Different Approaches to Mixed-Occupancy Buildings?

MeyerFire University | LS101.05

By Chris Campbell, PE

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TRANSCRIPT

The IBC offers 3 approaches to a mixed-occupancy building: accessory, non-separated and separated. In this module, we’ll cover all three of these approaches and the specific requirements related to each one.  

First, we'll start with non-separated occupancies. 

In a non-separated occupancy approach, there is no requirement for a fire-rated separation between adjacent occupancies. The key requirement in this approach is that the most-restrictive requirements for the allowable height area number stories, as well as the most-restrictive Fire Protection requirements for all the occupancies involved must be applied through the entire building.  

So, for example, if your building has two occupancy types and one of them requires a voice fire alarm system, a non-separated occupancy approach would mean that you need to provide a voice fire alarm system throughout the whole building.  

If your building falls within the allowable height area and number of stories for the occupancy types involved, and you can meet the Fire Protection requirements of the most restrictive occupancy, a non-separated mixed occupancy approach is the easiest way to go from a design standpoint because there's no additional requirements for rated separations between occupancies.  

The general code approach for a non-separated mixed occupancy building is first to determine the maximum allowable height, area, number of stories for each occupancy in the building and then apply the most restrictive of these requirements through the entire building.  

You are then also required to comply with the most restrictive Fire Protection requirements of Chapter 9 and apply those to the whole building.  

Finally, all other code requirements such as means of egress are applied to each individual occupancy classification involved for that portion of the building.  

Note that even if you are taking a non-separated occupancy approach, there are specific requirements for buildings containing groups H, I ,or R for rated separations between those occupancies and adjacent spaces.  

You can review those requirements in IBC Chapter 4.  

The second option is separated mixed occupancies, which is typically provided when a non-separated approach is not available or 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 this specific rating requirement.  

Note that a rated separation may not be required between occupancies of a similar hazard level.  

The allowable height, area and number of stories is based on each individual occupancy type.  

For example, if your construction type limits a Group A occupancy to four stories, your building could be taller as long as the Group A occupancy is not located above the fourth story. 

Similarly, 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 individually to each separated occupancy in the building.  

The general approach for code compliance in a separated mixed occupancy building is first to determine the minimum required fire resistance rating between adjacent occupancies.  

Second is to verify that the building does not exceed the maximum area for the type of construction involved. 

Third is to verify that the locations of occupancies do not exceed their allowable height or number of stories based on the type of construction, and then finally all other code requirements, such as a means of egress or Fire Protection requirements, are applied to each individual occupancy where they just locate it in the building.  

The final approach to mixed occupancy is an accessory occupancy approach.  

This method is applied where there's a small portion of building or space that is subsidiary to the main occupancy.  

When that smaller space is 10% or less of the total area of the story and accessory occupancy approach can be used. There is no required separation between the accessory and main occupancy.  

And the building height area number stories are based on the main occupancy, not the accessory occupancy.  

Within the accessory occupancy, other code requirements such as means of egress and Fire Protection systems are based on the accessory occupancy classification, not the main one.  

In summary, there are three approaches to mixed occupancy buildings, non-separated, separated and accessory.  

All buildings that contain multiple occupancies must use one or more of these approaches for code compliance.  

I'm Chris Campbell. This is MeyerFire University.