Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
(OP)
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?





RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
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
www.mfpdesign.com
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
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?
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
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.
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
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.
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
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.
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
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?
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
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 :)
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
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.
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
RE: Composite Wood Joists & The Dreaded 3000 sq.ft. Calc
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.