2015 IBC section 506.2.3....stupid formula

[Tim Mailloux]

The allowable area formula for a single occupancy one story building (506.2.3) includes the variable (Sa) for the actual number of stories above grade plane. But at the end of the section the last sentence reads "No individual story shall exceed the allowable area (Aa) as determined by Equation 5-2 using the value of Sa = 1.

So whats the point of including this variable if it always equals 1?

 

[RLGA]

It doesn't always equal one. This section is for "Single-occupancy, multistory buildings." For each individual story, you use "1" because that will determine the allowable area for any single story. However, for the total allowable building area, Sa cannot exceed "3." Thus, if you have a 3-story or taller building, you must use Sa=3 (except for buildings using NFPA-13R, then Sa=4 for buildings four stories maximum). If you have a 2-story building, then Sa=2.

 

[Tim Mailloux]

whats the point of even bringing the number of stories into the equation? Allowable Area (Aa) is to determine the maximum allowable floor plate, not overall building area.

 

[RLGA]

For Equations 5-1 and 5-2, Aa is the total (i.e., building) allowable area since they are addressing single-occupancy buildings. With mixed occupancies, the application is a bit different since the total building area is not based on a fixed square footage, but rather the sum of ratios, which must be 1 or less.

Thus, for a single story mixed-occupancy building, Equation 5-1 is used for each occupancy group. Equation 5-3, for multistory mixed-occupancy buildings, is really Equation 5-1 with brackets added (why they felt they needed to add Equation 5-3 is beyond me--this is the true "stupid formula" in my opinion). For multistory mixed-occupancies, you follow the requirements for single-story mixed-occupancy buildings for each story in the building. The difference is that the total sum of ratios for the entire building cannot exceed 3 (4 for buildings sprinklered per NFPA 13R). If the building was only two stories, then the sum of ratios cannot exceed 2.

I've mentioned this back when the 2015 IBC came out stating that this "new" method didn't make determining allowable height and area any easier--the old method was just fine.

 

[steveray]

I've mentioned this back when the 2015 IBC came out stating that this "new" method didn't make determining allowable height and area any easier--the old method was just fine.

Exactly...

 

[Chi]

It doesn't always equal one. This section is for "Single-occupancy, multistory buildings." For each individual story, you use "1" because that will determine the allowable area for any single story. However, for the total allowable building area, Sa cannot exceed "3." Thus, if you have a 3-story or taller building, you must use Sa=3 (except for buildings using NFPA-13R, then Sa=4 for buildings four stories maximum). If you have a 2-story building, then Sa=2.

Hi RLGA,
Thank you for your explain, but I am still a little bit confused here. If we have a 5-story, single ccupancy buidling, then the total allowable square footage is only [At + (NS x If)] X 3. Does it mean even if the zoning square footage requirement is more than this number, we still need to stick on this?

For example, the owner brought a property in a city, no frontage considered. The property area is the same as At (listed in the IBC 506.2), and the local zoning code allow a 5.0 FAR, and the max allowable floor is 5.
Then the max zoning area per zoning code = 5 x At
But the max actual building area per IBC 506.2 = 3 x At

 

[Chi]

Hi RLGA,
Thank you for your explain, but I am still a little bit confused here. If we have a 5-story, single ccupancy buidling, then the total allowable square footage is only [At + (NS x If)] X 3. Does it mean even if the zoning square footage requirement is more than this number, we still need to stick on this?

For example, the owner brought a property in a city, no frontage considered. The property area is the same as At (listed in the IBC 506.2), and the local zoning code allow a 5.0 FAR, and the max allowable floor is 5.
Then the max zoning area per zoning code = 5 x At
But the max actual building area per IBC 506.2 = 3 x At

I think I get the answer. Just read the newer version of IBC (2021) and it illustrate this very clear. Thank you though.

 

[RLGA]

I think I get the answer. Just read the newer version of IBC (2021) and it illustrate this very clear. Thank you though.

I am glad you figured it out. Zoning regulations and building codes are two separate things. If your building is of Type VB, the building code would likely be the limiting regulation. If your building is of Type IA construction, then the zoning ordinance would likely be the limiting regulation.