Remote Area Width Based on Original/Adjusted?

3/15/2023

Is the minimum length for a remote area determined by the original Density/Area (such as 1,500 sqft so it would be 46.5 ft), or is it based on a modified/adjusted area (such as 900 sqft = 36 ft).

After doing this for years, I was trained and always took is as a given that we use the length based on the original area. I cannot seem to find an actual reference backing this up.

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11 Comments

Pete H

3/15/2023 06:47:20 am

I don't have any code reference on hand, but the way I've been trained is the longest distance in your remote area is orientated to run parallel to the branches, and the shorter distance of your rectangle to run perpendicular to that.

Granted, room shapes, building shapes, and pipe arrangements don't always make for perfect rectangles, and if the building size doesn't allow the full 1.2 multiplied by the square root of the design area length to run in the same orientation as the branch lines, then you make the perpendicular measurement in your rectangle larger to make the required area.

Apply this logic as necessary for alternate arrangements that do not allow for a perfect rectangle.

Dan Wilder

3/15/2023 07:32:42 am

NFPA 13 - 22'

Area is taken from Section 19.2.3.1.1 (single point or density/area curves) and adjusted accordingly based on (not a complete list):
-Building construction (roof/ceiling slope 19.2.3.2.4)
-Sprinklered/unsprinklered allowances (19.2.3.1.5.2)
-QR sprinklers (19.2.3.2.3.1)
-High temp sprinklers (19.2.3.2.6)
-System type (dry or DI preaction 19.2.3.2.5)
-Orifice Size (19.2.3.2.7)
-Multiple variations of the adjustments based on the original area of operation, including the section 19.2.3.2.8.2 for unsprinklered combustible concealed spaces. As they are multiplicative, there is no order of operations, the end answer will always be the same (decrease 20% & increase 30% is the same as increase 30% and decrease 20%).

28.2.4.2.1* Where the design is based on the density/area method, the design area shall be a rectangular area having a dimension parallel to the branch lines at least 1.2 times the square root of the ***area of sprinkler operation (A) used***, which shall permit the inclusion of sprinklers on both sides of the cross main.

Emphasis on the "area of sprinkler operation used***. This does not reference the minimum, standard, or maximum square footage of the remote area....just that the calculation is to take into account the area used x 1.2 as found in Section 19.2.3.1.1.

Like much of NFPA, the earlier it happens in the book typically means that is a starting point and is modified after via subsequent and/or subsections. The book provides area/density tables, then modifications up and/or down, then tells how to perform the hydraulic calculation based on the information.

danefre

3/15/2023 08:22:38 am

Dan and Pete nailed it.

A bit unrelated to this - one of the common mistakes we see in dry systems with pitched roofs is the way the 30% increases are taken.

30% for dry & 30% for pitched is NOT a total of 60% increase

David Kendrick

3/15/2023 09:47:36 am

NFPA 13 - 2016 Annex

A.11.2.3.2.7 Example 1. A dry pipe sprinkler system (OH2) in a
building with a ceiling slope exceeding 2 in 12 in. (50 in 300mm)
(16.6 percent slope). The initial area must be increased 30 percent
for the dry pipe system and the resulting area an additional
30 percent for the roof slope. If the point 0.2 gpm/ft2 (8.1 mm/
min) over 1500 ft2 (139m2) is chosen from Figure 11.2.3.1.1, the
1500 ft2 (139 m2) area is increased 450 ft2 (42 m2) to 1950 ft2
(181 m2), which is then further increased 585 ft2 (54 m2). The
final discharge criterion is then 0.2 gpm/ft2 (8.1 mm/min) over
2535 ft2 (235 m2).

So the first 30 percent add to 1500 square feet is 1950. The second add is 1950 times 30 percent for 2535 square feet.

danefre

3/15/2023 10:24:56 am

Thanks for the reference. I'm glad the annex has an example.

Surprisingly, I have seen boilerplate spec's actually say increase by 60% for dry system and pitched roof.

Glenn Berger

3/15/2023 08:15:02 am

I am not quite sure what your question is implying. If you have a renovation project that requires sprinklers to be replaced / relocated and with piping reconfigured then you need to verify that the original conditions are not affected from the design.

Anthony

3/15/2023 10:30:13 am

As others have said, and I agree, the length of the longest side of the remote area along a branch line should be based on the modified area.

In your example yes you should use 36'-0.

sean

3/15/2023 12:03:38 pm

The final area is what determines the width.

Franck

3/15/2023 12:07:34 pm

It has to be done on your final design area.
If it is modified to 900, this is what you have to use.
Otherwise you won’t have the same required shape for your calculation. It might end up with a more favorable result (if your length along the branchline is proportionally smaller than with 1.2 factor) or unfavorable if the length is longer.
In your case, using 46.5 instead of 36 ft for 900 sq ft would end up with a higher flow and pressure demand (enhanced friction loss contribution along the branch lines).

I used to work for an insurance company that used to require a 1.4 factor instead of 1.2 as a conservative approach… And it does make a difference on the end result…

DAVE

3/15/2023 12:07:53 pm

Agreed, it's 1.2 x the square root of the adusted. I have always explained to trainees that once the final size of the remote area is determined, the multiplier adjusts a theoretical perfectly square fire into a more demanding NFPA 13-required rectangular remote area, and aligning with the branch lines to pick up more sprinklers usually (but not always) creates a greater system demand.
Interesting about the 30% plus 30% for dry system, I have not yet found someone making that 60% mistake, but will watch for it.

Alex

3/16/2023 05:57:47 am

As others have mentioned, you use your final area when determining branch lines and number of heads. Same goes for dry or pitched systems.

Remote Area Width Based on Original/Adjusted?

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