sprinklernotes
Calculating Sprinklers Above and Below Obstructions
Question: “I have complete floor coverage using standard coverage sprinkler
protection at the roof deck of a light hazard occupancy. There are
ceiling clouds below that complete floor coverage as well using extended
coverage sprinklers with flex drops. I am not omitting any sprinklers by
using NFPA 13 section 8.15.24. I have provided a calculation for each of the
areas (one for above sprinklers and one for below sprinklers). The AHJ
has demanded I include all of the sprinklers in a single calculation. Is there
any rule in NFPA 13 that allows me to only provide calculations to the
most demanding area, especially when I am not omitting any sprinklers above or
below, because each compliment of sprinklers would fully cover the
floor area by themselves?”
Answer: We have reviewed NFPA 13, Standard for the Installation of Sprinkler
Systems, 2016 edition as the applicable standard. Our informal
interpretation is that a calculation is required to include only the sprinklers
at a single ceiling level. Since extended coverage sprinklers with flexible
drops are being utilized, in addition to the calculation of the standard
coverage sprinklers at the deck, a separate calculation should be provided
including only the sprinklers in the cloud ceilings.
The AHJ might be referencing section 23.4.4.6.4 in this situation, but it isn’t
applicable. This section tells us when sprinklers are installed above and
below a ceiling from a common supply, the calculation shall be done for the
largest water demand. This is because each area is a separate
compartment and it’s confirming that the calculation only addresses one
compartment (the most demanding). But this is not two ceilings, so section
23.4.4.6.4 is not applicable. This is a ceiling (at the upper roof deck) with
multiple large obstructions (the floating clouds). There is the case (your
referenced section s.15.24 – Cloud Ceilings) where the collection of
obstructions complies with multiple restrictions and can be treated as a
ceiling.
But, as you already stated, that only addresses whether or not sprinklers can
be omitted from the upper deck.
When we complete hydraulic calculations, we are looking for how many sprinklers
are likely to open in a single fire event with a safety factor.
Research shows us a single fire event is commonly controlled by three to four
sprinklers activating, not all the sprinklers within a design area. It is
also known when sprinklers are installed above and below an obstruction
typically both sprinklers will not activate dependent on different factors.
This is explicitly addressed by section 23.4.4.7.3 which states: “The
requirements of 23.4.4.7.1.1 to include every sprinkler in the design area shall
not apply where sprinklers are provided above and below obstructions such as
wide ducts or tables.” This is further clarified by section 23.4.4.7.3.1
which states: “Sprinklers under the obstruction shall not be required to be
included in the hydraulic calculation of the ceiling sprinklers.” There is a
caveat to the preceding guidance on not including sprinklers beneath an
obstruction in the calculation. The intent of section 23.4.4.7.3.2 is readily
understood. It states: “Where the piping to sprinklers under obstructions
follows the same sizing pattern as the branch lines, no additional hydraulic
calculations shall be required for sprinklers under obstructions.” This is also
discussed in the NFPA 13 Handbook commentary for section
23.4.4.7.3.1 where it states: “When spray sprinklers or CMSA sprinklers, which
are covered in 23.4.4.6.3, are being used and sprinklers are installed
both above and below obstructions, all sprinklers above and below the
obstructions are unlikely to open. Rather than force the user to calculate both
sets of sprinklers simultaneously, the user is directed to prove the most
demanding situation with calculations. In most cases, ceiling sprinklers will
represent the greatest demand.” In this case, since a different sprinkler type
with a different coverage area and higher friction loss through the
flexible drop is used, it would be prudent to prove to the AHJ that,
independently, both levels work hydraulically.
