Open web wood trusses

[carlosgw]

2X4 open web wood trusses without out a ceiling below, more than 18" deep, less than 24" on center. I need sprinklers above the bottom of the joist but can not get 3X 3.5" away from the joist members. I think I need sprinklers between each joist (with a shield directly between the sprinklers). Am I missing something?

 

[skdesigner]

At 24" OC, I believe that would fall into the obstructed construction category.

NFPA 13 2010 3.7.1 Obstructed Construction: Panel construction and other construction where beams, trusses, or other members impede heat flow or water distribution in a manner that materially affects the ability of the sprinklers to control or suppress a fire.

NFPA 13 2010 8.8.4.1.2 Under obstructed construction, the sprinkler deflector shall be located in accordance with one of the following arrangements:

(1) Installed with the deflectors within the horizontal planes of 1 in. to 6 in. below structural members and a maximum of 22 in. below the ceiling / roof deck.

 

[carlosgw]

They are open web trusses.
Looking at a elevation of the truss I would have said the openings were at least 70%. I just did some measuring and the openings may actually be less than 70% (which would make if obstructed).

 

[carlosgw]

On the other hand the wood truss is specifically used as an example of unobstructed construction in the annex.
I am not sure what they mean by the depth not exceeding the least dimension.

 

[skdesigner]

NFPA 13 2010 3.7.2 Unobstructed Construction: Construction where beams, trusses or other members do not impede heat flow or water distribution in a manner that materially affects the ability of a sprinkler to control or suppress a fire. Unobstructed construction has horizontal members that are not solid, where the openings are at least 70 percent of the cross-section area and the depth of the member does not exceed the least dimension of the openings, or all contstruction types where the spacing of the structural members exceeds 7 1/2'.

So... If the openings are less than 70% - Obstructed. If greater than 70% - unobstructed.

To answer your latest question, think of it this way: The opening in the truss are going to be triangular. Your least dimension would be the shortest side of the triangle (likely the "height"). You've stated that the webs are made of 2x4's. The depth of the 2x4 member is what you now take into consideration. Is the 2x4 deeper than the height of the triangle?

Of course, the above is my interpretation, and I may be corrected by some of the more knowledgeable members.

 

[carlosgw]

If it is the height - the depth of the member (I assume they mean the entire truss), the hieght of the opening could never be greater then the depth. If that is what they mean, it is impossible so why write it.
My trusses look just like the bottom truss in Fig. A.3.7.2(c). The length of the two equal sides (as opposed to the height) of the larger triangle are less than the depth.

I just did a better area calculation (based on the architects drawing which may or may not be accurate). It came out to 71%.

 

[skdesigner]

So, we've deduced that it is unobstructed construction. Standard heads need to be between 1" and 12" from the deck.

Take a look at Figure 8.6.5.2.1.3. You have to be 10.5" from any member, but this shouldn't be a problem since your branchline is running through the open space in the truss to begin with. Set the line in the widest part of the opening, and put your sprinkler dead center between the trusses and be done with it.

Keep in mind Table 8.6.2.2.1(a) requires that even if light hazard, your heads need to be spaced for a protection area of 130 sq. ft.

 

[ryano669]

You might look into these

http://www.tyco-fire.com/TD_TFP/TFP/TFP632_07_2007.pdf

to increase spacing to 144sq.ft.

 

[carlosgw]

Even if the trusses are 24" on center I can not get 10.5" from the trusses. It is more likely the trusses will be 16" OC.
I think I need sprinklers between each truss.
The concealed combustible space sprinklers may work, but this particular application is not concealed - it is exposed from below.

 

[skdesigner]

If they were 24" OC, would you not be 12" from each truss, measured to the centerline of the head, if the head is centered between the trusses?

You have to think of this in 3d. By looking at a plan view of a 16" OC truss layout, then no, you couldn't get 10.5" away. However, the fitter will hang the line through the opening in the truss, so your measurement would be taken diagonally from the nearest member. If the truss is 18" deep as you state, call out your branchline el. at 9" below the deck. Running through the widest open space, you'll be 8" horizontally from each truss, plus the diagonal distance from any member.

I can agree that words alone are hard to interpret. If you want to sprinkle between every truss (sprinklers spaced at 16" intervals!!) be my guest. Even if you used the QR reduction, and you got the design area down to 900 sq. ft., you would have something like 50 sprinklers in your design area, all flowing a minimum of 7 psi. Ouch.

 

[carlosgw]

I don't "want" to put sprinklers between each joist.
The vertical members are 2x4s also. If they are 24"OC that leaves 20.5" between the inside edges. Maybe at 24"OC there is a spot even with the middle of the large triangle that gives me the clearance. The architect is showing 14"OC. I asked him and he said more likely 16" but maybe 24".
I suggested solid joist but then the space would be lost to running other trades. (plus they slope so maybe they can not make a wood beam with a slope)
I haven't figured out why they are using wood yet - cheap.