Allowable area

[Sifu]

I have a 4 story, VA, separated mixed use (R1, A3, A2, B, S1) building with an NFPA 13 system. The aggregate sum of the allowable areas exceeds 3. They are allowed to not exceed 4 if separated and an NFPA 13R system is installed. Why is the threshold higher for a 13R system? It seems counter-intuitive, but I am not a sprinkler expert.

 

[RLGA]

It is not really. Since building area increases are not permitted when using an NFPA 13R system, you are allowed an extra story since the 13R sprinkler system is permitted for predominately residential buildings up to four stories.

 

[steveray]

How big is the building?

 

[Sifu]

4 stories, each at 28,000sf² to 30,000sf², total of 115,859sf². The allowable areas come up to 3.19, which exceeds the allowable of 3 per 506.2.4. Still confused, the exception says, if I read the code correctly, they wouldn't be allowed to exceed 3 because they have a 13 system instead of a 13R system? I am using the 2018 codes.

 

[RLGA]

4 stories, each at 28,000sf² to 30,000sf², total of 115,859sf². The allowable areas come up to 3.19, which exceeds the allowable of 3 per 506.2.4. Still confused, the exception says, if I read the code correctly, they wouldn't be allowed to exceed 3 because they have a 13 system instead of a 13R system? I am using the 2018 codes.

I assume this is a hotel and all the other occupancy groups are associated with that primary use, which would allow you to use an NFPA 13R system.

If you were to use the NFPA 13R system, then the "S13R" value would need to be used for Group R-1, and for the other occupancy groups the "NS" values would need to be used. This will greatly reduce the allowable area permitted per story and may exceed the maximum of 1 for the sum of ratios for one or more stories.

Without knowing the details of occupancy group distribution and size on each story, it is difficult to say exactly how the two sprinkler methods would differ.

 

[Sifu]

I assume this is a hotel and all the other occupancy groups are associated with that primary use, which would allow you to use an NFPA 13R system.

If you were to use the NFPA 13R system, then the "S13R" value would need to be used for Group R-1, and for the other occupancy groups the "NS" values would need to be used. This will greatly reduce the allowable area permitted per story and may exceed the maximum of 1 for the sum of ratios for one or more stories.

Without knowing the details of occupancy group distribution and size on each story, it is difficult to say exactly how the two sprinkler methods would differ.

You are correct, it is a hotel with A3, A2, S1 and B associated with it. It is a separated strategy and the entire building is sprinklered with a 13 system.

Here is their ration calculation.

 

[RLGA]

You are correct, it is a hotel with A3, A2, S1 and B associated with it. It is a separated strategy and the entire building is sprinklered with a 13 system.

Here is their ration calculation.

View attachment 9102

As I suspected, if using NFPA 13R, each floor would have exceeded 1 for the sum of ratios just on the Group R-1 alone. I did a quick calculation based on the areas provided as if the building was sprinklered using NFPA 13R, and the sum of ratios would be 9.5--more than twice the allowable area of 4. In other words, the building would need to be 60% smaller to use NFPA 13R without fire walls using the same construction type.

This is why you get four stories (or an allowed sum of ratios not greater than 4) when using NFPA 13R.

 

[Jay Smith]

4-story Type VA hotels usually have frontage all around. There might be more allowable area available.

 

[RLGA]

4-story Type VA hotels usually have frontage all around. There might be more allowable area available.

Probably, but it won't get you a 135% increase.

 

[Jay Smith]

Probably, but it won't get you a 135% increase.

They aren’t 135% short. Their total building area is more than 3x floor allowable, but only by a relatively small amount.

 

[RLGA]

They aren’t 135% short. Their total building area is more than 3x floor allowable, but only by a relatively small amount.

I was looking at it from the perspective of using the NFPA 13R system. But, you are correct, based on what they show as allowable areas appear to be per Table Table 506.2 for "SM" without any frontage increase. They just might be able to overcome the small difference between 3 and 3.19.

 

[Sifu]

They do not provide a frontage calculation. It is an L shaped building, and entirely bound on the entire inside of the L, the both ends. I will let them do the math.