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Table 1604.5 of the 2009 International Building Code (IBC) sets forth Occupancy Category descriptions for buildings and other structures. The basic underlying principle in assigning Occupancy Category is to recognize the impact of a structural failure. An obvious illustration of this is the very different impacts of structural failures of an uninhabited building and a crowded building. IBC Section 1602 defines Occupancy Category as follows: A category used to determine structural requirements based on occupancy.

Occupancy Category is used in the IBC for a number of purposes, including determination of Importance Factor as set forth in ASCE 7; as well as requirements for structural integrity for exit and elevator hoistway enclosures (IBC Section 403.2.3); general structural integrity (IBC Section 1614); glazing for wind design (IBC Section 1609.1.2); determination of Seismic Design Category (IBC Section 1613.5.6); and special inspection and structural observation (IBC Chapter 17).

There are four Occupancy Categories (OC), ranging from lowest hazard to human life (OC I) to highest hazard to human life (OC IV). OC I is assigned to structures with a small number of occupants or because they have a limited period of exposure to extreme environmental loading. OC II is assigned to structures other than those in OC I, III, and IV, which are sometimes referred to as “ordinary” for the purpose of risk exposure. OC III is assigned to structures that have large numbers of occupants and are designed for public assembly — structures where physical restraint or something else hinders the occupant’s ability to move or evacuate, or structures that are part of the infrastructure, such as power generating stations or water treatment facilities, where a failure may not create an unusual life-safety risk, but can cause large-scale economic impact and/or mass disruption of day-to-day civilian life. OC IV is assigned to structures designated as essential facilities that are intended to remain operational in the event of extreme loading, such as hospitals or fire stations. Also included are structures required for the operation of OC IV facilities during an emergency, such as facilities to maintain water pressure for fire suppression. Furthermore, structures holding extremely hazardous materials also are included because of the potentially devastating effect of release of those materials in the environment.

In the case where there are multiple occupancies in a structure, IBC Section 1605.4.1 indicates the highest (or most restrictive) OC is to be assigned to the structure unless the portions are structurally separated. In other words, when a lower group impacts a higher group, the higher group must either be seismically independent of the other, or the two must be in one structure designed seismically to the requirements of the higher group. In the case in which the two uses are seismically independent but functionally dependent, both portions are required to be assigned to the higher OC.

IBC Table 1604.5, in some cases, defines the OC with the help of occupancy classifications defined in IBC Chapter 3. For instance, hospitals and health care facilities are referred to as Group I-2 occupancies in order to specify the assignment of OC III or IV, so that the care facilities in Group I-1 classification are not mistakenly assigned to OC III or IV.

Footnote “a” to IBC Table 1604.5 makes a distinction between the calculation of occupant load for the purpose of determining whether a building qualifies for OC III (because the occupant load exceeds 5,000) and that for the purpose of IBC Table 1004.1.1. The calculation of the occupant load for the purpose of Table 1604.5 can be based on the net floor area, even when Table 1004.1.1 requires gross floor area to be used. The net floor area does not include corridors, stairways, elevators, closets, accessory areas, or structural walls and columns. This is because of the basic difference in the purposes the two tables serve. Table 1004.1.1 provides minimum occupant loads for the purpose of egress design, and egress design is determined on a floor-by-floor basis, assuming that the maximum occupant load can occur on a certain floor at a given time. However, for the purpose of Table 1604.5, occupant loads are calculated for the whole building, and it is very unlikely that all floors of a building would have the maximum occupant load at the same time. This is the same reason the live load reduction provisions are included in the code.

There also has been some confusion about whether or not the occupant loads noted in the descriptions for OC III are based on an individual floor occupant load or the total occupant load on all floors. The intent is for the total occupant load of all floors to be used unless, for some reason, all floors cannot be occupied simultaneously. The intent of the 300-occupant load trigger is often questioned. In this case, yes, the total occupant load is to be used; however, one must first be sure that the primary occupancy of the structure is public assembly, and like many provisions in the code, this requires judgment. Note that IBC Section 303.1 indicates that “Group A occupancy includes, among others, the use of a building or structure, or a portion thereof, for the gathering of persons for purposes such as civic, social, or religious functions; recreation, food or drink consumption, or awaiting transportation.”

It should be noted that the term “occupancy category” will be replaced with “risk category” in the 2012 IBC for consistency with the term used in ASCE 7-10. This change was made because it was determined that the use of the word “occupancy” was misleading in that it implied the category was directly tied to occupancy classifications in the code, whereas the word “risk” communicates more accurately that the category is based on acceptable risk of failure.

S.K. Ghosh Associates Inc., is a structural seismic and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. Presidents S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at skghosh@aol.com and susandowty@gmail.com, respectively, or at www.skghoshassociates.com.