Supporting noncombustible secondary framing on conmbustible primary

[Hoboneer]

Is there any kind of code stipulation that prevents supporting secondary non-combustible framing members on primary combustible member? For example supporting C-joist roof framing on an LVL middle girder that's further supported on steel columns?

Further, is there code stipulation that prevents or allows such "mixed" material construction?

If allowed, would it require an upturned sprinkler line running down the LVL girder?

 

[phamENG]

As long as your construction type allows for combustible materials, you can use them. You're always allowed to provide something better than the code minimum.

 

[KootK]

I struggle with fire stuff so take my response as conversation fodder rather than any kind of definitive advice. I feel that the crux of this is the philosophical difference between:

1) The anticipated fire load which which informs regulation on the use of combustible materials. There are just some situations in which you do not want your building made from kindling such that it would burst into a raging inferno in short order. This is about available fuel for the fire.

2) Measures taken to slow the spread of fire and delay the structural damage caused by it with the intent of buying time for occupants to escape. This includes all of the stuff in our fire rated assemblies as well as, I think, sprinklers (flashover etc).

Consequently, I feel that even a sprinklered wood beam contributes to the fire load since, ultimately, sprinklers are not to be relied upon to actually put fires out wholesale in most situations. They are primarily just measures taken to buy time. This leads me to suspect that you cannot use combustible structural elements within structures that are code mandated to be non-combustible.

Rationally, if 95% of your building material mass is non-combustible, does the addition of a handful of combustible beams really add anything meaningful to the fire load? Surely not. However, you may have to seek some manner of building official dispensation to make such a setup street legal. And, usually, that kind of hassle is best avoided.

As an example that is, admittedly, a bit of a reach, check out the system proposed in this paper: Fire Protection of Windows Using Sprinklers. There, the sprinklers are used to make a fire rated assembly out of a material (glass) not normally suited to that function. But, again, this has not changed the fire load. It's still just about buying time for folks to high tail it out the door.

 

[DBronson]

Hoboneer:

The answer to your first two questions is no - mixed material types are permitted and indeed, used all the time. The code only prohibits the use of combustible materials for a couple of building types: high hazard facilities (H-1) and some institutional facilities (I-2). The code limits the allowable building area by material type - non-combustible buildings can have much larger areas than combustible ones. The building use is what generally drives the requirement for sprinklers. They are also used to increase the allowable area of the building - up to 200%. So the answer to your third question is maybe - depending on the use/area of your building. Note that, if sprinklers are required by code, they will need to be installed throughout the space (or the entire building). They would not be installed just to protect the LVL. All this assumes you're here in the US and using the IBC.

Regards,
DB

 

[Hoboneer]

Let me give a little bit more info: this is an existing warehouse space with masonry walls and steel columns, beams/trusses supporting the roof. There's no fireproofing on the beams. Sections of the roof need to be replaced and the client wants to go with LVLs and (now) wood joists. The space will be sprinklered below per code, but also all the wood framing will be sprinklered directly. I don't like the idea but at the same time cannot directly point to a section in the code that would prohibit this.

@KootK: I fully share the sentiment of not involving the building official in this decision.

@DBronson: correct, the sprinklers will be installed throughout the space per code AND also to protect the wood framing.

 

[phamENG]

So you're changing your construction type from Type II (Noncombustible, Unprotected) to Type III (Ordinary, Unprotected). So you'll need to confirm with the code official that the change is acceptable for the proposed occupancy, building height, etc.

 

[dhengr]

Hoboneer:
Actually, heavy timber frames, heavy timber beams and columns, and GluLam members perform quite well vs. steel columns, beams and open web steel joists, under fire conditions. The steel will almost invariably yield and buckle under the high heat before the heavy wood does. I don’t think I’ve ever seen the same testing results for LVL’s, 2x joists and their hardware, they are just too thin to develop the charring/insulating protection achieved with the heavier wooden members. This lighter framing has to be covered with sheetrock (some fire rating mat’l.) to start to march up. I assume this decision, on the contractor’s part, is being driven by the immediate availability of the LVL’s and 2x’s, as apposed to some lead time for bar joists, etc. and the assumption that any cheap carpentry crew can do the wood roof system. But, then add the initial cost and ongoing maintenance cost of a sprinkler system, and I wonder if that’s in the best interest of the bldg. owner. Structural Engineering certainly has changed over the years. They used to come to us for our engineering advice and design. Now, they seem to be saying, this is what I’m going to do, in my infinite wisdom and want for quick completion and cheap first cost, you (the SE) make it work, stamp it, act as the insurer of last resort, or I’ll find someone else who will, do you want the engineering job, or not?

 

[phamENG]

Now, they seem to be saying, this is what I’m going to do, in my infinite wisdom and want for quick completion and cheap first cost, you (the SE) make it work, stamp it, act as the insurer of last resort, or I’ll find someone else who will, do you want the engineering job, or not?

You know...I'm beginning to realize this is NOT as prevalent as we often think it is. It's certainly out there and more common than it should be, but I'm finding more and more people who are on board with the 'engineers are actually good for the project' idea. I think this board invites people who find themselves in this position because:
1) people who come here for help are probably not getting help where they should - their bosses/mentors. That's because they're probably at poorly run companies with no training system in place. Which means they probably can't attract and keep the good clients, either. Which means they're stuck with a lot of pushy clients who fit this bill.
2) several of us regulars are solo or small shops, so we've had times where we had to 'take what was available'. And everyone else probably has had at least one run in with these characters, so we all have our stories.

So it's a problem, but I think it's a little overinflated here.

Alright, we can resume our original thread now...

 

[Hoboneer]

@dhengr: it's not a lousy client by any means and normally they follow and appreciate my advice. This is just one of those times where a combination of various factors lead us to this issue and yes, time is of the essence.

@phamENG: I'm a solo-dolo shop and this is literally the only time so far when the client is politely asking me for a favor of sorts.

While never dismissing constructive input, I've never allowed any client to dictate the design to me and I agree with you that such a thing isn't prevalent at all.

 

[JLNJ]

Ask the architect - it's their realm. As phamENG said, some construction types allow this, some don't. It all depends on the area, height, use, building separation, sprinkler density, etc (and which building code is enforced, of course).

Maybe the building is short enough and small enough that it doesn't need to be all non-combustible construction. I've seen plenty of all-wood warehouses.