Frontage Increase

[Mech]

2009 IBC

1. Section 506.2.1: If a building has a uniform W value of 40 around the entire building, can I use a value of 40 in equation 5-1 (If = [F / P - 0.25]W / 30), or must it be reduced to 30? I haven’t found a maximum allowable value for a uniform W value.

2. Suppose a type 2B, single story 200 ft x 200 ft structure has an open space of 30 ft, and it also has a fire wall separating the structure into two separate buildings. The fire wall is 150 ft from one end of the structure and the first building is group B with 30,000 sf and the second building is S-1 with 10,000 sf.

Am I correct in my reasoning: for the group B building, the perimeter (P) is 200 (outside wall) + 150 (outside wall) + 150 (outside wall) + 200 (shared fire wall) = 700 ft. The building perimeter that fronts on an open space (F) is 500 ft, because the 200 ft shared fire wall has a building next to it and is therefore not open. From eq. 5-1, If = [ 500 / 700 – 0.25] (30) / 30 = 0.464 (an allowable area increase of 46.4%.)

And then the S-1 perimeter (P) is 200 + 50 + 50 +200 (fire wall) = 500 sf and the perimeter fronting an open space (F) is 200 + 50 + 50 = 300 sf? If = [300 / 500 – 0.25] (30) / 30 = 0.35 (an allowable area increase of 35%.)

Someone in my office is saying that the perimeter (P) and perimeter fronting the open space (F) is 200 + 200 + 200 + 200 = 800 ft, so the value of F/P is 800 / 800 = 1 for the structure. If = [800 / 800 – 0.25] (30) / 30 = 0.75. They want to increase the allowable area for group B building 75% and the group S building 75%. Is this correct? I think it is not.

Thanks in advance.

 

[texasbo]

1. You can actually use a value of 60 for W if the building meets the exception (all requirements of 507 except 60' yard requirement). Otherwise, W cannot exceed 30' per 506.2.1.

2. I allow the entire building to be considered when calculating frontage. The code says that a fire wall is permitted to constitute separate buildings. it doesn't say it has to. Others will argue with me.

 

[Mech]

texasbo,

1. What if the building does not meet the exception (all requirements of 507 except 60' yard requirement)? Suppose I only have 40 feet of open space to property lines in all directions, with no extra dedicated space on adjacent property?

2. What if the fire wall is needed to separate the structure into two buildings?

 

[Plans Approver]

With a firewall, you are creating 2 separate buildings and each building must meet the requirements of Table 503 and the open area increase. The entire structure is 2B construction.

The 10,000 sf, B building does not require an area increase since it is permitted to have an area of 23,000 sf by Table 503.

The 30,000 sf, S-1 building is permitted to have only 17,500 sf area by Table 503. The open space area increase applies here.

The open perimeter is 150 + 200 + 150 = 500 lf. The maximum width of open space is 30' by 506.2.1, "Where the value of W exceeds 30 feet, a value of 30 feet shall be used in calculating the weighted average, regardless of the actual width of the open space." You don't comply with requirements of 507, specifically 507.5. Your S-1 space exceeds 24,000 sf and needs a sprinkler system (903.2.9) which will give you a 3x area increase so you don't really need an open area increase.

The words "weighted average" can be confusing. Since the maximum open width is 30' (excluding exceptions), the "weighted average" only applies where the open width is less than 30'.

T-bo, I guess I'm one of the others "706.1 General. Each portion of a building separated by one or more fire walls that comply with the provisions of this section shall be considered a separate building. The extent and location of such fire walls shall provide a complete separation. Where a fire wall also separates occupancies that are required to be separated by a fire barrier wall, the most restrictive requirements of each separation shall apply."

 

[Inspector 102]

Nonseparated building under 508.3.2 would allow a 75% increase for frontage if 30 feet is provided completed around the building. 30,625 SF total.

Separated Use would get you 46.42% for the B occupancy or 25,625 SF and 23,625 SF for the S-1 area for total allowable building size of 49250 SF with 30 feet around 3 sides of the building and common firewall providing no setback along one side.

I think I got this correct, lets see what others think.

 

[Examiner]

I have a spread sheet that does all the calulations for area increases. You do not get the firewall side in either building. So instead of a perimeter increase of 75% max, if all sides had clearances of 30-ft or more, your perimeter percentage would be lower. In my spreadsheet you have to input the lenght and width of the building for perimeter. Then input each of the sides length that has a clearance of 30-feet or more from the property line. In that part of the spread sheet the firewall's length would be zero. You input the Table's value for the Occupancy and Construction type and input increases for sprinklers. It then calculates the max allowable area per floor and the max building area. To get the max building area you have to fill in the number of stories. The spreadsheet has information to aid one in fill in the blanks. I can share it if you want to try it.

 

[Mech]

Thanks for the responses.

So the only allowable values for W are between 20 and 30, and exactly 60?

 

[Plans Approver]

Thanks for the responses.So the only allowable values for W are between 20 and 30, and exactly 60?

Almost, between 20 and 30 feet, 60 feet minimum, 40 feet minimum if applicable by 507 and 507.5.

 

[Gene Boecker]

You can use a factor between 20 and 60, weighed average based on each allowance - 30 to 60 if it could be a 507 unlimited area building except for frontage; 20 to 30 otherwise.

The question is whether or not a building subdivided by a fire wall meets the intent of the exception. Certainly the fire produces a zero frontage on that side if each side is being reviewed as a separate building. Does that also allow for use of the exception or is it still applicable because the frontage is to another building on the same lot? I would argue that it would apply as well.

I'm not sure that was the intent of the exception but I don;t see it being off limits based on the text.

 

[EPrice]

Thanks for the responses.So the only allowable values for W are between 20 and 30, and exactly 60?

The allowable values are between 20 and 30 if 506.2.1 exception cannot be used, or between 20 and 60 if the exception can be used.

The exception does not require the separation to be 60'. That is the only part of the unlimited area qualifications that the building can fail to meet and still use the exception.