as being more accurately an expansion joint rather than a basement. We may have to sprinkler basements because they can be occupied hence the risk and a need to mitigate the risk. We wouldn't need to sprinkler a 2" expansion joint as it won't have the same risk.

Based on how it is constructed though, there are other risks that you may want to look at more so because you have a cavity and moisture and/or vapor drive or even insects and vermin may become a concern.

He beat me too it, moisture acumilation could be a concern.

 

[Enri Code]

He beat me too it, moisture acumilation could be a concern.

Based on how everything is configured, thermal and acoustic may be a concern too. Thermal may not be that big an issue (check energy code though as you may not have a choice in the matter about the need to insulate) but I can imagine how the sound of cars going through the concrete tunnel may transfer through the concrete and onto the wood floor with the cavity allowing reverberation (like a drum!) . It may actually be worth it to look into filling the cavity with a compressible insulation to take care of that. It will still allow the structural movement/ separation if it is compressible.