Hydraulically most demanding area shape, regarding walls

[fpst]

NFPA 13 says to use 1.2 * sqrt(min remote area) to find the L dimension of the remote area rectangle. I need to know if this means we just ignore walls and assume the fire teleports through them, or if we use them as barriers and can then make square or "tall rectangle" remote areas, see the pdf for graphical representation.

 

[Sdpaddler50]

fpst, I am not a sprinkler designer, but I dust off my hydraulic program a few times per year to run calcs, more for verification purposes in consulting, not final design. If i had a one hour, floor to ceiling wall, i would stop the most remote area at that point. The 1.2 rule (1.4 for FM i believe) is just to ensure designers dont calc unreasonable demand areas. The 1.2 dimension assures we calc the area most representative of the expected fire growth area. If the area is bounded by rated walls, then using those boundries is allowable. But, i will let the people who design for a living correct me if I am off base here.

 

[skdesigner]

It depends on what calculation method you have elected to use. NFPA 13 does not say multiply 1.2 by the square root of the design area to determine the length of the design area, NFPA 13-2010 22.4.4.1.1.1 does (i.e only one part of the standard). 22.4.4.1.1.1 does not tell you to stop at a wall, it says "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 used."

22.4.4.1.2 Room design method states "Where the design is based on the room design method, the calculation shall be based on the room and communicating space, if any, that is hydraulically most demanding".

In this case, you would stop at the walls. All depends on which method best suits your particular design.

 

[fpst]

So skdesigner, are you saying that if I wasn't doing the room design method, I would not stop at the walls? I'm having trouble interpreting your message

 

[skdesigner]

I'm saying that if you're using the density/area method, your design area length must be at least 1.2 times the square root of the design area, regardless of building features.

Keep in mind that NFPA 13-2010 22.4.4.1.1.3 states "In systems having branch lines with an insufficient number of sprinklers to fulfill the 1.2 requirement, the design area shall be extended to include sprinklers on adjacent branch lines supplied by the same cross main." Meaning, the only time your design area is permitted to be shorter than 1.2*sqrt(DA) is when the branch lines are not physically long enough to fulfill the requirement.

 

[fpst]

Thank you skdesigner, was right there in the standard, I just didn't want it to be that way so I guess I was blind to it *facepalm*
Can anyone put forward a probable theory as to why it's this way, when clearly a fire is going to be hindered by a wall, especially one without a door or a CMU fire wall. NFPA, I want to understand you!

 

[stookeyfpe]

It's a factor of safety in anticipation that an opening protective such as a fire door does not close. If the fire occurs at the room adjacent to the opening, we may see flame rollover out of the opening until water is discharged. The 20% increase compensates for the door not closing, or not being closed if a fire-resistance rated door assembly is not required (which is generally the case).

 

[cdafd]

""""""""""" How you treat walls when the design basis is the same, though, depends on whether you're designing a new system or modifying an existing system. In designing new systems, you can ignore walls in the portion of the building with the same design basis and select the remote area simply based on the most demanding adjacent fire sprinklers. For existing systems, however, you must treat walls as boundaries. The ease of placement is no longer the driving factor, and you should consider the impact walls will have on a fire's heat flow. Is it possible to create a more demanding remote area by ignoring walls? Certainly. Since this is another gray issue, however, the AHJ must again define what he or she will allow."""""""""""""

http://www.nfpa.org/categoryList.asp?categoryID=714&URL=Publications/NFPA Journal&cookie_test=1#3

 

[fpst]

Hmm, interesting link cdafd, just goes to show how easily one can get confused in all of these rules!
I can't follow that article as I'm new to sprinkler design and haven't even seen the circa 2003 nfpa 13, and all of the sections he refers to for that quote are missing/different now in nfpa 13 2013. From what I gathered, however, he is saying to sometimes treat walls as boundaries as a safety factor, because it could cause elongated remote area shapes along branch lines and make the hazard more demanding, which is the opposite of my case. Treating walls as barriers in my case would mean more of a square shape for a remote area.