By John “Buddy” Showalter, P. E., M. ASCE, M. NCSEA, and Sandra Hyde P. E., M. ASCE, M. NCSEA
This multi-part series discusses significant structural changes to the 2024 International Building Code (IBC) by the International Code Council (ICC). Part 5 includes an overview of changes to IBC Chapter 16 regarding risk categories. Only a portion of the chapter’s total number of code changes is discussed in this article. More information on the code changes can be found in the 2024 Significant Changes to the International Building Code available from ICC.
ASCE 7-22
ASCE/SEI 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures includes provisions for risk categories and corresponding importance factors in Tables 1.5-1 and 1.5-2 respectively. However, IBC Table 1604.5 provides more comprehensive coverage of buildings assigned to various risk categories. The ASCE 7 Commentary provides background for the disparity. “The reason for this generalization is that the acceptable risk for a building or structure is an issue of public policy rather than purely a technical one…Individual communities can alter these lists when they adopt local codes based on the model code, and individual owners or operators can elect to design individual buildings to higher occupancy categories based on personal risk management decisions…[this] also provides individual communities and development teams the flexibility to interpret acceptable risk for individual projects.”
While wind and seismic loads have for some time been based on the risk category of the building, changes in ASCE 7-22 to snow, ice, and rain loads along with new provisions for tornado loads use the building’s risk category as well to determine load magnitudes (see Part 6 of this series in the April 2024 issue of STRUCTURE for more details). According to the ASCE 7-22 Commentary, prior to ASCE 7-22, Importance Factors were presented for snow, ice thickness, and seismic loads for the four risk categories identified. The specific Importance Factors differed according to statistical characteristics of the environmental loads and the way a structure responds to those loads. Now that snow load and ice thickness hazard maps incorporate consideration of less probable loads for structures assigned to higher risk categories, Importance Factors for those loads are no longer needed. However, the system reliabilities for earthquake are different from those for other environmental hazards because the design philosophy of ASCE 7 is to prevent system collapse in a risk-targeted maximum considered earthquake. Accordingly, ASCE 7 Table 1.5-2 which lists importance factors based on risk category was revised as follows:
| Risk Category from Table 1.5-1 | I | Seismic, Ie | ||
| I | 1.00 | |||
| II | 1.00 | |||
| III | 1.25 | |||
| IV | 1.50 |
IBC Risk Category Revisions
IBC Section 1604.5 and IBC Table 1604.5 provide a joint presentation of the risk categories of buildings and other structures. The risk category (RC) reflects the relative seriousness of potential failure. Categories vary from the lowest hazard to human life (RC I) to the highest hazard (RC IV) and serve as a threshold for a variety of code provisions related to earthquake, rain, flood, snow, ice, tornado, and wind loads. Loads increase with the importance of the facility. The result is a more robust structure that would be less likely to sustain damage under the same conditions than a structure assigned to a lower risk category. The intent is to enhance a structure’s performance based on its use or the need to remain in operation during and after a design event.
In the 2024 IBC, most parking garages are assigned to RC II, a multiple occupancy trigger was added for RC IV, power-generating stations are separated into RC III or IV based on power unit ratings, and photovoltaic (PV) panel systems are clarified for all four risk categories. New to RC IV are public utility facilities providing power generation, potable water treatment, or wastewater treatment. All Group I-2 occupancies and most of Group I-3 occupancies are also moved to Risk Category IV.
1604.5 Risk category. Each building and structure shall be assigned a risk category in accordance with Table 1604.5. Where a referenced standard specifies an occupancy category, the risk category shall not be taken as lower than the occupancy category specified therein. Where a referenced standard specifies that the assignment of a risk category be in accordance with ASCE 7, Table 1.5-1, Table 1604.5 shall be used in lieu of ASCE 7, Table 1.5-1.
Exceptions:
1. The assignment of buildings and structures to Tsunami Risk Categories III and IV is permitted to be in accordance with Section 6.4 of ASCE 7.
2. Freestanding parking garages not used for the storage of emergency services vehicles or not providing means of egress for buildings or structures assigned to a higher risk category, shall be assigned to Risk Category II.
| Risk Category | Nature of Occupancy |
| III | Buildings and other structures that represent a substantial hazard to human life in the event of failure, including but not limited to: –Group I-3, Condition 1 occupancies. –Any other occupancy with an occupant load greater than 5,000.a –Power-generating stations with individual power units rated 75 MWAC (megawatts, alternating current) or greater, water treatment facilities for potable water, wastewater treatment facilities and other public utility facilities not included in Risk Category IV. |
| IV | Buildings and other structures designated as essential facilities and buildings where loss of function represents a substantial hazard to occupants or users, including but not limited to: –Group I-2 occupancies, —Group I-3 occupancies other than Condition 1. –Public utility facilities providing power generation, potable water treatment, or wastewater treatment. |
1604.5.1 Multiple occupancies. Where a building or structure is occupied by two or more occupancies not included in the same risk category, it shall be assigned the classification of the highest risk category corresponding to the various occupancies. Where buildings or structures have two or more portions that are structurally separated, each portion shall be separately classified. Where a separated portion of a building or structure provides required access to, required egress from or shares life safety components systems, designated seismic systems, emergency power systems, or emergency and egress lighting systems with another portion having a higher risk category, or provides required electrical, communications, mechanical, plumbing, or conveying support to another portion assigned to Risk Category IV, both portions shall be assigned to the higher risk category.
(No changes to Section 1604.5.1 Exception)
1604.5.2 Photovoltaic (PV) panel systems. Photovoltaic (PV) panel systems and elevated PV support structures shall be assigned to a risk category as follows:
1. Ground-mounted PV panel systems serving only Group R-3 buildings shall be assigned to Risk Category I.
2. Ground-mounted PV panel systems other than those described in items 1 and 5 shall be assigned to Risk Category II.
3. Elevated PV support structures other than those described in Items 4, 5, and 6 shall be assigned to Risk Category II.
4. Rooftop-mounted PV panel systems and elevated PV support structures installed on top of buildings shall be assigned to the same risk category as the risk category of the building on which they are mounted.
5. PV panel systems and elevated PV support structures paired with energy storage systems (ESS) and serving as a dedicated, stand-alone source of backup power for Risk Category IV buildings shall be assigned to Risk Category IV.
6. Elevated PV support structures where the usable space underneath is used for parking of emergency vehicles shall be assigned to Risk Category IV.
Change Significance:
Parking Structures
The new exception to IBC Section 1604.5 addresses an anomaly whereby large parking structures that serve airports, shopping centers, and other large buildings potentially triggered a RC III designation when floor areas exceeded 1 million square feet based on the parking garage occupant load factor. IBC Table 1604.5 designates “Any other occupancy with an occupant load greater than 5,000” as an RC III structure. The occupant load factor for parking garages is 200 gross square feet per occupant per IBC Table 1004.5. There are circumstances where even when deducting drive aisles and the other non-usable space to establish an occupant load based on the net area that the total occupant load can still exceed 5,000. The occupancy for parking garages is classified as a Group S-2 light hazard storage area and the occupant load density tends to be low due to the intermittent nature of the storage occupancy. It is unlikely that a large parking garage will have 5,000 occupants simultaneously entering or exiting a garage concurrent with a significant natural hazard event. Therefore, the new Exception 2 allows all parking garages to be assigned to RC II unless the building is used for emergency vehicles or is part of an egress path from a RC III or IV building – a highly unlikely design approach.
The modification to IBC Table 1604.5 footnote “a” clarifies that floor area for drive aisles in parking garages can be deducted when determining net floor area for the assignment of risk category. While occupants use drive aisles to access parked motor vehicles, they are not commonly identified as “aisles” based on the IBC definition (associated with the path of egress). Unlike mechanical access parking garages, vehicular aisles are an integral part of a garage’s function, and motor vehicle occupants are not concurrently located within the drive aisle.
Multiple Occupancies
In IBC Section 1604.5.1, the phrase “electrical, communications, mechanical, plumbing, or conveying” refers to the requirements of IBC Chapters 27 through 30, respectively. The references are intended to be generic, just like the provision’s references to undefined “life-safety systems” and to egress, access, and structural separation. As with many IBC provisions, it is appropriate to leave project-specific details to the project team with approval by the code official, in this case, to determine which aspects of those systems must function post-disaster for the RC IV building or utility.
Power-generating Stations
Since there is no definition in the IBC for “power-generating stations” a modification was made to IBC Table 1604.5 to define the power generation threshold for categorization as a RC III structure. Note that 75 MWAC is established as the smallest power-producing unit of a power-generating station in North American Electric Reliability Corporation (NERC) Docket No. RR15-4-000, Order on Electric Reliability Organization Risk Based Registration Initiative and Requiring Compliance Filing (issued March 19, 2015). Therefore, power generation under the 75 MWAC limit does not trigger a Risk Category III or IV assignment.
Public Utilities
New to RC IV are public utility facilities providing power generation, potable water treatment, or wastewater treatment. Instead of drawing a line between normal operations and emergency backup, this change makes the distinction between public utilities (typically designated by a state or local commission) and other utilities such as communications or public transit. The discussion of Institutional Facilities also addresses public utilities and changes to the Risk Category IV designation.
Photovoltaic Panel Systems
New IBC Section 1604.5.2 resolves confusion about the assignment of risk categories for PV projects. Two new definitions for PV systems help clarify risk category assignments. A ground-mounted PV system is defined as an independent system without useable space underneath, installed directly on the ground. An elevated PV system is defined as an independent PV panel support structure designed with useable space underneath, a minimum clear height of 7.5 feet, and intended for secondary uses such as providing shade or parking for motor vehicles.
Ground-mounted PV panel systems meet the description of RC I, as they “represent a low hazard to human life in the event of failure” and are assigned to this category as a low-risk structure. Elevated PV support structures are often constructed on the ground surface over parking spaces. In this application, the elevated PV support structures do not use any space that is not already used as a parking lot, and they provide the added benefit of shade or weather protection for vehicles. These structures can also be constructed to supply shade for other uses such as picnic areas. In all these cases, elevated PV support structures are assigned to Risk Category II as having a typical hazard risk similar to most buildings (Figure 2).
Where PV panel systems are mounted on building roofs, whether attached or unattached, they are assigned the same risk category as the building on which they are mounted. Where PV panels plus an energy storage system (ESS) are the only direct source of backup power for an essential services facility (RC IV) – with a transfer switch or other equipment enabling the ESS to operate independently from the grid during a time of grid power outage – the panels and ESS are assigned to RC IV. If PV panels plus ESS are not designed to operate in the event of a grid power outage, then they may be assigned the same risk category as the building to which they supply power. This assignment of risk category can also apply when power switching enables the use of either the PV plus ESS or a generator interchangeably.
There are cases where elevated PV support structures are installed on the same site as a Risk Category IV building over surface parking spaces that are designated for emergency services vehicles. Whether or not those elevated PV support structures are serving as part of a backup power source, the elevated PV support structures must be assigned as RC IV as damage to the panels or support structure can take the emergency vehicles out of service.
Institutional Facilities
The code defines essential facilities as those that need to remain operational during and after an extreme event, for example, a large earthquake, flood, wind, or snow event. Prolonged downtime – which can be weeks or months after an earthquake or hurricane for RC II or III structures – may pose a risk for vulnerable users. A Risk Category IV designation is a common approach for providing functional facilities soon after an extreme event. In terms of post-event functionality, there is a wide gap between RC II or III facilities (which have identical requirements for nonstructural systems) and RC IV facilities.
This performance gap is most acute for occupancies that depend on functional nonstructural systems and special design provisions to serve vulnerable users. For example, a hospital has individuals “rendered incapable of self-preservation,” when anesthesia is used. Therefore, evacuating the building is very rare. Instead, compartmentalization into smoke zones or fire compartments is a strategy to keep building occupants safe in case of a fire or emergency event. Because institutional facilities are specially designed, their services and occupants cannot be quickly relocated to other buildings. Therefore, other institutional facilities with special design features and vulnerable users are occasionally designated as RC IV buildings exceeding minimum code requirements.
The scope of RC IV is expanded in the 2024 IBC from “essential facilities” to include “buildings where loss of function represents a substantial hazard.” This “substantial hazard” can be life-threatening where, for example, a 24-hour medical facility, residential care facility, public water or power utility, or a detention center with impeded egress is out of service for weeks or longer.
Accordingly, medical care facilities assigned to Group I-2 are all now classified as RC IV. Many design professionals assume all hospitals, typically assigned to Group I-2, are already assigned to RC IV, but that was only true for facilities that provide emergency surgery or emergency treatment. Even “in-patient stabilization,” which is part of what defines Group I-2 Condition 2, did not qualify as RC IV. Some Group I-2 facilities, which include hospitals, nursing homes, and detoxification facilities, were assigned to RC II or RC III, even though they provide 24-hour medical care for patients who are incapable of self-preservation, and even though they are already required to meet special design requirements for corridors and egress plans in IBC Section 407. Group I-2 facilities with fewer than 50 patients were not even assigned to RC III.
While most Group I-2 facilities are reassigned to a higher risk category, it is important to note that this change does not affect:
- Medical care facilities for 5 or fewer residents as IBC Section 308.3, Group I-2, applies only to larger facilities.
- Outpatient or ambulatory care facilities (buildings addressed in IBC Section 422), including “urgent care” businesses, dialysis centers, dentists, optometrists, or similar clinics. These buildings are typically assigned to Group B. Ambulatory care facilities with emergency surgery or emergency treatment facilities are already assigned to RC IV.
- Pharmacies or drug stores are typically Group M.
- Medical office buildings are typically Group B.
- Medical supply or equipment manufacturers, and warehouses or stores are typically Group F, S or M.
- Adult family homes and other care facilities designed per the IRC.
Another change to IBC Table 1604.5 reassigns four of the five Conditions under Group I-3 to RC IV. Except for Condition 1, which remains in RC III, all Group I-3 facilities have egress and free movement impeded by locks, rendering occupants incapable of self-preservation (Figure 3). Because of this restraint, the uniqueness of Group I-3 facilities, and the implications of long repair times, Risk Category IV is appropriate. This change does not affect:
- Group I-3, Condition 1 since these facilities allow free movement for occupants and are even eligible for design as residential occupancies.
- Facilities with fewer than 6 people under restraint – per IBC Section 308.4, Group I-3, applies only to larger facilities. This would exempt typical holding cells in small court facilities.
- Halfway houses assigned to Group I-1 or R-4.
Group I-1 facilities are placed in RC II, unless another trigger moves them to RC III or higher, for example having an occupant load greater than 5,000 people in one building or having Group I-2 facilities in the same building without fire separation between the two portions of the building.
Conclusion
Structural engineers should be aware of significant structural changes in the 2024 IBC Chapter 16 for risk categories. Most parking garages are assigned to RC II, a multiple occupancy trigger was added for RC IV, power-generating stations are separated in RC III or IV based on power unit ratings, and photovoltaic (PV) panel systems are clarified for all four risk categories. New to RC IV are public utility facilities providing power generation, potable water treatment, or wastewater treatment. All Group I-2 occupancies and most of Group I-3 occupancies are also moved to Risk Category IV. ■
John “Buddy” Showalter, P. E., (bshowalter@iccsafe.org) is Senior Staff Engineer of ICC’s Consulting Group.
Sandra Hyde, P. E., (shyde@iccsafe.org) is Managing Director of ICC’s Consulting Group.