Plywood subfloor intersecting 2 hr wall

[BayPointArchitect]

Given:

I am the authority having jurisdiction.

I have a Type 3B exterior bearing wall that is 2 hour fire-rated and consists of noncombustible materials and fire-retardant-treated wood framing - except for the plywood subfloor.

Question:

Does the 3/4" floor sheathing highlighted in orange (see attachment or use your imagination) also need to be fire-retardant-treated? Or is it permissible to be regular plywood?

Thank you

ICC Certified Plan Reviewer
NFPA Certified Fire Plan Examiner

 

[Sifu]

I think the answer is no based on a presentation given by the AWC that I think answers your question, it even has nice illustrations. Give me your email and I will send it to you. Or you can check their website and look for BCD 501.

 

[Paul Sweet]

My main concern would be the floor trusses and band joist. Are they fire-retardant-treated?

 

[BayPointArchitect]

Looking at the BCD 501 presentation given by the American Wood Council, pages 69 through 72, only the framing within the exterior wall is fire-resistive-treated. The untreated rim joist (part of the untreated floor truss) is protected with untreated sacrificial wood blocking. According to that presentation, the floor sheathing and truss system do not need to consist of FRTW - provided that they are encapsulated within a fire-rated floor-ceiling assembly. In my case, I have a healthy layer of gypcrete over the top of the plywood subfloor.

 

[Bryant]

Didn't want to hijack this thread with another topic, but this configuration you presented is similar and not much different than what we see the architects attempt to do to shaft walls , for stair enclosures and elevators in 4 story 5A const. R2"s. My concern has always been, how do you maintain the continuity of the (shaft) wall without a membrane penetration into gypsum/wood frame construction?

 

[BayPointArchitect]

Bryant,

Yes, I have dealt with that before. I often recommend more sacrificial blocking that is thoughtfully put into place where the floor system (stair landing) intersects the shaft wall. The idea being that we do not build a shaft wall and then smash ledger boards into the drywall skin in order to support the floor framing. Structural framing takes precedence followed by blocking inserted according to thoughtful detailing. When all of that is complete, then the drywallers can take over the building and cover everything with the necessary amount of 5/8" Type "X" gyp.bd.

 

[redeyedfly]

we do not build a shaft wall and then smash ledger boards into the drywall skin in order to support the floor framing.

We routinely install ledgers over two layers of gyp. There are several fasteners designed for this specific application. In fact, I see this detail 90% of the time.

You are also allowed membrane penetrations into shaft walls for beam pockets, it only needs to be firestopped. That is the typical detail in non-combustible.

To the original question, I haven't encountered a jurisdiction that allows platform framing in Type III exterior walls. Typically they are balloon framed w/ FRT and the trusses are top chord bearing or installed w/ through gyp rated hangers. DCA 3 is rejected. The AHJ always wants to see a continuous FRTW load path.
A few AHJs require the top chords and ply extending into the wall assembly to also be FRTW, but that is more rare.

I hope someday the IBC will address this issue definitively. For now there is no clear direction either way.

 

[Bryant]

I think the real question to ask is, is the continuity for a shaft wall required to be self supporting of its own design or is it relying upon supporting construction, in that case a horizontal floor assembly provides the lateral restraint? If to imply the floor assembly is providing primary lateral support, then would not the floor assembly also need to be rated as the rating of the shaft? Certainly at 4 stories you are buying a 2 hour rating, that to me would imply the floor rating needs to be the same or some portion of the supporting tributary width of the floor system would need to be protected likewise in some fashion or form.
The best analogy I can think of is a shaft constructed of CMU's being grouted with bound beams restraining the lateral forces as applied. Truly the makings a firewall as the definition would imply.
Wasn't intending to dive down this rabbit hole, but the OP's pic kinda teed it up for a discussion. I think when looking thru the lenses of a means of egress component, it becomes more apparent and critical why the fire resistance is required, including the continuity of the stair shaft in this case. I would say the code path ( ICC/VCC Ch. 7 & 10) is circuitous for gaining compliance in the above scenario...

 

[redeyedfly]

The detail from the OP would not be acceptable for shaft walls. Shaft walls are fire barriers and must extend through the floor framing and up to the underside of the sheathing above. Fire barriers are substantially different from FRR exterior walls. They are also not fire walls which must be structurally independent.
Shafts must be supported by FRR assemblies with an equal or greater FRR than that of the shaft down to the foundation. There are negligible lateral loads on a shaft inside a building. The 5psf interior load cancels out on opposing sides.

 

[Bryant]

The detail from the OP would not be acceptable for shaft walls. Shaft walls are fire barriers and must extend through the floor framing and up to the underside of the sheathing above. Fire barriers are substantially different from FRR exterior walls. They are also not fire walls which must be structurally independent.
Shafts must be supported by FRR assemblies with an equal or greater FRR than that of the shaft down to the foundation. There are negligible lateral loads on a shaft inside a building. The 5psf interior load cancels out on opposing sides.

understand that, But what we see the architects typically attempt to do, is to provide a draconian UL design to mimic a shaft wall with a gypsum 2 hour rated design 4 stories or 1 hr <3 stories. The question then becomes one of lateral restraint, probably not a good word to use since it implies top or bottom as typical restraint, but rather provides the resistance to lateral loads hence the floor system comes into play to provide the check to that load. The problem with those designs is the UL designs are only for a section of the vertical wall, 8, 9, 10 feet, not stacked concurrently to form a true shaft. So they attempt to mimic the sacrificial wood or fluff as a check and balance. That in my opinion invades the continuity of the shaft and can create a membrane penetration outside of what is allowed, sprinkler ,electrical, etc.
But I think the question again becomes what of the primary supporting construction in this case the floor assembly? If it is supporting or checking the lateral load of the shaft since it is not self supporting, seems to me the floor assembly would need to be rated as well. Again the tried and true method is CMU shaft we're done, if the floor assembly is wood and burns the egress component is compromised whereas the CMU is still standing and provides a path to safety for that portion of the building. If the shaft collapses, well not a good scenario.
I have digressed, the question was about IIIB & VB exterior walls of which could also become compromised in the case of fire separation distance where the rating would require the exterior as well as the interior to be rate...

 

[redeyedfly]

understand that, But what we see the architects typically attempt to do, is to provide a draconian UL design to mimic a shaft wall with a gypsum 2 hour rated design 4 stories or 1 hr <3 stories. The question then becomes one of lateral restraint, probably not a good word to use since it implies top or bottom as typical restraint, but rather provides the resistance to lateral loads hence the floor system comes into play to provide the check to that load. The problem with those designs is the UL designs are only for a section of the vertical wall, 8, 9, 10 feet, not stacked concurrently to form a true shaft. So they attempt to mimic the sacrificial wood or fluff as a check and balance. That in my opinion invades the continuity of the shaft and can create a membrane penetration outside of what is allowed, sprinkler ,electrical, etc.
But I think the question again becomes what of the primary supporting construction in this case the floor assembly? If it is supporting or checking the lateral load of the shaft since it is not self supporting, seems to me the floor assembly would need to be rated as well. Again the tried and true method is CMU shaft we're done, if the floor assembly is wood and burns the egress component is compromised whereas the CMU is still standing and provides a path to safety for that portion of the building. If the shaft collapses, well not a good scenario.
I have digressed, the question was about IIIB & VB exterior walls of which could also become compromised in the case of fire separation distance where the rating would require the exterior as well as the interior to be rate...

I think you should read chapter 7 again. Rating of lateral bracing is discussed for exterior walls and fire walls but there are no requirements for fire barriers. Those requirements are consistent with the purposes of the different FRR assemblies. Again, there are no appreciable lateral loads on an internal shaft, there are on exterior walls and fire walls. To which ASCE-7 loads are you referring?

There is no question, FRR lateral bracing of fire barriers is not required, there is no reason to require it. The code writers didn't just forget when they got to fire barriers.

There are thousands of wood framed stair shafts; CMU is expensive, unnecessary, and doesn't work well with seismic loads in a wood framed building. If I saw a CMU stair shaft in a wood framed building I would assume the designers are inexperienced in light frame construction.
I haven't the slightest idea why you consider UL assemblies "draconian" and that they "mimic a shaft wall"; U301 & U305 are standard shaft wall assemblies.

 

[Bryant]

I think you should read chapter 7 again. Rating of lateral bracing is discussed for exterior walls and fire walls but there are no requirements for fire barriers. Those requirements are consistent with the purposes of the different FRR assemblies. Again, there are no appreciable lateral loads on an internal shaft, there are on exterior walls and fire walls. To which ASCE-7 loads are you referring?

There is no question, FRR lateral bracing of fire barriers is not required, there is no reason to require it. The code writers didn't just forget when they got to fire barriers.

There are thousands of wood framed stair shafts; CMU is expensive, unnecessary, and doesn't work well with seismic loads in a wood framed building. If I saw a CMU stair shaft in a wood framed building I would assume the designers are inexperienced in light frame construction.
I haven't the slightest idea why you consider UL assemblies "draconian" and that they "mimic a shaft wall"; U301 & U305 are standard shaft wall assemblies.

Wind & seismic loads, but that's not the factor I was eluding too, gravity & lateral loads are always present in some form or another.
uh, because the organization is over 100 years old and there is no design for stacking the same UL design 301 or 305 4 stories without an engineering judgment collectively as one measure, we are diverging here that's fine.
The OP focused in on exterior wall and afforded rating & protection with FRTW.

All I am saying is when you follow the code path for shafts and shafts that enclose interior stairs, the pathway runs from 1023.2, 707.3.1 bouncing to 713.5 back to 707.5 for continuity now the kicker 707.5.1 supporting construction, very confusing I must say, hopeful later codes will clean it up a bit
If these UL designs are being supported by the floor assembly, the floor assembly needs to be rated same as the shaft, period. Now how one gets around that really is what my whole point was. I can imagine seeing different modifications playing out from state to state...
Its really not about the continuity per say, its about the supporting construction, that's it. how one achieves that varies from jurisdiction to jurisdiction from what I see and hear.
The industry is getting onboard with light frame construction, especially with the shaft dilemma for interior stair, elevator, trash enclosures from what I'm seeing. anyway weekend is here

 

[redeyedfly]

Wind & seismic loads, but that's not the factor I was eluding too, gravity & lateral loads are always present in some form or another.
uh, because the organization is over 100 years old and there is no design for stacking the same UL design 301 or 305 4 stories without an engineering judgment collectively as one measure, we are diverging here that's fine.
The OP focused in on exterior wall and afforded rating & protection with FRTW.

All I am saying is when you follow the code path for shafts and shafts that enclose interior stairs, the pathway runs from 1023.2, 707.3.1 bouncing to 713.5 back to 707.5 for continuity now the kicker 707.5.1 supporting construction, very confusing I must say, hopeful later codes will clean it up a bit
If these UL designs are being supported by the floor assembly, the floor assembly needs to be rated same as the shaft, period. Now how one gets around that really is what my whole point was. I can imagine seeing different modifications playing out from state to state...
Its really not about the continuity per say, its about the supporting construction, that's it. how one achieves that varies from jurisdiction to jurisdiction from what I see and hear.
The industry is getting onboard with light frame construction, especially with the shaft dilemma for interior stair, elevator, trash enclosures from what I'm seeing. anyway weekend is here

UL designs must be supported by a floor assembly? What are you smoking? That is absolutely not true.
U301 & U305 are load bearing assys, you can definitely stack them. No one is questioning that but you. I have done projects all across the US and this has NEVER ONCE been questioned. Probably because the code is very clear, there is nothing to clean up. There is no shaft dilemma for anyone except you.
Fire barriers are not required to be structurally independent, nor are any bracing elements required to meet or exceed the FRR of the shaft wall. There is no need for the requirements, that's why it's not in the code.

Gravity and lateral loads are always present??? Please stop playing engineer. That is also not true. I assure you that no engineer has ever done lateral calcs on an interior stair shaft unless it is part of the lateral system for the building.

Also: "alluding"

 

[Bryant]

Smoking? Sounds a bit racist. So UL tests 60 ft tall wall assemblies 301 & 305? Don’t think it will fit in the Steiner oven.
if you open your ears and mind , all I was saying is it’s beyond the tested assembly for the above designs it’s not tested in the totality of the assembly, hence the interstitial space between wall assemblies and the floor assembly. Someone has to quantify the designs of different assemblies, it’s not the architect but the EOR.
I am no engineer , but I work with one and another commercial plan reviewer that I would wholeheartedly pit against anyone in this business. More the reason Virginia has its own codes

 

[redeyedfly]

Racist? How would I know your race?

There is no interstitial space in fire barriers. They are required to go through the floor system and be attached to the underside of the deck. That connection is required to be firestopped. I still can't find how this is confusing.

You're not required to test full scale assemblies, they test wall/floor assemblies and put them together with joint firestopping details that are also tested. 301&305 are load bearing assys,; that means they are tested with a load at the capacity of the wall. I have no idea what you mean by "quantify the designs of different assemblies". But, architects design the FRR assemblies and details, not engineers. Fire engineers get involved with EJs and the actual testing only.

You sound very inexperienced and certainly lack an understanding of the theory behind the requirements in Chapter 7. You're inventing problems where they don't exist.

 

[Bryant]

Racist? How would I know your race?

There is no interstitial space in fire barriers. They are required to go through the floor system and be attached to the underside of the deck. That connection is required to be firestopped. I still can't find how this is confusing.

You're not required to test full scale assemblies, they test wall/floor assemblies and put them together with joint firestopping details that are also tested. 301&305 are load bearing assys,; that means they are tested with a load at the capacity of the wall. I have no idea what you mean by "quantify the designs of different assemblies". But, architects design the FRR assemblies and details, not engineers. Fire engineers get involved with EJs and the actual testing only.

You sound very inexperienced and certainly lack an understanding of the theory behind the requirements in Chapter 7. You're inventing problems where they don't exist.

You sound like a dog barking at the moon, whatever dude. redeyedfly, not the stellar pseudo name, besides there a nuisance around here...

 

[BayPointArchitect]

Imagine building a four story exterior wall with balloon framing, applying two layers of 5/8" Type "X" and everything necessary for a two-hour fire-rated exterior wall. The prudent inspector is please to see forty feet in height 2 x 6 framing, continuous drywall, and no interior floor system. Fantastic! Then the framers show back up to attach floor trusses on that same wall like you hang a picture on the wall. RedEyeFly - is this what you are seeing as being normal? And where do I find 40 ft long 2 x 6 studs?

 

[SH225]

In my experience, jurisdictions are allowing platform framing in Type III. The primary question has been how to deal with the 1-hour floor-ceiling assembly interrupting the 2-hour wall. Our most common solution has been to frame the space between the floor trusses with FRT studs and cover the inboard face of that with two layers of 5/8-inch gypsum sheathing. We are sometimes required to make the floor deck FRT and sometimes not.

 

[Bryant]

I would only add this. where have you seen a 4 story platform framed shaft being built before the floor and wall assemblies were built? When have you seen a CMU shaft built before the floor ands wall assemblies were built? I know we are not talking fire walls , but fire barriers and I would assume base on the ability of the CMU shaft to stand on its on merits versus having to engage a gyp/wood assembly shaft with the floor system. What happens when you have a 1 hour rated floor assembly engaging a 2 hour rated gyp/wood assembly?
That was really the only point I was making....

 

[SH225]

In my experience, jurisdictions are allowing platform framing in Type III. The primary question has been how to deal with the 1-hour floor-ceiling assembly interrupting the 2-hour wall. Our most common solution has been to frame the space between the floor trusses with FRT studs and cover the inboard face of that with two layers of 5/8-inch gypsum sheathing. We are sometimes required to make the floor deck FRT and sometimes not.

As a clarification, this comment is in regard to exterior walls. Unrelated to shaft enclosures.

 

[redeyedfly]

Imagine building a four story exterior wall with balloon framing, applying two layers of 5/8" Type "X" and everything necessary for a two-hour fire-rated exterior wall. The prudent inspector is please to see forty feet in height 2 x 6 framing, continuous drywall, and no interior floor system. Fantastic! Then the framers show back up to attach floor trusses on that same wall like you hang a picture on the wall. RedEyeFly - is this what you are seeing as being normal? And where do I find 40 ft long 2 x 6 studs?

I'm starting to doubt the credentials you listed. I feel sorry for the architects you're reviewing. You seem to have no experience in Type III & V. My first post gave you more credit. I assumed you kind of knew what was going on, this time I have pictures. Really it's my fault; the gypcrete going under the sole plate in your sketch should have been a bigger red flag.

To the original question, I haven't encountered a jurisdiction that allows platform framing in Type III exterior walls. Typically they are balloon framed w/ FRT and the trusses are top chord bearing or installed w/ through gyp rated hangers. DCA 3 is rejected. The AHJ always wants to see a continuous FRTW load path.
A few AHJs require the top chords and ply extending into the wall assembly to also be FRTW, but that is more rare.

I hope someday the IBC will address this issue definitively. For now there is no clear direction either way.

DCA 3 provides the opinion of the AWC. https://awc.org/docs/default-source/dca/awc-dca3_20210209_awcwebsite.pdf?sfvrsn=4f5c39bc_2

As I already wrote, most AHJs reject DCA 3 detailing and require balloon framed (or "semi-balloon framed" if you want to be pedantic). All that means is that the wall framing continues through the floor system. Here are the two most typical details. Top chord bearing is far more common since exterior walls typically don't need to be firestopped. The through gyp hangers are most often used at shafts where you do need to firestop w/ a listed system. The hangers are listed without any additional firestopping required. They are not inexpensive so we try to limit their use.

And to the question of smashing ledger boards into gyp: http://embed.widencdn.net/pdf/plus/ssttoolbox/xgzdxspvcc/C-F-2017-p296-299.pdf. Simpson probably sells millions of these. The loads are more restricted so you'll see these where corridor floors or landings frame into shafts or other fire barriers. Trusses bearing on fire barriers will always get the the hangers.

 

[redeyedfly]

In my experience, jurisdictions are allowing platform framing in Type III. The primary question has been how to deal with the 1-hour floor-ceiling assembly interrupting the 2-hour wall. Our most common solution has been to frame the space between the floor trusses with FRT studs and cover the inboard face of that with two layers of 5/8-inch gypsum sheathing. We are sometimes required to make the floor deck FRT and sometimes not.

That sounds like a lot of work!! See the DCA 3 link I posted above. There are several details that should save you labor if the AHJs in your area allow platform construction.

 

[BayPointArchitect]

RedEyedFly: Your right about two things. In my quick sketch, I should not have had my gypcrete running under the sill plate. Like I said, it was a quick sketch. And I do have limited experience with Type III construction. In our neighborhood, the local architects started using Type III again about ten years ago to go along with the podium style construction. Considering that I have only twenty-six years of experience, I am still learning. Those many years prior to the 26 while doing wood-framed construction were not enough.

Its a good thing we have the "ignore" button. I will make good use of that feature starting today.

ICC Certified Plan Reviewer
NFPA Certified Fire Plan Examiner