sprinklernotes
Lateral Zones of Influence
You asked the following question: “For apartment building projects, we
typically run a main in a corridor and then have branch lines that tie into the
heads of the units. When calculating the lateral brace loads, it is obvious
that we must include the loads of the branch lines that run perpendicular to
the main. Do we also need to include the load of the pipes that run off the
branch line that are parallel to the main?”
Answer: We have reviewed NFPA 13, Standard for the Installation of Sprinkler
Systems, 2019 edition as the applicable standard. Our informal interpretation
is that the orientation of the sprinkler pipe is irrelevant. If the pipe meets
the definition of a branch line, drop, or sprig and it is tributary to the
main, it must be calculated.
The zone of influence includes the weights of branch lines, arm-overs, drops,
and sprigs tributary to the main. The direction of the pipe, whether parallel
or perpendicular to the main, does not matter. While section 9.3.5.9.6 states,
“The zone of influence for lateral braces shall include all branch lines and
mains tributary to the brace, except branch lines that are provided with
longitudinal bracing or as prohibited by section 9.3.5.9.6.1,” the 2019 edition
of NFPA 13 clarifies the intent of this section by revising it to the following:
18.5.9.6* The zone of influence for lateral braces shall include all branch
lines, drops, sprigs, and mains tributary to the brace, except branch lines
that are provided with longitudinal bracing or as prohibited by 18.5.9.6.1.
Although not mentioned in the section above, the arm-over should also be
included. In summary, the weight of the water-filled pipe for determining
horizontal loads within the zone of influence of lateral braces should include
arm-overs, drops, return bends, and sprigs. The annex for this indicates that
the zones of influence do not have to be symmetrical. Zones of influence should
include all applicable pipe from the brace to halfway between the next brace
(or end of line). It should be noted that the annex material at the end of
section A.9.3.5.9 indicates that the factor of 1.15 is intended to approximate
the additional weight of all valves, fittings, and other devices attached to
the system. It is the intent of NFPA 13 that the chosen zone of influence be
the worst-case load scenario.
