American Fire Sprinkler Association [http://www.firesprinkler.org/]
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AFSA Technical Update
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A newsletter dealing with codes and other technical issues
July 11, 2014
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Wood Trusses as Unobstructed Construction
"A basement has wood sub-floor over 18 in. deep open-web wood trusses spaced 16
in. on-center. The bottom and top cords and the webbing of trusses are
constructed
of 2 x 4's. We contend that this is unobstructed construction and the
sprinklers
should be located in accordance with Section 8.6.4.1.1 and Figure
8.6.5.2.1.3(B).
The AHJ is looking for confirmation that this design is correct. Are wood
trusses
unobstructed construction?"
We have reviewed NFPA 13, 2013 edition as the applicable standard. Our informal
interpretation is that wood trusses with a top chord less than 4 in. deep are
unobstructed
construction.
This was specifically addressed in A.3.7.1(9) Truss Construction (wood or steel)
and A.3.7.1(10) Bar Joist Construction (wood or steel) as new text for
obstructed
construction. These two descriptions have top and bottom chord members greater
than 4 in. in depth. The unobstructed construction descriptions in A.3.7.2(5)
for
Truss Construction (wood or steel) has top and bottom chord members not
exceeding
4 in. in depth. Note that the bottom chord member depth is irrelevant since
that
has minimal to no effect on heat flow at the roof or floor deck. So this
application
is combustible, unobstructed construction.
Regarding the previous editions of NFPA 13, the definitions in Section 3.7 does
not specifically quantify how deep is deep enough to affect the heat flow.
Granted
any amount will have an affect but the issue is when will there be a significant
effect. There is some guidance in Annex A.3.7.1 and A.3.7.2. The discussion on
beams says greater than 4-in. nominal thickness and joist are 2 to 4 in. in
nominal
width. These are both cross sectional dimensions. Joist specifies up to a
maximum
14-in. depth but no minimum. Under unobstructed, the dimensions identified is
for
bar joists and trusses and that is a 4 in. depth. This, though, is the cross
section
of the chord as shown in Figures A.3.7.2(a) and (c). The only indirect guidance
available is the figure for trusses. The chord in the figure appears to be a 2
x 4 on edge. Using a 4-in. depth (distance down from the ceiling) is a
reasonable
point of separation for obstructed and unobstructed construction. Thus Section
8.6.4.1.1 would apply for the deflector distance from the ceiling would be 1 in.
minimum and 12 in. maximum throughout the area of coverage of the sprinkler.
Section
8.6.5.2.1.3 would also apply (nick-named the 3 times rule) for obstructions.
Section
8.6.5.2.1.6 can be applied as an exception to locate the sprinkler in center in
between the trusses if the 3 times rule cannot be met.
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Technical Update is prepared by the Technical Services Dept. of the AFSA: Vice
President
of Engineering and Technical Services Roland Huggins, a PE registered in fire
protection
engineering; Phill Brown, a NICET IV certified automatic sprinkler technician
and
NFPA Certified Fire Protection Specialist (CFPS) and Tom Wellen, a PE registered
in fire protection engineering. This is provided with the understanding that
the
AFSA assumes no liability for this opinion or actions taken on it and they are
not
to be considered the official position of the NFPA or its technical committees.
Copyright 2014, American Fire Sprinkler Association.
All Rights Reserved
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