Occupancy Categories

Question

What is meant by the term Occupancy Category? As a truss designer, I see Occupancy Category II noted on residential jobs but recently received plans from a new client with a Category I designation. Who is responsible for determining the Occupancy Category and how does it affect the loads for a structure?

Answer

The Occupancy Category designates the intended use for a building, which affects the required design loads for the structure. These loads include flood, wind, snow, earthquake and ice loads. The truss designer is not responsible for determining the Occupancy Category but it’s useful to understand the concept and the role it plays in determining loads for a structure.

The authority having jurisdiction over a project assigns the Occupancy Category, for both new construction and additions and alterations to existing structures, based on the building permit application. Established on the function a building is meant to serve, the Occupancy Category’s Importance Factor determines whether the loads for that structure can be reduced, or need to be increased from a base reference design load.

There are four Occupancy Categories: I, II, III and IV (see Table 1). From lowest to highest, the Occupancy Category describes the potential hazard to human life a structure would pose in the event of failure or serviceability issues. The authority having jurisdiction assigns a category ranging from a low hazard to human life to a significant hazard or essential structure. For example, an agricultural facility with an Occupancy Category of I would pose a low hazard to human life, whereas a school with an Occupancy Category of III would pose a significant hazard to many people if the building were to fail.

Each Occupancy Category has an associated Importance Factor, which is taken into account when determining a building’s structural reliability. There is a direct relationship between the Occupancy Category and the Importance Factor—a low category will result in lower loads and a high category will call for an increase in loads. It’s important to note that it is possible for a structure to be assigned multiple Occupancy Categories for different parts of the building. While the Importance Factor is used to modify a number of different design loads, we will look at the Importance Factors that are specifically used to adjust snow loads in Table 2.

Category II is the “default,” and therefore, has an Importance Factor of 1.0. Using this as a baseline, anything below Category II would have a reduction in loads and anything higher than Category II would have an increase in loads. ASCE 7-05 – Minimum Design Loads for Buildings and Other Structures defines Category II as, “[a]ll buildings and other structures except those listed in Occupancy Categories I, III and IV.” Essentially, Category II is a catchall for any structures that don’t fall into the other categories. This is also the category that applies to most residential homes.

Let’s take a look at an example and how it relates to the loading required by Table 2.

Given, Ground snow load (P_g) = 30 psf, Thermal factor (C_t) =1.0, and Exposure factor (C_t) = 1.0.

For snow loads, the required flat roof design load is defined in ASCE 7-05 as:

Where:

Since C_t and C_e are 1.0 in this example, the flat roof design snow (P_f) load is 30 (I)(1.0)(1.0) or 30(I). The resulting design snow load for the various Occupancy Categories is then shown in Table 3.

As you can see, the Occupancy Category of a building can affect the required design loads dramatically. Be sure to check the construction documents for the appropriate category to use, or consult with the building designer.

Note: The discussion above relates to the 2006 and 2009 versions of the IBC and IRC and the referenced standard ASCE 7-05. The 2012 editions of these codes reference an updated version of the standard, ASCE 7-10, where a number of changes have been made. Discussion of the 2012 versions will be addressed in a future article.

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