Allowable area calculation

[BayPointArchitect]

Given:

Four story apartment building, type VA construction, 13R sprinkler system, divided in half with a two-hour fire-rated wall from foundation to the underside of the non-combustible roof deck. Thirty feet clear separation on all four sides.

2018 IBC Section 506.2.3

Question:

What is the maximum allowable fire area (including all four floors with a single fire compartment) on each side of the fire wall?

Please check my math and let me know if I miscalculated.

As always, thanks in advance.

ICC Certified Plan Reviewer
NFPA Certified Fire Plan Examiner

 

[steveray]

Well you get 12,000 ft per story plus whatever you might get as a frontage increase...You do not get the full 75% as you are not open on the firewall (middle) side.....Can't have your cake and eat it too...

 

[walker.t]

Please check my math and let me know if I miscalculated.

I think you made a mistake in calculating the frontage increase. You can only use the full value of 0.75 when the entire perimeter of the building has open space, you have two buildings separated by a fire wall so I(f) must be less than 0.75 because only three sides of each building are open.

If we assume each side of each building is the same length we have:

For F, use a value of 1 for each side.
P is therefore 4.
W = 30

You’ll have to calculate the exact value based on the actual length of each side of the building but this is just an example.

I(f) = [F/P - 0.25] x W/30
I(f) = 3/4 - 0.25] x 30/30
I(f) = [0.75 - 0.25] x 1
I(f) = 0.50

Plug that into Equation 5-2:

A(a) = [A(t) + {NS x I(f)}] x S(a)
A(t) = 12,000
NS = 12,000
S(a) = 4
I(f) = 0.50

A(a) = [12,000 + (12,000 x 0.50)] x 4
A(a) = [12,000 + 6000] x 4
A(a) = 18,000 x 4
A(a) = 72,000 s.f.

 

[walker.t]

Thirty feet clear separation on all four sides.

Forgot to mention that when determining which sides of the building actually have frontage you need to make sure that you meet the requirement in 506.3.1 that the open space be accessed from a street or fire lane. If your building is on a lot with a street on the front and private lots on the other three sides you only have frontage on the front and the two sides accessed from the street, you can’t count the open space behind the building unless there’s a fire lane beside the building providing access to the back of the building. If your building is on a corner lot then you could count the open space on all four sides without the need for a fire lane.

 

[RLGA]

Just FYI, this has nothing to do with "fire area"--what you are asking about is "building area."

Both of these are defined in Chapter 2.

 

[BayPointArchitect]

Yes. I was thinking that the separation distance at the fire wall should be zero.
Yes. The open space on the backside of the building (not facing the public way) is accessible by a combination of fire truck and long water hose.

Thank you everyone!

 

[Yikes]

Forgot to mention that when determining which sides of the building actually have frontage you need to make sure that you meet the requirement in 506.3.1 that the open space be accessed from a street or fire lane. If your building is on a lot with a street on the front and private lots on the other three sides you only have frontage on the front and the two sides accessed from the street, you can’t count the open space behind the building unless there’s a fire lane beside the building providing access to the back of the building. If your building is on a corner lot then you could count the open space on all four sides without the need for a fire lane.

Interesting that in older codes, the open space was not about fire vehicle access, it was simply about its function as a fire break between structures. Here's an excerpt from the 1997 UBC Handbook, where they speculated that a flood control channel could function as an open space. I wonder when it changed to requiring the connection?

 

[steveray]

Interesting that in older codes, the open space was not about fire vehicle access, it was simply about its function as a fire break between structures. Here's an excerpt from the 1997 UBC Handbook, where they speculated that a flood control channel could function as an open space. I wonder when it changed to requiring the connection?

View attachment 15342

Yeah...is it an intended change or just creep....IIIB construction is another one I think gets perverted as we have discussed here before...