This installment of Navigating the Building Code examines Occupant Load, how to calculate the Occupant Load, and how that calculation can affect the building design.
What is Occupant Load?
Occupant Load is defined by the International Building Code as “the number of persons for which the means of egress of a building or portion thereof is designed.” So, what does that actually mean? In the most general terms, it means it is the maximum amount of people that can occupy a space so that each person can safely exit the space as required.
How is the Occupant Load Determined?
In order to determine the occupant load, you first must calculate the area of the space in question. Typically, you must breakdown the project as a whole into smaller parts and determine the area of each individual part. Each individual part would then be classified by the function of the space.
For example, if within a library there were reading rooms and library stacks, the floor plan would need to be viewed in terms of each space. So, if you had 2 reading rooms that are 30’x40′, you would have 2,400 sf of reading rooms (2X30X40). Then, if the library stacks area was 50’x100′, you would have 5,000 sf of library stacks. After determining the area of each space then you would reference the load factor in the code, in the IBC it is shown in Table 1004.5 (figure 1.1). Then you simply divide the area by the occupant load factor with the result being the occupant load for that space. Finally, continue determining the the occupant load for each space and then add all of the occupant loads together to determine the occupant load for the project.
From our example above, we can determine the library stacks occupant load by finding the load factor in the table above (100 gross) and the known area (5,000 sf). So our occupant load for the the stacks area would be 5,000 sf/100 = 50 Occupants. Then, we would just continue with each space for the entire project and add all the occupants together.
One thing to keep in mind is whether the load factors are calculated using gross or net. Gross is the total area of a space, where as net is calculated using only occupiable space. The determination of whether a space is considered net, gross, or any other special cases are laid out in the code, but is more detail then we need to explore at this time.
How Does Occupant Load Affect the Design?
Occupant Load is the driving force of many aspects of design. Some of the elements that are directly tied to the number of occupants are the required number of exits, required number of restroom plumbing fixtures, the width of stairs, the clear width of doors, the width of the clear egress path, types of door hardware, corridor fire-resistance, and a number of other items to ensure the health, safety, and welfare of people within the structure.
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