Building Area Increase Permitted for NFPA 13R?

1/13/2020

Will you please clarify why a Building Area Increase is NOT permitted for an NFPA 13R design?

We are working on a project and the architect has stated that the calculations following IBC T506.2 allow for an area increase up to 135,000 sf. We are working in IBC 2015. The Owner would like confirmation that the 4 story R-2 Type V-A building can be protected under NFPA 13R when it is over 12,000 sf.

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8 Comments

Wes

1/13/2020 07:18:34 am

There is no credit for an NFPA 13R system versus no system at all for an R-2 occupancy. If you check the Table 506.2 for R-2 Occupancy (IBC 2015), all of the tabular allowable building areas are the same for "NFPA 13R system (S13R)" and "No System (NS)".

The architect may be increasing the allowable building area based on open frontage around the building, which is another part of the allowable area determination. For a single-occupancy multi-story building, this calculation would be under Section 506.2.3 (Equation 5-2), which is Aa = [At + (NS x If] x Sa.

This may be what the architect is referring to, which is an allowable area increase based on building frontage but not the NFPA 13R system.

Jeffrey Hugo link

1/13/2020 10:48:07 am

Building area increases are attributed to NFPA 13 because sprinklers are installed in occupied and non-occupied spaces at a density (0.10 gpm/sf min.) proven for fire control and property protection.

NFPA 13R does not get an area increase because sprinklers are only installed in occupied areas and the density is half (0.05 gpm/f=sf) providing only a life safety system.

Marc Vieno

1/13/2020 11:21:01 am

That area calc is pretty wild, are you sure it's 135,000 sf and that's not a typo? 5A construction allows for 12,000 sf with 13R, 48,000 sf with a full 13 system.

As Wes mentioned you can also get bonus for frontage, but the max for that is 9,000 sf in either case, so I'm really not sure how the architect got to 135,000 sf.

Also, ditto on what Jeffrey said in regards to the effectiveness of the different systems.

Brian Gerdwagen FPE

1/13/2020 05:34:04 pm

There is no area increase allowed for a 13R system for two reasons, the first is as Wes said, it isn't a full coverage system. The second is that the IBC since 2009 requires a sprinkler system in all R occupancies, so the area shown already expects to have a 13R system as a minimum.

Jean Martin

1/14/2020 10:32:34 am

Please pardon my ignorance. I still don't see a code reference that definitively states that for NFPA 13R an area increase is not allowed in this case..

Under equation 5-2 under the listing of variables it states that:
At = Tabular allowable area factor (NS, S13R or SM value, as applicable) in accordance with Table 506.2.
This would lead me to believe that formula could be used on an NFPA 13R protected building.

I don't completely understand Wes's last statement regarding an allowable building area increase based on frontage, but not being allowed for the NFPA 13R system. How does that work?

Can someone please further explain and point me to a code reference that I can show my client to state the position that NFPA 13 must be used?

Thank you.

Wes

1/14/2020 10:58:54 am

You're right - you can use Equation 5-2. Just follow those steps and you'll get to the same conclusion that an NFPA 13R system doesn't benefit the allowable building area for this project.

Let's work through this example:

For No System At All:

Aa = Allowable area (square feet).
At = Tabular allowable area factor (NS, S13R or SM value, as applicable) in accordance with Table 506.2. = 12,000 sqft [Note here that the At value from Table 506.2 is the same for a non-sprinklered building as a 13R building]
NS = Tabular allowable area factor in accordance with Table 506.2 for a nonsprinklered building (regardless of whether the building is sprinklered). = 12,000 sqft
If = Area factor increase due to frontage (percent) as calculated in accordance with Section 506.3. = 0.75 [Assuming the most increase allowed under 506.3]
Sa = Actual number of building stories above grade plane, not to exceed three. For buildings equipped throughout with an automatic sprinkler system installed in accordance with Section 903.3.1.2 (NFPA 13R), use the actual number of building stories above grade plane, not to exceed four. = 4

Aa = [At + (NS x If)] x Sa
Aa = [12,000 + (12,000 x 0.75)] x 4 = 84,000 sqft total
Aa = [12,000 + (12,000 x 0.75)] x 1 = 21,000 sqft per floor

For an NFPA 13R System:

Aa = Allowable area (square feet).
At = Tabular allowable area factor (NS, S13R or SM value, as applicable) in accordance with Table 506.2. = 12,000 sqft [Note here that the At value from Table 506.2 is the same for a non-sprinklered building as a 13R building]
NS = Tabular allowable area factor in accordance with Table 506.2 for a nonsprinklered building (regardless of whether the building is sprinklered). = 12,000 sqft
If = Area factor increase due to frontage (percent) as calculated in accordance with Section 506.3. = 0.75 [Assuming the most increase allowed under 506.3]
Sa = Actual number of building stories above grade plane, not to exceed three. For buildings equipped throughout with an automatic sprinkler system installed in accordance with Section 903.3.1.2 (NFPA 13R), use the actual number of building stories above grade plane, not to exceed four. = 4

Aa = [At + (NS x If)] x Sa
Aa = [12,000 + (12,000 x 0.75)] x 4 = 84,000 sqft total
Aa = [12,000 + (12,000 x 0.75)] x 1 = 21,000 sqft per floor

For an NFPA 13 System:

Aa = Allowable area (square feet).
At = Tabular allowable area factor (NS, S13R or SM value, as applicable) in accordance with Table 506.2. = 48,000 sqft
NS = Tabular allowable area factor in accordance with Table 506.2 for a nonsprinklered building (regardless of whether the building is sprinklered). = 12,000 sqft
If = Area factor increase due to frontage (percent) as calculated in accordance with Section 506.3. = 0.75 [Assuming the most increase allowed under 506.3]
Sa = Actual number of building stories above grade plane, not to exceed three. For buildings equipped throughout with an automatic sprinkler system installed in accordance with Section 903.3.1.2 (NFPA 13R), use the actual number of building stories above grade plane, not to exceed four. = 3

Aa = [At + (NS x If)] x Sa
Aa = [48,000 + (12,000 x 0.75)] x 3 = 171,000 sqft total
Aa = [48,000 + (12,000 x 0.75)] x 1 = 57,000 sqft per floor

Ultimately, there is no difference between no system and NFPA 13R for allowable building area. There is a big difference when introducing an NFPA 13 system.

Brian Gerdwagen FPE

1/14/2020 04:02:32 pm

In older IBC editions they were very hidden and difficult to decipher. For example, in the 2012 edition, Section 504 allows a height increase of 20' was given for systems complying with 903.3.1.1 and an increase of 20' up to 60' with a maximum of four stories for a 903.3.1.2 system. You need to read the Chapter 9 section to see that 903.3.1.1 is a 13 system and a 903.3.1.2 is a 13R system.

Section 506.3 allows an area increase for a system complying with 903.3.1.1, but not a 903.3.1.2 system. Most people read this as "a sprinkler system" but not that it only applies to 13, not 13R. I am so glad they clarified this in the successive versions.

Jean Martin

1/14/2020 05:34:41 pm

I think the confusion revolves around the exact definition of the verbiage, "area increase."

I believe the code official is reading it to mean that the building area cannot ever be larger than 12,000sf/floor. I think he believes the variable lf - "Area factor increase" due to frontage is synonymous with an "area increase."

The architect believes they are synonymous also. So his position is that an "area increase" is permitted and he is asking for code sections from the AHJ telling him that an area increase is not allowed for NFPA 13R. He has produced his calcs and they match the calcs the Wes provided. TY!!!

I cannot find the definition of "area increase." Can anyone give me an official definition of "area increase?"

Thanks for all your help!

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