Allowable Height for Mixed Occupancy Buildings

[Markmax33]

If you have a mixed occupancy building with multiple fire areas, how do you calculate the allowable number of stories for the building or portions of the building? Can anyone confirm my logic please!!:inspctr

The IBC 2012 code sections read:

SECTION 508 MIXED USE AND OCCUPANCY

508.3 Nonseparated occupancies.

508.3.2 Allowable building area and height.

The allowable building area and height of the building or portion thereof shall be based on the most restrictive allowances for the occupancy groups under consideration for the type of construction of the building in accordance with Section 503.1.

508.4 Separated occupancies.

508.4.3 Allowable height.

Each separated occupancy shall comply with the building height limitations based on the type of construction of the building in accordance with Section 503.1.

1. It seems clear that for a separated building you can build the number of stories for each occupancy based on each separated occupancy.

2. It seems clear that for a non-separated building you can build the number of stories for lowest allowable stories in the building among the occupancies.

An example:

If fire area A had 2 occupancies and one occupancy allowed a building of 3 stories and the other allowed 4 stories for a given construction type is this correct?

3. Find the smallest allowable number of stories for the two occupancies in the fire area in question and then say that fire area could only have 3 stories? (this would follow the logic on how allowable area is calculated in a fire area)

*I'm aware of all of the allowable height increases for the fire sprinkler system, etc.

Thanks for any advice in advance!

 

[RLGA]

Markmax33:

Separated Occupancies: The building may have the number of stories permitted for the occupancy that allows the largest number of stories, provided that each individual occupancy is not located on a story that is higher than what is permitted for that story. For example, let's assume a a nonsprinklered Type IIB building with Groups B, A-3, and S-2. Groups B and S-2 are allowed three stories whereas the Group A-3 is permitted 2 stories. If the occupancies are separated, then the building is permitted to be three stories; however, Group A-3 occupancies can only be located on the first and second stories.

Nonseparated Occupancies: The building may have the number of stories permitted by the most restrictive occupancy group. Using the same example as above, if the occupancies are not separated, then the entire building is a Group A-3 fire area and the number of stories for the building is limited to two stories.

I think that is what you meant, but I wasn't sure.

 

[Builder Bob]

Ron Geren post has hit the answer right on the head......

 

[Markmax33]

Markmax33:Separated Occupancies: The building may have the number of stories permitted for the occupancy that allows the largest number of stories, provided that each individual occupancy is not located on a story that is higher than what is permitted for that story. For example, let's assume a a nonsprinklered Type IIB building with Groups B, A-3, and S-2. Groups B and S-2 are allowed three stories whereas the Group A-3 is permitted 2 stories. If the occupancies are separated, then the building is permitted to be three stories; however, Group A-3 occupancies can only be located on the first and second stories.

Nonseparated Occupancies: The building may have the number of stories permitted by the most restrictive occupancy group. Using the same example as above, if the occupancies are not separated, then the entire building is a Group A-3 fire area and the number of stories for the building is limited to two stories.

I think that is what you meant, but I wasn't sure.

Yes that's what I meant. What's your take if you have fire areas? You just treat each fire area like it's own non-separated building for number of allowable stories? Thanks for answering my questions!

 

[RLGA]

Yes that's what I meant. What's your take if you have fire areas? You just treat each fire area like it's own non-separated building for number of allowable stories? Thanks for answering my questions!

You always have fire areas, whether it's one fire area (i.e. the entire building) or multiple fire areas.

However, if you use a combination of separated and nonseparated uses, then the nonseparated portions (i.e. two or more occupancy groups within a fire area that are not separated) are limited in height to that of the most restrictive occupancy within the fire area.

Using the example in my earlier post, let's assume the Group S-2 is on the first story, the Group A-3 is on the second story, and the Group B is on the third story. The Group A-3 is separated from the Group S-2, but is not separated from the Group B. This situation would not be acceptable, since the Group B and Group A-3 are part of the same fire area, which places a Group A-3 fire area on a third story. Since the Group A-3 is limited to two stories, it is the most restrictive and cannot be located on a third story. To make this situation work, the designer would need to either sprinkler the building throughout or separate the Group A-3 from the Group B.

 

[cda]

Maybe should not be using the term fire area in this answer? since not used in 508?

Because you can have multiple fire areas in a story, but you cannot have multiple stories in a fire area??? Is that correct?

 

[mtlogcabin]

Because you can have multiple fire areas in a story, but you cannot have multiple stories in a fire area??? Is that correct?

No

This situation would not be acceptable, since the Group B and Group A-3 are part of the same fire area, which places a Group A-3 fire area on a third story.

I would have to disagree with this application of fire areas to Chapter 5.

 

[cda]

NoSplain Lucy????

Because you can have multiple fire areas in a story, but you cannot have multiple stories in a fire area??? Is that correct?

NO???

 

[mtlogcabin]

but you cannot have multiple stories in a fire area??

This is not a correct statement A multi story building with no horizontal fire barrier separation is one fire area therefore you can have multiple stories within a single fire area

[F] FIRE AREA. The aggregate floor area enclosed and bounded by fire walls, fire barriers , exterior walls or horizontal assemblies of a building. Areas of the building not provided with surrounding walls shall be included in the fire area if such areas are included within the horizontal projection of the roof or floor next above.

 

[cda]

This is not a correct statement A multi story building with no horizontal fire barrier separation is one fire area therefore you can have multiple stories within a single fire area

[F] FIRE AREA. The aggregate floor area enclosed and bounded by fire walls, fire barriers , exterior walls or horizontal assemblies of a building. Areas of the building not provided with surrounding walls shall be included in the fire area if such areas are included within the horizontal projection of the roof or floor next above.

Ok I will give you one

It sounded good

 

[Markmax33]

Ron did a really good write up back in 2008 on fire areas which is where I really learned most of the info I know about them. Here's a link for anyone trying to learn more about them:

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

 

[RLGA]

Although "fire areas" may not be the best term to use, since the fire ratings required for fire barriers used to create fire areas vary slightly from the fire ratings required for occupancy separation. However, the concept remains the same.

Prior to the 2009 IBC, fire barriers and horizontal assemblies used for occupancy separation also created fire areas. But the IBC did have a section and table for fire ratings of fire barriers and horizontal assemblies used to to create fire areas within a single occupancy (used primarily to separate large single occupancy fire areas into smaller fire areas to avoid sprinkler installation). Thus, separated occupancies that required fire-resistive assemblies for the occupancy separation also created fire areas. Therefore, my statement regarding fire areas is true when applied to those earlier IBC editions.

However, starting in the 2009 IBC, the fire area section and table in Chapter 7 was revised to include fire ratings for all fire barriers and horizontal assemblies used to create fire areas and it differed from the fire ratings required for occupancy separation, especially when a sprinkler system is installed (occupancy separations have a reduced rating when a sprinkler system is installed, but fire area separations do not have a similar sprinkler trade-off). Therefore, the fire-resistive construction used for occupancy separation does not necessarily mean a fire area has been created, hence cda's and mtlogcabin's comments regarding the application of fire areas in Chapter 5 are true.

Regardless of whether it's a fire area or not, if a portion of the building uses nonseparated occupancies, then the most restrictive occupancy within the nonseparated area is applied to the extent of the nonseparated area--just as it would be when this method is applied to an entire building if the entire building consisted of nonseparated occupancies. In my example, the Group A-3 and Group B, on the second and third stories, respectively, are not separated with a two-hour horizontal assembly as required by Table 508.4 (2012 IBC) for separated occupancies; thus, the requirements for the most restrictive occupancy must apply to both floors. Since a Group A-3 cannot exist on a third story for that construction type (Type IIB), then the condition is not permitted and either a sprinkler system or a 2-hour horizontal assembly must be provided.

 

[cda]

They you for commentaries, consultants and FPE's

Need them all to figure out what we wrote

 

[mtlogcabin]

Regardless of whether it's a fire area or not, if a portion of the building uses nonseparated occupancies, then the most restrictive occupancy within the nonseparated area is applied to the extent of the nonseparated area--just as it would be when this method is applied to an entire building if the entire building consisted of nonseparated occupancies.

I am still not following your logic. Non-separated occupancies have to meet the most restrictive requirements from 403 and Chapter 9 for the entire building.

508.3.1 Occupancy classification.

Nonseparated occupancies shall be individually classified in accordance with Section 302.1. The requirements of this code shall apply to each portion of the building based on the occupancy classification of that space except that the most restrictive applicable provisions of Section 403 and Chapter 9 shall apply to the building or portion thereof in which the nonseparated occupancies are located.

Are you thinking of 508.3.2 2009 Edition?

508.3.2 Allowable building area and height.

The allowable building area and height of the building or portion thereof shall be based on the most restrictive allowances for the occupancy groups under consideration for the type of construction of the building in accordance with Section 503.1.

I have been taking the "portion thereof" to mean as long as the "A occupancy" was on a permitted level within Table 503 the building and uses are code compliant

Example

2 story V-B building

2,000 sq ft on each floor

A-2 on the first and a "B" offices on the second

Sprinklers and alarms throughout the building.

1 tenant on the first floor with multiple tenants on the second

OL less tan 250 for the entire building

1 fire area

My understanding from what you are saying is this is not permitted.

I believe it is because the "A" portion is on the 1st level and the "B" portion is permitted on the second level.

 

[RLGA]

I am still not following your logic. Non-separated occupancies have to meet the most restrictive requirements from 403 and Chapter 9 for the entire building.

Yes, I am talking about Section 508.3.2: For nonseparated occupancies, the IBC does not require the most restrictive occupancy to be applied to the entire building in regard to height and area--only Section 403 when high rise buildings are a consideration per Section 508.3.1. However, Chapter 9, as Section 508.3.1 states, only applies to "the total nonseparated occupancy area" and not the entire building (unless Chapter 9 requires protection through a building, such as in the case of Group R occupancies).

Section 508.1 (2012 IBC): "General. Each portion of a building shall be individuallyclassified in accordance with Section 302.1. Where abuilding contains more than one occupancy group, the buildingor portion thereof shall comply with the applicable provisionsof Section 508.2 [Accessory occupancies], 508.3 [Nonseparated occupancies] or 508.4 [separated occupancies], or a combination ofthese sections."

Section 508.3 (2012 IBC): "Nonseparated occupancies. Buildings or portions of buildings that comply with the provisions of this section shallbe considered as nonseparated occupancies."

Section 508.3.2 (2012 IBC): "Allowable building area and height. The allowablebuilding area and height of the building or portionthereof shall be based on the most restrictive allowancesfor the occupancy groups under consideration for the typeof construction of the building in accordance with Section503.1."

Per your example, it would be permitted since the sprinkler system installed throughout would allow two stories for the Group A-2 per Table 503 and Section 504.2.