This article appears in the April 2024 issue of STRUCTURE Magazine as Part 6 of 9. Reprinted with permission.
This multi-part series discusses significant structural changes to the 2024 International Building Code (IBC) by the International Code Council (ICC). This article includes an overview of changes to IBC Chapter 16 for environmental loads including snow, rain, wind, tornado, and earthquake. 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 (Figure 1). [For all images, See PDF or View in Full Issue.]
ASCE 7-22 and Hazard Tool
ASCE/SEI 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures includes a significant number of revisions for nearly all environmental loads. Changes throughout the 2024 IBC, most significantly in IBC Chapter 16, match changes to ASCE 7. To easily determine new environmental loads based on location and risk category, use the ASCE Hazard Tool (asce7hazardtool.online) – a free resource.
Snow Loads
New ground snow load (GSL) maps based on ASCE 7-22 are included in the IBC and are now based on the risk category of the building (Figure 2).
1608.2 Ground snow loads. The ground snow loads to be used in determining the design snow loads for roofs shall be determined in accordance with the reliability-targeted (strength-based) ground snow load values in Chapter 7 of ASCE 7 or Figures 1608.2(1) and through 1608.2(24) for the contiguous United States and Table 1608.2 for Alaska. Site-specific case studies shall be determined in accordance with Chapter 7 of ASCE 7 and shall be approved by the building official. made in areas designated “CS” in Figures 1608.2(1) and 1608.2(2). Ground snow loads for sites at elevations above the limits indicated in Figures 1608.2(1) and 1608.2(2) and for all sites within the CS areas shall be approved. Ground snow load determination for such sites shall be based on an extreme value statistical analysis of data available in the vicinity of the site using a value with a 2-percent annual probability of being exceeded (50-year mean recurrence interval). Snow loads are zero for Hawaii, except in mountainous regions as approved by the building official.
Additional related technical and editorial changes throughout the IBC are not shown for brevity.
Change Significance: ASCE 7-22 includes updated national GSL datasets in electronic and map form. The new snow loads are based on 25 years of additional snow load data and updated procedures for estimating snow loads, as well as using strength design-based values. Additionally, this approach incorporates advanced spatial mapping that has reduced the number and size of case study regions in mountainous areas significantly and eliminates discontinuities in design values across state boundaries.
Given that GSL values have been provided as allowable stress loads up to this point, many provisions within the IBC and the IRC rely on allowable stress design (ASD) values. Therefore, a new Section 1608.2.1 is added to provide a conversion from the strength-based values now provided in the IBC reliability-targeted GSL maps to an equivalent ASD value. Note that the ASCE Hazard Tool also provides ASD ground snow loads.
Wind Loads
Wind speed maps and associated provisions are updated to the newly referenced ASCE 7-22 load standard.
SECTION 202
DEFINITIONS
BASIC WIND SPEED, V. Basic design wind speeds. The wind speed used for design, as determined in Chapter 16.
WINDBORNE DEBRIS REGION. Areas within hurricane-prone regions located:
(1.) Within 1 mile (1.61 km) of the mean high-water line where an Exposure D condition exists upwind at the waterline and the basic design wind speed, V, is 130 mph (58 m/s) or greater; or (2.) In areas where the basic design wind speed, V, is 140 mph (63 m/s) or greater.
For Risk Category II buildings and structures and Risk Category III buildings and structures, except health care facilities, the windborne debris region shall be based on Figure 1609.3.(1) 1609.3.(2). For Risk Category III health care facilities, and Risk Category IV buildings and structures and Risk Category III health care facilities, the windborne debris region shall be based on Figure 1609.3(2) 1609.3(3) and Figure 1609.3(4), respectively.
WIND DESIGN GEODATABASE. The ASCE database (version 2022-1.0) of geocoded wind speed design data. The ASCE Wind Design Geodatabase of geocoded wind speed design data is available at https://ascehazardtool.org/.
1609.3 Basic design wind speed. The basic design wind speed, V, in mph, for the determination of the wind loads shall be determined by Figures 1609.3(1) through 1609.3(12) 1609.3(4). The basic design wind speed, V, for use in the design of Risk Category I II buildings and structures shall be obtained from Figures 1609.3(1), 1609.3(5) and 1609.3(6). The basic design wind speed, V, for use in the design of Risk Category II III buildings and structures shall be obtained from Figures 1609.3(2), 1609.3(7) and 1609.3(8). The basic design wind speed, V, for use in the design of Risk Category III IV buildings and structures shall be obtained from Figures 1609.3(3), 1609.3(9) and 1609.3(10). The basic design wind speed, V, for use in the design of Risk Category IV I buildings and structures shall be obtained from Figures 1609.3(4), 1609.3(11) and 1609.3(12). Basic wind speeds for Hawaii, the US Virgin Islands, and Puerto Rico shall be determined by using the ASCE Wind Design Geodatabase. The ASCE Wind Design Geodatabase is available at https://ascehazardtool.org, or an approved equivalent.
The basic design wind speed, V, for the special wind regions indicated near mountainous terrain and near gorges shall be in accordance with local jurisdiction requirements. The basic design wind speeds, V, determined by the local jurisdiction shall be in accordance with Chapter 26 of ASCE 7. In nonhurricane-prone regions, when the basic design wind speed, V, is estimated from regional climatic data, the basic wind speed, V, shall be determined in accordance with Chapter 26 of ASCE 7.
Additional related technical and editorial changes throughout the IBC are not shown for brevity.
Change Significance: These changes to IBC wind load provisions include technical updates as well as editorial corrections and reorganization. Technical updates to the wind speed maps within ASCE 7-22 include new hurricane coastline wind speed contours from the Carolinas through Texas, as well as new Special Wind Regions in Southern California and Northern Colorado (Figure 3). All updates are based on recent wind studies conducted in those areas.
Along with the continental United States, the wind speeds for the U.S. Virgin Islands and Puerto Rico were also updated based on recent wind studies of these islands. The resulting wind speeds account for the steep terrain of these islands and create a very dense contour map that is not easily read in the IBC. Therefore, wind speeds for the U.S. Virgin Islands, Puerto Rico, and Hawaii are only included in the ASCE Wind Design Geodatabase and are no longer represented by maps in ASCE 7-22. These maps have also been removed from the IBC and replaced with a pointer to the ASCE Wind Design Geodatabase. Wind speeds in the updated Special Wind Regions also are available for designers and code officials in the ASCE Hazard Tool.
Tornado Loads
Design provisions for tornado loads are now required for Risk Category III and IV buildings in defined areas. See Figure 4 for the average annual frequency of tornadoes per state; most tornadoes occur in central and southeastern states.
1609.5 Tornado Loads. The design and construction of Risk Category III and IV buildings and other structures located in the tornado-prone region as shown in Figure 1609.5 shall be in accordance with Chapter 32 of ASCE 7, except as modified by this code.
Additional related technical and editorial changes throughout the IBC are not shown for brevity.
Change Significance: Tornado hazards have not previously been required in the design of conventional buildings, even though tornadoes and tornadic storms cause more fatalities and more catastrophe-insured losses than hurricanes and earthquakes combined. This gap is addressed for the first time in ASCE/SEI 7‐22 which now includes requirements for tornado loads. ASCE 7-22 requirements for tornado loads apply to Risk Category III and IV buildings only sited in the tornado-prone region, which is roughly defined in IBC Figure 1609.5 as the area of the U.S. east of the Continental Divide.
Tornado loads specified in the new Chapter 32 of ASCE 7 provide reasonable consistency with the reliability delivered by wind criteria in ASCE 7 Chapters 26 and 27 for the Main Wind Force Resisting System (MWFRS). The same mean recurrence intervals (MRI) are used for tornado wind speeds as the basic wind speeds in Chapter 26 for Risk Category (RC) III and IV facilities (MRI = 1,700 and 3,000 years, respectively). At return periods of 300 and 700 years (used for wind speeds with RC I and II structures), tornado wind speeds are generally so low that tornado loads will not control over ASCE 7 Chapter 26 wind loads. Therefore, design for tornadoes is not mandated for RC I and II buildings.
ASCE 7-22 tornado design speeds for RC III and IV structures range from 60 to 138 mph depending on geographic location and effective plan area (which is a function of the building or multiple buildings’ footprint size and shape). This generally corresponds to wind speeds for Enhanced Fujita (EF) Scale EF0-EF2 tornadoes, which are the most common. From 1995 to 2016, over 89% of all reported tornadoes were EF0-EF1, and 97% were EF0-EF2.
Buildings and other structures classified as Risk Category III or IV and located in the tornado-prone region, including the MWFRS and all components and cladding (C&C), are to be designed and constructed to resist the greater of the tornado loads or the straight-line wind loads determined per ASCE 7-22. This means a check of both tornado and wind loads is required. However, if the tornado wind speed is less than 60 mph, design for tornado loads is not required. Also, if the tornado wind speed is less than a certain percentage of the straight-line wind speed as a function of exposure, design for tornado loads is also not required.
The intent of ASCE 7 and the IBC is to increase wind speeds in locations where the increase is reasonable and can be applied to most buildings that are in RC III and IV. This change doesn’t require buildings to add a storm shelter, rather the MWFRS and C&C are designed to resist both straight-line winds and EF2 tornadoes. In any given area, the wind speeds of the straight-line wind may control the wind design or the EF2 tornado could control the wind design based on the worst-case combinations of geographic location, exposure, effective plan area, mean roof height, enclosure classification, building shape, and other parameters.
To make it clear that the ASCE 7 tornado provisions are not intended to protect from the most violent tornadoes, a “User Note” on the first page of the ASCE 7 Tornado Load chapter advises readers in part as follows:
“…A building or other structure designed for tornado loads determined exclusively in accordance with Chapter 32 cannot be designated as a storm shelter without meeting additional critical requirements provided in the applicable building code and ICC 500, the ICC/NSSA Standard for Design and Construction of Storm Shelters…”
Tornado hazard criteria for ICC 500 and FEMA P-361 Safe Rooms for Tornadoes and Hurricanes – Guidance for Community and Residential Safe Rooms are much more stringent than ASCE 7, reflecting the purpose to provide “near-absolute life-safety protection” as described by FEMA P-361. For example, the tornado shelter design wind speed in the central U.S. is 250 mph. This compares to ASCE 7 tornado wind speeds of approximately 80-125 mph for Risk Category III and 95-140 mph for Risk Category IV.
Rain Loads
The design storm return period for determination of the hydraulic head is now to be based on risk category. Other ponding provisions are updated to be consistent with ASCE 7-22.
1603.1.9 Roof rain load data. Design rainfall Rain intensity, i (in/hr) (cm/hr), and roof drain, scupper and overflow locations shall be shown regardless of whether rain loads govern the design.
1608.3 Ponding instability. Susceptible bays of roofs shall be evaluated for ponding Ponding instability on roofs shall be evaluated in accordance with Chapters 7 and 8 of ASCE 7.
1611.1 Design rain loads. Each portion of a roof shall be designed to sustain the load of rainwater as per the requirements of Chapter 8 of ASCE 7. Rain loads shall be based on the summation of the static head, ds, hydraulic head, dh, and ponding head, dp, using Equation 16-19. The hydraulic head shall be based on hydraulic test data or hydraulic calculations assuming a flow rate corresponding to a rainfall intensity equal to or greater than the 15-minute duration storm with return period given in Table 1611.1. Rainfall intensity shall be determined in inches per hour for 15-minute duration storms for Risk Category given in Table 1611.1. The design rainfall shall be based on the 100-year 15-minute duration event, or on other rainfall rates determined from approved local weather data. Alternatively, a design rainfall of twice the 100-year hourly rainfall rate indicated in Figures 1611.1(1) through 1611.1(5) shall be permitted. The ponding head shall be based on structural analysis as the depth of water due to deflections of the roof subjected to unfactored rain load and unfactored dead load.
R = 5.2 (ds + dh + dp) (Equation 16-19)
where:
dh = hydraulic head equal to the depth of water on the undeflected roof above the inlet of the secondary drainage system for structural loading (SDSL) required to achieve the design flow in inches (mm) Additional depth of water on the undeflected roof above the inlet of secondary drainage system at its design flow (in other words, the hydraulic head), in inches (mm).
dp = ponding head equal to the depth of water due to deflections of the roof subjected to unfactored rain load and unfactored dead load, in inches (mm)
ds = static head equal to the depth of water on the undeflected roof up to the inlet of the secondary drainage system for structural loading (SDSL) in inches (mm) Depth of water on the undeflected roof up to the inlet of secondary drainage system when the primary drainage system is blocked (in other words, the static head), in inches (mm).
R = Rain load on the undeflected roof, in pounds per square foot (kN/m2). Where the phrase “undeflected roof” is used, deflections from loads (including dead loads) shall not be considered when determining the amount of rain on the roof.
SDSL is the roof drainage system through which water is drained from the roof when the drainage systems listed in ASCE 7 Section 8.2 (a) through (d) are blocked or not working.
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Risk Category
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Design Storm Return Period
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I&II
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100 Years
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III
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200 Years
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IV
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500 Years
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IBC Table 1611.1 Design [Rain] Storm Return Period by Risk Category
1611.2 Ponding instability. Susceptible bays of roofs shall be evaluated for ponding Ponding instability on roofs shall be evaluated in accordance with Chapters 7 and 8 of ASCE 7.
Change Significance: The primary change to IBC Section 1611.1 is the addition of the ponding head (dp) directly into the rain load calculation (Figure 5). In ASCE 7-16 and earlier editions, there was a requirement to perform a ponding analysis, yet limited guidance was provided on how to perform that analysis. The term “secondary drainage system for structural loading (SDSL)” is consistent with ASCE 7-22. Activation of the SDSL is intended to serve as a warning that the primary drainage system is blocked. Per ASCE 7, the elevation of the SDSL must be at least 2 inches above that of the primary drainage system so that the SDSL is not frequently activated, which would decrease the urgency of the warning and also make the SDSL more susceptible to blockage.
IBC Figures 1611.1(1) through 1611.1(5) were removed because they were 100-year “hourly” rainfall maps, which did not provide rainfall intensities for the required 15-minute duration storms. Furthermore, rainfall rates must now be determined based on the building’s Risk Category. New Table 1611.1 defines the design storm return period by Risk Category consistent with the determination of rainfall intensity per ASCE 7-22. Note that return periods are now 200 years and 500 years for Risk Category III and IV structures, respectively. The ASCE Hazard Tool provides both 15-minute and 60-minute rainfall intensities.
Sections 1608.3 and 1611.2 refer to the defined term “Susceptible Bay” for ponding instability evaluation. ASCE 7-22 has dropped this term but still takes ponding into account for snow and rain loads.
Earthquake Loads
IBC Section 1613 includes requirements for determining a building’s seismic design category (SDC). The balance of the earthquake design requirements is contained in ASCE 7. These changes bring the 2024 IBC up to date with new provisions of ASCE 7-22 and determining the SDC is simplified.
1613.2 Determination of seismic design category Seismic ground motion values. Structures shall be assigned to a seismic design category based on one of the following methods unless the authority having jurisdiction or geotechnical data determines that Site Class DE, E or F soils are present at the site:
1. Based on the structure risk category using Figures 1613.2(1) through 1613.2(7).
2. Determined in accordance with ASCE 7,
Where Site Class DE, E or F soils are present, the seismic design category shall be determined in accordance with ASCE 7. Seismic ground motion values shall be determined in accordance with this section.
Sections 1613.2.1 through 1613.2.5.1 have been deleted without substitution and are not shown for brevity. New Figures 1613.2(1) through 1613.2(7) replace existing Figures 1613.2(1) through 1613.2(10). Additional related technical and editorial changes throughout the IBC are not shown for brevity.
Change Significance: These changes simplify IBC Section 1613 by providing SDC maps that users can reference to quickly determine a project’s SDC based on default site conditions (Figure 6). These maps replace current ground motion response acceleration maps in the IBC and have been derived based on new multi-period response spectra procedures of ASCE 7-22.
The SDC maps are one of two methods provided in the IBC to determine SDC. Users are still allowed to determine the SDC following ASCE 7 provisions, where more refined information such as site-specific soils data can be considered.
These new maps will allow building officials, non-structural engineers, component manufacturers, and others to quickly identify a conservative SDC based on location alone. The ASCE Hazard Tool can be used to determine seismic design parameters, including SDC, based on location, soil class, and risk category.
Areas with SDC D for buildings in all Risk Categories based on the new maps include:
- Most of the state of Nevada except for the northeastern portions.
- Areas within an approximate 150-mile radius from New Madrid, Missouri (except for higher SDCs along the New Madrid fault line).
- Areas within an approximate 75-mile radius of Charleston, South Carolina.
Conclusion
Structural engineers should be aware of significant structural changes in the 2024 IBC Chapter 16 for environmental loads. Updates provide consistency between the IBC and ASCE 7-22. Most loads are now based on the risk category of the structure and use strength design values. Changes to snow and rain load provisions reflect this risk-based approach to design. New provisions for tornado loads apply to Risk Category III and IV structures. Updates to wind and seismic provisions harmonize with ASCE 7. The ASCE Hazard Tool (ascehazardtool.org) is a free resource for determining environmental loads based on location and risk category.
John “Buddy” Showalter, PE, M. ASCE, M. NCSEA (bshowalter@iccsafe.org) is a Senior Staff Engineer and Sandra Hyde, PE, M. ASCE, M. NCSEA (shyde@iccsafe.org) is Managing Director of ICC’s Consulting Group.