Three Approaches to Mixed Occupancy Buildings

The IBC offers 3 approaches to mixed occupancy buildings: accessory, nonseparated and separated. Any building with two or more occupancy types must choose one or more of these approaches for compliance. In this post, we will cover all three of these approaches and the specific requirements related to each.

Many urban buildings are mixed occupancy buildings and use one of these described approaches.

Nonseparated Mixed Occupancy

In a nonseparated occupancy approach, there is no requirement for a fire-rated separation between adjacent occupancies. However, the key requirement in a nonseparated occupancy approach is that the most restrictive requirements for the allowable height, area and number of stories, as well as the most restrictive fire protection requirements must be applied throughout the entire building. For example if your building has two occupancy types and one of them requires a voice fire alarm system, a nonseparated occupancy approach would mean that you need to provide a voice fire alarm system throughout the building.

If your building falls within the allowable height and area for the occupancy types involved and you can meet the fire protection requirements of the most restrictive occupancy, a nonseparated mixed occupancy approach is the easiest from a design standpoint because there is no additional requirements for rated separations.

The general approach for code compliance in a non separated mixed occupancy building is:
1. Determine the maximum allowable height, area and number of stories for each occupancy involved and verify that your building meets the most restrictive requirements.
2. Determine the fire protection requirements from IBC Chapter 9 for each occupancy involved and apply the most restrictive requirements throughout the building. If the building is a high-rise, the requiremetns of Section 403 would also apply throughout.
3. Apply all other code requirements, such as means of egress, to each individual occupancy classification individually for that portion of the building.

Note that even if you are taking a nonseparated occupancy approach there are specific requirements for buildings containing Groups H, I and R for a rated separation between those occupancies and adjacent building spaces you can review those requirements in IBC Chapter 4.

Separated Mixed Occupancy

The separated mixed occupancy approach is typically provided when the non-separated approach is not an option. With a separated mixed occupancy approach, a fire-rated separation is typically provided between each occupancy type. IBC Table 508.4 is referenced for the specific rating requirement, though a rated separation may not be required between occupancies of a similar hazard level.

The allowable height and number of stories is applied for each individual occupancy type. For example, if your construction type limits a Group A occupancy to 4 stories, your building could be taller as long as the Group A occupancy is not located above the fourth story.

The allowable area is based on the sum of the ratios of the actual floor area for each occupancy divided by the allowable floor area of that occupancy. The sum of these ratios cannot exceed one for any given story. Requirements for egress and fire protection systems are applied invidivualy to each separated occupancy in the building.

The general approach for code compliance in a separated mixed occupancy building is:

1. Determine the minimum required fire resistance rating between adjacent occupancies.
2. Determine the actual area and allowable area for each occupancy involved.
3. Verify that the sum of the actual to allowable area ratios is not more than 1 for any story and not more than 3 for the entire building (assuming an NFPA 13 sprinkler system).
4. Verify that the locations of occupancies do not exceed their allowable height or number of stories based on the type of construction.

All other code requirements such as means of egress or fire protection are applied to each individual occupancy where it is located in the building. Note that Chapter 9 may require sprinkler protection throughout the building, even with separated occupancies, so be sure to review the requirements that apply to your specific situation.

Accessory Occupancy

The final option is an accessory occupancy approach. This method is applicable where there is a small portion of a building or space that is subsidiary to the main occupancy. When that smaller space is 10% or less of the total area of a story, an accessory occupancy approach can be used.

There is no required separation between the accessory and main occupancy, and the building height, area and number of stories are based on the main occupancy, not the accessory occupancy. If the building contains a Group H, I or R occupancy, those separation requirements found in Chapter 4 would still apply.

Within the accessory occupancy, other code requirements such as egress and fire protection systems are based on the accessory occupancy classification, not the main occupancy. This is crucial point that confuses many people. Essentially, the accessory occupancy designation allows you to ignore the height, area and number of stories requirements for the accessory occupancy, but all of the other code requirements for the accessory occupancy still apply.

For example, a large conference room located within an office building could be considered an accessory occupancy, so the limitations for height, area and number of stories for Group A-3 occupancies would not apply. But all of the other code requirements for Group A-3, including means of egress and fire protection requirements would still apply!

Bonus: Fire Walls

One final option - if you are able to separate multiple occupancies using a fire wall, you create two separate buildings from a code application standpoint and you can consider each building indvidually. A fire wall is the most robust separation option available though, so its likely to be the most disruptive from a design standpoint.

Conclusion

In summary, there are three approaches to mixed occupancy buildings: non separated, separated and accessory. All buildings with multiple occupancies must use one or more of these approaches. If your building contains three mor more occupancy types, it's possible to use a combination of the approaches, such as designating an accessory occupancy and then taking a nonseparated approach for the remaining occupancies.