• Welcome to The Building Code Forum

    Your premier resource for building code knowledge.

    This forum remains free to the public thanks to the generous support of our Sawhorse Members and Corporate Sponsors. Their contributions help keep this community thriving and accessible.

    Want enhanced access to expert discussions and exclusive features? Learn more about the benefits here.

    Ready to upgrade? Log in and upgrade now.

Rated but Exposed Structure Floor Assembly in VB Small Apartment Project

E

ETThompson

SAWHORSE
Joined
May 19, 2017
Messages
190
Location
Columbus, Ohio
Hi

I'm working on a small apartment project, about a dozen units, 3 stories, type VB construction, new construction. The owner would really like to have no ceilings leave structure and ductwork exposed and paint out. Not that it relates to the question (I think), but we have a small-ish retail space on part of the ground floor. We also have a small rooftop amenity. We are under the Ohio Building Code which follows the 2015 IBC mostly.

We have a type 13 sprinkler system throughout, so per Section 711 I can have 1/2 hour only between dwelling units, per the exception in 711.2.4.3. And per 711.2 exception 2 the supporting structure does not have to be protected for type VB construction. I take this to mean i could, in theory, have the full fire rating in the floor assembly above the trusses.

However, so far as I've been able to find, there is no tested UL assembly for 1/2 hr with exposed wood trusses below, tried the UL Directory, USG Design Studio and other resources.

So, I've been looking at either the prescriptive (section 721) or calculated (section 722).

In 721, the closest I see is Item 13, double wood floor over joists (as a side note, this includes asbestos paper. I would assume that I cannot use open web wood trusses in this scenario. Anyway, all these options still have plaster or gwb ceilings - which I would also assume rules out putting this layer on top of the structure.

The sections I see with "none" listed under ceiling construction are lightweight concrete on steel deck - so does not apply.

I do see item 21 which has wood trusses, but unless i can put the gwb "ceiling" on top of the structure, that doesn't work either.

So, I ruled this approach out.

Then I looked at section 722. The wood section there 722. 6. 722.6.2.1 tells me that if I use a membrane it has to be on the exposed side - not clear if this rules out any possibility of putting the membrane on top, between the structure and the floor assembly. If so, would kind of knock this approach out too. 722.6.2.4 tells me that I have to have a regular wood subfloor and finish floor (which we'd have) or I can use any membrane in 722.6.2 [1] as long as it's more than 15 minutes. However, it also says, in Note a, that this only applies to joists, not trusses. Could I use TJIs with 1 layer of type X GWB (40 min per 722.6.2 [1] in between the joists, say screwed to the underside of the subfloor? If not, do I have any other options here?

The only other thing I can see is 722.1 tells me I could calculate based on a Wood Association manual. I don't have this, is this likely to provide a solution?

So with all this, ultimately my question is, is there any way to do a 1/2 hr wood truss floor assembly with exposed structure / no ceiling?

Thanks, Eric
 
Not into assemblies, ......


Have you looked for one hour wood
assembly??

Not sure why 1/2 ever was put in the code, but someone wanted it.


Does your ceiling have to be one hour??????
 
The problem is that the IBC does not define what constitutes “supporting construction.” I look at this as primary versus secondary structural frame. The primary structural frame (defined in Chapter 2) is not required to be protected per Table 601. However, the floor construction, per Table 601, includes the secondary members (also defined in Chapter 2). My understanding is that the primary frame and bearing walls supporting the horizontal assembly do not require protection, but the joist and deck above do require protection. The fire rating test for horizontal assemblies is from below the assembly, so no amount of protection on the top side will protect the assembly from the bottom side.
 
Look at heavy timber construction and then calculate the fire rating.

Also look at the testing done to justify fire rating of cross laminated timber.
 
cda said:
Not into assemblies, ......


Have you looked for one hour wood
assembly??

Not sure why 1/2 ever was put in the code, but someone wanted it.


Does your ceiling have to be one hour??????
Click to expand...

Yes, we looked at a lot of 1 hr assemblies...could not find any that did not have a membrane (usually Type X GWB or plaster) below the joists.
 
Can you build the 1/2 or one hour at the ceiling/deck,,,

Than put the pretty wood below it????
 
RLGA said:
The problem is that the IBC does not define what constitutes “supporting construction.” I look at this as primary versus secondary structural frame. The primary structural frame (defined in Chapter 2) is not required to be protected per Table 601. However, the floor construction, per Table 601, includes the secondary members (also defined in Chapter 2). My understanding is that the primary frame and bearing walls supporting the horizontal assembly do not require protection, but the joist and deck above do require protection. The fire rating test for horizontal assemblies is from below the assembly, so no amount of protection on the top side will protect the assembly from the bottom side.
Click to expand...

OK thanks, I had not considered that, but good point. Seems like I have to tell the owner they can't do it.
cda said:
Once again not into assemblies::


https://www.constructionspecifier.com/design-of-fire-resistive-exposed-wood-members/
Click to expand...

Interesting, may try that if I can figure it out for my project. There's a reason I did not become an engineer!
 
ETThompson1 said:
How would that help? A membrane that achieves 1/2 hr is not hard - I could use a single layer of Type X. The question is where it has to be, above or below the structure, no?
Click to expand...



Rgla says below


You said not enough hieght, that is why the shaftliner

Or a piece of sheetrock
 
cda said:
Rgla says below


You said not enough hieght, that is why the shaftliner

Or a piece of sheetrock
Click to expand...

Well, no matter how thin the assembly, I don't have enough height currently to put the "pretty" wood below, if I'm understanding what you're saying.

I have a 10'-0" floor to floor. Subtract the 14-16" structure, plus the subfloor and floor above and we're probably down to 8'-6". Even if I use a piece of Type X below that, I would be getting pretty close to 8'-0" or lower clear, which is too small.
 
I believe steveray suggested your most practical solution (intumescent coated), and the truss/I-joist manufacturers may even carry pre-treated units for you. It's certainly worth looking into.
 
https://www.fireengineering.com/art...tructural-collapse-under-fire-conditions.html

TRUSS SYSTEM PERFORMANCE UNDER FIRE CONDITIONS
Truss system manufacturers and public safety officials have debated the performance of truss systems under real-life fire conditions for some time. Literature reviews and case studies show the need for additional research to provide the ability to effectively evaluate collapse potential. Engineering shows the fire performance of these systems under controlled conditions. Exposed truss members fail in as little as five to 10 minutes. Protected truss members show fire resistance ratings of one to two hours. Since actual fire conditions are an uncontrolled environment, truss system failure becomes less predictable.

Few indicators of imminent structural failure are available to firefighters and incident commanders. Building construction is a small part of the basic firefighting course curriculum; this usually includes the differences between structural loads (live loads, dead loads, impact loads), different construction types (fire resistive, noncombustible, ordinary, heavy timber, wood frame), construction materials (wood, concrete, steel), and a brief section on building collapse. Most firefighting textbooks identify collapse indicators as the following:
 
If considering the intumescent option, be careful about the product or products you select. Some manufacturers will say their product is usable for wood, but they're not clear as to what purpose it has been tested on wood. In most cases, the intumescent coating is approved on wood for getting a Class A rating per ASTM E 84, but has not been tested (or failed testing) for compliance with ASTM E 119. You have to make sure the coating has been tested per ASTM E 119 for the particular application (horizontal assembly, in this particular case).
 
  • Like
Reactions: JBI
On a separate but related note to your design, IBC 1207 also requires airborne sound insulation of STC 50 and impact sound insulation of IIC 50 between dwelling units.
Most tested assemblies I've worked with have used layers of gyp board for mass and batt insulation for deadening. You'l want to check and see if you can find tested assemblies for open trusses - - and/or consult with an acoustical engineer.
 
You must log in or register to reply here.
Back
Top