Composite Wood Joists & The Dreaded 3000 sq.ft. Calc 2

[Haldorson]

It seems that for any NFPA 13 project that uses composite wood joists a 3000 square foot calculation is required unless the composite wood joist volume is broken up into smaller 160 cubic foot volumes using 1/2" GWB or better on all faces. In other words, the joist web construction is not adequate.

Is there any other way around a 3000 sq.ft. calc besides sprinklering each joist channel?

 

[TravisMack]

We often see this in apartments. When the TJI is between 2 full height walls, it is rare to see them exceed 160 cubic feet volume.

However, in the times when we have been required to do the 3000 sq ft calc, you have a couple choices. Nothing says you can't go up the design curve. If this is light hazard, your new density is 0.07 / 3000. If you go to 4.2k sprinklers, you are at 158 sq ft with 7 psi minimum pressure. That may help to mitigate some of the issues. If you are in OH1 or OH2, you can still go up the curve, but you will need to stick with 5.6k sprinklers. You will be at 0.12 or 0.17 gpm / sq ft. It does seem to help with pipe sizing by going up the curve in these cases.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[Haldorson]

Here is the catch with the 160 cubic foot volume as it reads to me: the volume calculation is not limited to composite wood joist channels! Therefore unless the constructor specifically installs 1/2"GWB within the composite wood joist channels creating 160 cubic foot volumes then basically every project with TJI's will require a 3000 sq ft calc. See 11.2.3.1.4.(4).(j) in NFPA 13 (2010).

Thoughts?

 

[SprinklerDesigner2]

Haldorson, if I understand you correctly I don't think you're right.

 

[Haldorson]

I hope I'm wrong. Can you explain why?

 

[SprinklerDesigner2]

I don't think it has to be gypsum board but anything compatible with what the composite wood joist is made from such as plywood. I've never had situation where the voids exceeded 160 cu ft so I would want to confirm first bu what is wrong with using a couple 2x4 and amall piece of plywood as long as it was the same grade and thickness used in the construction of the joists?

Typically you could create eight stops from a single piece of plywood and eight pieces of cheap low grade 2x2. A lot cheaper than installing 8 lines with maybe 60 heads.

Caution, run it by the building professional first... don't think they would have a problem with the approach but make sure before you go with it.

 

[Haldorson]

No, it explicitly states that the composite wood joist webbing is not adequate and a 1/2" GWB is required to avoid the 3000 sq.ft. calc.

 

[SprinklerDesigner2]

Haldorson,

So I learn something new every day!

Never really had to look at it closely because so far I've never had one that exceeded the 160 cu ft.

 

[Haldorson]

But here is the thing, if you have a TJI 12" deep, and the floor plan covers more than 160 sq ft, then right away you have exceeded 160 cubic feet. This is practically every project. The joist channel compartments don't factor into the 160 cubic foot calculation at all. In other words, the TJI webbing should be all but forgotten when calculating the volume. The 160 cubic foot calculation is from drywall ceiling to floor, and from 1/2" GWB to 1/2" GWB lengthwise and 1/2" GWB to 1/2" GWB widthwise.

Without the GWB, to avoid the 3000 sq.ft. calculation it must either be a residential occupancy, or the concealed space is completely filled with non-combustible insulation or sprinklered (not practical). I don't see any other way around it.

Now, does the 3000 sq.ft. calc occur on the floor above the combustible concealed space or below? If above, then what about if the TJI is at the roof assembly? If below, then what about for 8.15.6, spaces under ground floors?

 

[SprinklerDesigner2]

Haldorson, It's obvious it's something I never looked at closely because I've never encountered TJI construction in anything but an 13R system.

I'm learning, Georgia just adopted the 2013 edition of NFPA #3 on January 15th this year making the jump from 2002. Eleven years, I suppose it is about time. I have my new copy of the 13 handbook and I read a few pages each night. Lots of exciting new stuff to learn!

8.15.1.2.8 "provided that in composite wood joist construction the joist channels are firestopped into volumes each not exceeding 160 ft3 (4.53 m3) to the full depth of the joist with material equivalent to the web construction, shall not require sprinkler protection." I am a newbie here but if I am reading this correctly it can be fire stopped with 1/2" plywood if the web is 1/2" as long as we use the same grade plywood?

But even if it is fire stopped we're looking at the dreaded 3,000 sq ft. if I am reading this correctly.

Oh, I got it....

"Light or ordinary hazard occupancies where noncombustible or limited-combustible ceilings are attached to the bottom of composite wood joists either directly or on to metal channels not exceeding 1 in. (25.4 mm) in depth, provided the adjacent joist channels are firestopped into volumes not exceeding 160 ft3 (4.5 m3) using materials equivalent to 1⁄2 in. (12.7 mm) gypsum board and at least 31⁄2 in. (90 mm) of batt insulation is installed at the bottom of the joist channels when the ceiling is attached utilizing metal channels."

It's apparent I have some reading and catching up to do. Be back in a few days when I know what I am talking about

 

[skdesigner]

SprinklerDesigner2 - A jump from 2002 to 2013? That's incredible! So much change...

For what it's worth, I believe TJI's in 13R buildings are still an issue. NFPA 13R-2010 7.2 states "For areas outside the dwelling unit, the design discharge and design area criteria shall comply with NFPA 13, unless permitted by 6.4.7 and 7.2.2.

6.4.7 indicates areas where residential sprinklers are permitted outside the dwelling unit. 7.2.2 states that you can limit the design area to four sprinklers IF the subsequent conditions are met.

If your 13R building had, say, a large dining room, or a lobby in a motel that did not meet the requirements of 6.4.7 or 7.2.2, you would be stuck looking to NFPA 13 for the design requirements. NFPA 13-2010 11.2.3.1.4 (j) is going to throw the 3,000 sq ft design area at you if the second floor is framed with TJI's.

I ran into this issue a while back, so I posed it to NFPA. They confirmed (albeit informally and with a disclaimer) that the 3,000 sq. ft. DA would be required in situations like this.

 

[Haldorson]

Yes, that is frightening! I can tell you that a lot of projects that require a 3000 sq.ft. DA are installed with a 900 sq.ft. DA. But with the 3000 sq.ft. DA you do get to reduce the density and not flow sprinklers in bathrooms, closets, and other small compartments requiring only one sprinkler as per 22.4.4.6.2.

 

[skdesigner]

Haldorson - Yep, whatever it takes to keep that Q in the hazen-williams equation as small as possible!

 

[Haldorson]

SK - What about the calculation location? Do you flow the sprinklers on the floor above or below the combustible concealed space? If above, then how do you manage the TJI's located at the roof assembly? If below, how do you manage the combustible concealed space below the ground floor as per 8.15.6?

 

[skdesigner]

In a 13R building, I've never had to worry about second floor, as the space still falls into the res sprinkler criteria. I calc 3,000 on the main floor (doing EVERYTHING in my power to keep the flow down). If a roof is framed out of TJI's (not common in my part of the world), it is typically gyp'd underneath and packed full of insulation.

If it's too high to be a 13R building, it's typically non-combustible, and makes the concealed spaces a moot point.

With unsprinklered combustible spaces beneath ground floors, you got it - 3,000 sq. ft. The key word is adjacent meaning it could be above, below, to the left or to the right.

Bummer.

NFPA 13-2013 has altered the residential sprinkler section of chapter eleven from 3,000 sq. ft. to 8 res sprinklers. Could really help with a poor water supply and a salesman who allowed for 1" blaze lines throughout.