sprinklernotes
Small Design Areas and Adding Additional Flow
You asked the following questions: “We have a small three-story,
non-combustible, office building with small decks outside of each unit. Each
deck has one door that goes into the adjacent unit. The units & decks are
separated from the adjacent units by 8" poured concrete walls. The decks are
protected by a dry pipe system and the units by a wet pipe system. The largest
deck is 869 sq. ft., and the total area of the eight decks is 1,850 sq. ft. The
Engineer is saying that we have to calculate 1,950 sq. ft. for the dry system.
I was thinking that I would calculate the largest deck (5 extended coverage dry
pendent sprinklers), plus two heads off the wet system inside the one access
door. What is the correct way?” In response to your question, we have reviewed
NFPA 13, 2016 edition as the applicable standard. Our informal interpretation
is the calculation area should only cover the most demanding area of the dry
system.
To determine the size of your most demanding hydraulic calculation area we will
have to reference NFPA 13 Section 11.2.3.2.2.3 which states:
For extended coverage sprinklers, the minimum design area shall be that
corresponding to the hazard in Figure 11.2.3.1.1 or the area protected by five
sprinklers, whichever is greater.
Since the largest area is smaller than 1,950 sq. ft. (required by 11.2.3.1.1),
we need to determine how many sprinklers cover the 869 sq. ft. terrace. If
there are at least five sprinklers we meet the part of the requirements of
section 11.2.3.2.2.3. This aspect is discussed further in the Sprinkler
Handbook which states:
Paragraph 11.2.3.2.2.3 clarifies that EC sprinklers have a minimum design area,
namely the area that comprises not less than five sprinklers or the associated
area from the density/area curve. For a light or ordinary hazard occupancy, the
curve could be as low as 900 ft 2 (84 m 2) for quick-response sprinklers in a
10 ft (3.0 m) ceiling to 1500 ft 2 (139 m 2).
For a standard-response EC sprinkler listed for use in light hazard occupancies
and a protection area per sprinkler of 20 ft × 20 ft (6.1 m × 6.1 m), the
design area would be 2000 ft 2 (186 m 2) — that is, 400 ft 2 (37.2 m 2) per
sprinkler multiplied by five sprinklers.
For a quick-response EC sprinkler, the minimum area depends on the possible
modifiers to the operating area. If the reduction of the remote area by
11.2.3.2.3 is not applicable, such as for a dry pipe system, or the ceiling is
at least 20 ft (6.1 m) and the EC sprinkler is listed with a protection area of
16 ft × 16 ft (4.9 m × 4.9 m), the design area would be based on a 1500 ft 2
(139 m 2 ) area of operation. In this case, the sprinkler is listed to cover
256 ft 2 (25 m 2). Multiplying the minimum five sprinkler design area by 256 ft
2 (25 m 2) results in an area of operation of 1280 ft 2 (125 m 2). The
requirements of 11.2.3.2.2.3 require the larger area of 1500 ft 2 (139 m 2)
from the density/area curve or the five-sprinkler design area. If the reduction
of area is applicable such that the minimum area for the density/area was 900
ft 2 (84 m 2), then the 1280 ft 2 (125 m 2 ) area would be required. The
presence of sloped ceilings as dictated in 11.2.3.2.4 will also impact whether
the density/ area curve or the minimum of five sprinklers dictates the size of
the remote area.
Although you have a minimum of 5 EC sprinklers, you do not meet the requirement
to be as large as the area required by Figure 11.2.3.1.1 (in this case 1,950
sq. ft.). As for how to address areas smaller than the assigned size of the
remote area for the area/density method, there are several options available.
Technically, it’s always been required to meet the minimum size of the remote
area (unless using the room design method or one is covering the entire floor
area). Nonetheless, historically this issue has been somewhat ignored until
recently. This is especially true for small sub-systems like dry systems for
loading docks or preaction systems for a computer server room. The industry
norm was to define the area by the boundary of the loading dock as if it were a
stand-alone separate building. Back in the 2007 edition, text was added in the
annex (Section A.11.1.2) that defines how to hydraulically calculate a small
high hazard area. It states you have to have a full-sized remote area and that
the remote area can actually contain two different densities. That is option
one which doesn’t work for different types of sprinklers and/or systems. The
other option was added to the 2016 edition and is discussed in Section
23.4.4.2.4 and 23.4.4.2.5 which states:
23.3.3.2.4 Where the available floor area for a specific area/density design
criteria, including any extension of area as required by 11.1.2 and Section
12.3, is less than the required minimum design area, the design area shall be
permitted to only include those sprinklers within the available design area.
23.4.4.2.5 Where the total design discharge from these operating sprinklers is
less than the minimum required discharge determined by multiplying the required
design density times the required minimum design area, an additional flow shall
be added at the point of connection of the branch line to the cross main
furthest from the source to increase the overall demand, not including hose
stream allowance, to the minimum required discharge.
This option adds additional water flow such that the demand created by the
full-sized remote area is created. It is worth noting that there is no
additional flow reflecting overage experienced in a regular hydraulic
calculation. As a final note, for a system protecting an exterior canopy (or
such), if it is separated from the interior of the building by fire-resistant
rated construction, it’s appropriate to treat it as a room design method.
Furthermore, the annex section of 23.4.4.2.4 outlines the steps that need to be
taken to determine if you will need to add additional water or flow to your
calculations and provides an example.
