How do you calculate Unity Formula (Mixed Use-Area Ratio) with an Unlimited Area?

[annoyitated]

The original building permit application was made in 1989 so I am assuming it was constructed utilizing the 1987 BOCA code which would have been adopted by the City at that time. According to the permit, it is a type 2A construction which allowed 5 Stories, 65 feet, and 19,950 allowable square feet for an A-3 Assembly Use.

With the sprinkler and open perimeter modifications, using 1987 BOCA calculations, the allowed height and areas increased to 6 Stories (+1st for A.S.), 85 feet (+20 feet for A.S.), and 56,977 allowable square feet (15% Area Reduction for 5 story building = 16,957.5 + 16957.5 for A.S. + 23,062.2 for open perimeter).

The building is a six floor building with a ground floor footprint of 41,132 square feet.

We are looking to add two additions to the building and are evaluating our options from a code perspective.

If I calculate the existing building allowable height and area using the 2009 IBC I get 4 stories (3 allowable per Table 503+1 for A.S.), 85 feet (65 feet allowable per Table 503 + 20 feet for A.S.), and 57,040 allowable square feet (15,500 allowable per Table 503 + 31,000 for A.S. + 10, 540 for open perimeter).

Currently, we exceed the 57,040 allowable square feet with our additions on at least one floor. As I investigate ways to tackle this problem, including the obvious one of reducing the additions, I was working through the 2009 IEBC options. In looking at the compliance methods I was leaning toward the Work Area Compliance Method since it allowed the use of a fire separation wall, my thinking was similar to a separated mixed-use. I have two questions that I can’t seem to find an answer to.

Question No. 1 – On my first level I have an A-3 existing on one side of the separation and an A-3 new on the addition side, is the rating really 0-hr?

Question No. 2 – If I construct the addition utilizing IB type construction it would allow 11 stories with unlimited area. With the existing building being 2A it is causing me a problem because some of the assembly ratings were only 1-1/2 hour and so I am guessing the AHJ will only allow that building to be considered a IIA equivalent (1-hr assembly ratings). So my problem is that I have a IIA with an allowable area of 57,040 square feet on one side of the fire separation and an unlimited allowable area with the IB on the other side. How do you calculate the Unity Formula (Mixed-Area Ratio) with an unlimited area?

 

[cda]

Welcome

You will get some answers

How did you find us

 

[annoyitated]

I have sent emails to ron@specsandcodes.com and visited his website for various code questions. But I felt it was unfair to continue to burden only him with my questions and I noticed he had corresponded on several posts on this site.

 

[zigmark]

Look at Section 706 , specifically Table 706.4 and you will find the fire wall will need to be 3-hr rated. Pay close attention to the adjoining wall requirements as on building rises above the height of the lower one.

ZIG

 

[annoyitated]

Look at Section 706 , specifically Table 706.4 and you will find the fire wall will need to be 3-hr rated. Pay close attention to the adjoining wall requirements as on building rises above the height of the lower one.ZIG

Would it not be Section 707 since it is a Fire Separation wall not a Fire Wall? If so, would Table 707.3.9 and 2-hours be the answer I was looking for?

 

[zigmark]

Are your creating two separate buildings or two separate fire areas? If you are only creating two separate fire areas and not two separate buildings then the most restrictive for either type of construction and occupancy apply to both sides of the wall.

ZIG

 

[zigmark]

It would still all be one building and classified as II-B construction if a Firewall per IBC Section 706 was not used. Fire Barriers only serve a purpose in this case if you want to continue with the same type of construction and occupancy. Changing type of construction does you no good on the "other side" of a fire barrier to gain an additional area increase since it is still part of the same building and has to comply with the most restrictive for TOC and Occupancy.

ZIG

 

[annoyitated]

It would still all be one building and classified as II-B construction if a Firewall per IBC Section 706 was not used. Fire Barriers only serve a purpose in this case if you want to continue with the same type of construction and occupancy. Changing type of construction does you no good on the "other side" of a fire barrier to gain an additional area increase since it is still part of the same building and has to comply with the most restrictive for TOC and Occupancy.ZIG

ahh, and that answers Question No. 2. Thank you.

 

[zigmark]

Glad I could Help! It if indeed I did??

And, WELCOME TO THE FORRUM!

ZIG

 

[Markmax33]

The original building permit application was made in 1989 so I am assuming it was constructed utilizing the 1987 BOCA code which would have been adopted by the City at that time. According to the permit, it is a type 2A construction which allowed 5 Stories, 65 feet, and 19,950 allowable square feet for an A-3 Assembly Use.With the sprinkler and open perimeter modifications, using 1987 BOCA calculations, the allowed height and areas increased to 6 Stories (+1st for A.S.), 85 feet (+20 feet for A.S.), and 56,977 allowable square feet (15% Area Reduction for 5 story building = 16,957.5 + 16957.5 for A.S. + 23,062.2 for open perimeter).

The building is a six floor building with a ground floor footprint of 41,132 square feet.

We are looking to add two additions to the building and are evaluating our options from a code perspective.

If I calculate the existing building allowable height and area using the 2009 IBC I get 4 stories (3 allowable per Table 503+1 for A.S.), 85 feet (65 feet allowable per Table 503 + 20 feet for A.S.), and 57,040 allowable square feet (15,500 allowable per Table 503 + 31,000 for A.S. + 10, 540 for open perimeter).

Currently, we exceed the 57,040 allowable square feet with our additions on at least one floor. As I investigate ways to tackle this problem, including the obvious one of reducing the additions, I was working through the 2009 IEBC options. In looking at the compliance methods I was leaning toward the Work Area Compliance Method since it allowed the use of a fire separation wall, my thinking was similar to a separated mixed-use. I have two questions that I can’t seem to find an answer to.

Question No. 1 – On my first level I have an A-3 existing on one side of the separation and an A-3 new on the addition side, is the rating really 0-hr?

The fire separations listed in table 508.4 will create a new fire area in the building. It would not be used for separating the same occupancy on a floor or in a building. Occupancy separation walls help separate areas with very low allowable areas in the unity formula from the rest of the building. If you are separated with one of these barriers you can use the larger floor area in the denominator of your unity equation, thus creating a smaller ratio for the new fire area.

There is a good example here:

http://www.specsandcodes.com/Articles/The%20Code%20Corner%20No.%2027%20-%20Fire%20Areas.pdf

Question No. 2 – If I construct the addition utilizing IB type construction it would allow 11 stories with unlimited area. With the existing building being 2A it is causing me a problem because some of the assembly ratings were only 1-1/2 hour and so I am guessing the AHJ will only allow that building to be considered a IIA equivalent (1-hr assembly ratings). So my problem is that I have a IIA with an allowable area of 57,040 square feet on one side of the fire separation and an unlimited allowable area with the IB on the other side. How do you calculate the Unity Formula (Mixed-Area Ratio) with an unlimited area?

Fire barriers are generally used for avoiding a fire sprinkler system:

707.3.9 Fire areas. The fire barriers or horizontal assemblies, or both, separating a single occupancy into different fire areas shall have a fire-resistance rating of not less than that indicated in Table 707.3.9. The fire barriers or horizontal assemblies, or both, separating fire areas of mixed occupancies shall have a fire-resistance rating of not less than the highest value indicated in Table 707.3.9 for the occupancies under consideration.

Fire walls create new buildings. This is probably what you are looking for:

SECTION 706 FIRE WALLS

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.