Risk Category & Occupant Load

Good value engineering involves carefully determining risk categories and occupant loads

By Chris Pruitt, PE | Fire Protection Engineering | Honolulu Office

Determining the Risk Category

Determining the Risk Category for a new building is one of the most consequential decisions a design team can make, with effects that impact multiple design disciplines. The process begins with IBC Table 1604.5 (2018 edition, for those in Hawaii), which contains categories that range from Risk Category I: temporary, lowest hazard to human life, with few or no occupants; up to Risk Category IV: essential facilities such as fire stations or emergency shelters. It can be reasonably straightforward to assign a category when your building is clearly a temporary, unoccupied structure, but what about Categories II and III?

Risk Category II is often thought of as the “default,” since it includes any buildings that do not fall under one of the other three categories. The key differentiators that push a building up to Category III hinge on the number of occupants, the level of hazards within the building, and if the building’s occupants are impaired or restricted from responding to an emergency. Some of these differentiators have precise requirements in Table 1604.5, but one requirement can be overlooked: “Any other occupancy with an occupant load greater than 5,000. (a)”

Calculating an occupant load is read as a simple check, but any designer will tell you a fair amount of judgment goes into determining occupant loads.

Determining Total Occupant Load

The footnote at the end of this requirement should ring a bell that this needs a little extra thought. At the bottom of Table 1604.5, this section permits us to use net floor areas to determine the total occupant load (for the purposes of determining the Risk Category), where Table 1004.5 would normally require gross floor areas. Using net floor areas can make a huge difference for business, mercantile, and residential occupancies since it permits us to exclude corridors, closets, stairways, and the like from the calculation. Remembering to apply this footnote can make all the difference for some buildings.

Good Value Engineering

The value of good engineering judgment doesn’t stop there. An example that affected a recent project involved two attached high-rises and an attached parking structure that were analyzed as one building (as allowed by IBC 503.1.2). Simply adding up the occupant loads for each building would have put the total occupant load well over 5,000 people, even after applying the net floor area exemption from Table 1604.5. However, digging deeper revealed that access into the parking garage is restricted to persons using the other two towers. An occupant could not be in a tower and the parking garage at the same time; in other words, the parking garage does not experience simultaneous use with the high-rise towers. This meant that we double-counted occupants if we added the occupant loads for all three buildings. Removing the parking garage’s non-simultaneous occupant load from the risk category calculation brought the combined occupant load under 5,000 people, allowing us to properly assign the combined structure to Risk Category II.

Every project presents different challenges and requires each team member to ask the right questions. Although codes define certain requirements, it is just as important to understand the exceptions and when to apply them.