Table 1604.5 of the 2012 IBC sets forth Risk Category descriptions for buildings and other structures. The basic underlying principle in assigning Risk 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 202 defines Risk Category as follows: A categorization of buildings and other structures for determination of flood, wind, snow, ice and earthquake loads based on the risk associated with unacceptable performance.

Risk Category is used in the IBC for a number of purposes, including determination of Importance Factors as set forth in ASCE 7-10 Table 1.5-2; as well as requirements for structural integrity for exit and elevator hoistway enclosures (IBC Section 403.2.3); determination of wind speed (IBC Section 1609.3); protection of openings in wind design (IBC Section 1609.1.2); general structural integrity (IBC Section 1615); determination of Seismic Design Category (IBC Section 1613.3.5); and special inspection and structural observation (IBC Chapter 17).

There are four Risk Categories (RC), ranging from lowest hazard to human life (RC I) to highest hazard to human life (RC IV). RC I is assigned to structures with a small number of occupants or because they have a limited period of exposure to extreme environmental loading. RC II is assigned to structures other than those in RC I, III, and IV, which are sometimes referred to as “ordinary” for the purpose of risk exposure. RC 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. RC 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 RC IV facilities during an emergency, such as facilities to maintain water pressure for fire suppression. Furthermore, structures holding extremely hazardous materials are included because of the potentially devastating effect of release of those materials in the environment.

In cases where there are multiple occupancies in a structure, IBC Section 1605.4.1 indicates that the highest (or most restrictive) RC 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 designed seismically to the requirements of the higher group. In cases where the two uses are seismically independent but functionally dependent, both portions are required to be assigned to the higher RC. Here is a recent question we received on Section 1605.4.1:

Q: We have a question regarding the building Risk Category designation: If an office building has more than 5,000 total occupants but the building is structurally separated into two wings by a seismic joint with less than 5,000 occupants in each wing, can each wing of this building be assigned Risk Category II? There is a shared basement garage so the “structures” are connected below grade. In your opinion, does this change how we can classify the building Risk Category?

A: Given that the two office wings are not structurally independent at the lowest level, the building should be assigned RC III.