A revised approach to wind load calculations

The 2022 edition of ASCE 7, “Minimum Design Loads and Associated Criteria for Buildings and Other Structures,” became available in December 2021 and replaces the 2016 edition. ASCE 7, issued by the American Society of Civil Engineers, provides information that allows designers to determine various types of loads and load combinations on buildings. The 2022 edition will be referenced in the 2024 editions of the International Building Code® and International Residential Code,® as well as the 2023 edition of the Florida Building Code.

The new edition’s wind load provisions have not changed dramatically except for the inclusion of a new chapter addressing tornado loads.

For those familiar with calculating loads in accordance with the 2016 edition, other than new tornado provisions, the transition to the 2022 edition will be easy. Changes that were made in the 2010 and 2016 editions laid an important foundation for users of the 2022 edition. For those unfamiliar with the 2010 and/or 2016 changes, see "Mapping the 2010 wind changes,” August 2010 issue, and “How do I load thee?” October 2017 issue.

The primary 2022 changes that pertain to the roofing industry include:

• Relatively minor revisions to the wind speed maps for the U.S. mainland
• New wind speeds for Puerto Rico, U.S. Virgin Islands and Northern Mariana Islands
• Site-specific wind speeds for select special wind regions
• Wind speed-up at abrupt changes in topography
• Minor revisions to the velocity pressure coefficients (Table 26.10-1)
• Relocation of the wind directionality factor K_d
• Clarification of the wind-borne debris region
• Commentary on internal pressure coefficients
• Rooftop solar panel arrays
• Deletion of simplified tables in Chapter 30
• Zone changes for low-slope stepped roofs
• Gable and hip roofs
• Attached canopies on buildings higher than 60 feet
• Roof pavers
• Tornado loads

Following is an overview of these additions and changes. I do not address changes that only pertain to primary structural elements such as beams, columns, shear walls and diaphragms that provide overall support and stability for buildings.

Wind speeds

The basic (design) wind speeds for hurricane-prone regions are derived from modeling rather than historical records because of the infrequency of hurricanes. An updated hurricane simulation model was used to develop the wind speeds for the 2022 edition, which resulted in modest changes to the wind speed contours along the Atlantic Ocean and Gulf of Mexico. Speeds increased in parts of Texas and the Florida Panhandle. Speeds decreased in parts of Louisiana, along the Mississippi and Alabama coastlines, and in parts of the Northeast. The difference between the 2016 and 2022 basic wind speed at a specific location can be found quickly using the ASCE 7 Hazard Tool.

Wind speeds in America Samoa, Guam and Hawaii are unchanged.

For the first time, ASCE 7 includes speed for the Northern Mariana Islands, which matches that of nearby Guam. The Hawaii maps are not included in the print and online version of the 2022 edition; instead, the ASCE 7 Hazard Tool should be used to obtain Hawaii’s wind speeds. As with the 2016 edition, Hawaii’s speeds are microzoned “effective” wind speeds that include the wind speed-up effect of topography. Inclusion of topographic effects will simplify load calculations, particularly for sites with complex topography. However, designers still should examine local site conditions of finer toposcale, such as ocean promontories and local escarpments.

The 2022 edition includes new microzoned wind speed maps for the U.S. Virgin Islands and Puerto Rico. As with Hawaii, these maps include the wind speed-up effect of topography. However, these maps use a grid with a resolution of about 100 m, so most local site conditions won’t need examination by a designer. The ASCE 7 Hazard Tool also must be used to obtain these speeds. These new maps were developed after Hurricanes Irma and Maria struck these islands in 2017.

Outside the hurricane-prone regions, there are a few minor changes to the mainland wind speed contours. Alaska’s contours are unchanged.

The ASCE 7 Hazard Tool indicates special wind regions, which are mountainous areas. Site-specific values for the special wind regions in northern Colorado and Kern County, Calif., are included in the hazard tool. If site-specific values at other mainland areas are not provided, the authority having jurisdiction should be consulted for the basic wind speed.

Miscellaneous changes

A variety of other 2022 changes affect wind loads on roof systems and rooftop equipment. Most of the following have relatively minor effects on the roofing industry:

• Wind speed-up at abrupt changes in topography: Changes were made to Section 26.8 and Figure 26.8-1.
• Velocity pressure exposure coefficients (K_z), Table 26.10-1: K_z accounts for the exposure (B, C or D) and building height. Several of the exposure B and C coefficients have changed because of revisions to the height of the atmospheric boundary layer (also known as gradient height). Further background information regarding these changes is provided in the standard’s Commentary section.
• Velocity pressure calculation, equation 26.10-1: In earlier editions, the wind directionality factor (K_d) was included in the velocity pressure equation. In the 2022 edition, K_d was removed from that equation and placed in the individual wind pressure equations and force equations. So for rooftop equipment and rooftop solar panels, K_d is applied in Chapter 29, and for roof systems it is applied in Chapter 30.
• Wind-borne debris regions: These regions occur in portions of hurricane-prone regions. There are two criteria for defining the location of the wind-borne debris region, and changes only were made to the first criterion, which pertains to buildings within 1 mile of bodies of water. In previous editions, the first criterion was ambiguous, particularly in locations with complex shorelines. The first criterion has been clarified to remove the ambiguity. Incidentally, the only requirements related to wind-borne debris pertain to glazing. For new buildings, the wind-borne region does not affect loads on roof systems. However, the wind-borne region may affect roof system loads on existing buildings.
• Commentary on internal pressure coefficients: Section 26.12 provides criteria for determining internal pressure coefficients as a function of a building being classified as open, partially enclosed or partially open. The 2022 commentary has substantially more guidance to assist designers in selecting the enclosure classification and corresponding internal pressure coefficients. New information is given regarding breached glazing outside and within wind-borne debris regions; the influence of compartmentalization on the distribution of increased internal pressure following a breach of the building enclosure; lobby entry vestibules; and remedial work on existing buildings located in wind-borne debris regions.
• Rooftop solar panel arrays: Changes were made in Section 29.4.4 regarding the array edge factor. Also, the pressure equalization factor in Figure 29.4-8 was revised to account for different gap widths between panels.
• Deletion of simplified tables: Chapter 30 pertains to components and cladding loads. The 2016 edition had simplified methods to determine wind pressures. With the simplified methods, pressures were determined from tables in lieu of calculations. These methods were deleted from the 2022 edition.
• Zone changes for low-slope stepped roofs: The field, perimeter and corner zone layout shown in Figure 30.3-3 substantially changed in the 2022 edition because the figure was not updated in 2016 to reflect the low-slope zone changes made that year. For 2022, there are no changes to the zone layout or the pressure coefficients for roofs with slopes up to 1½-in-12.
• Gable and hip roofs: For slopes greater than 1½-in-12, the zones have been simplified and the pressure coefficients have been reduced for some zones. The 2016 edition had a figure for hip roofs with slopes of 27 to 45 degrees. That figure was replaced with one that only has a slope of 45 degrees. The new figure notes linear interpolation is required to determine pressure coefficients for slopes greater than 27 degrees and less than 45 degrees. This will result in higher values in zones 2 and 3 as slopes decrease. In the 2016 edition, the figures included roof overhangs. For 2022, overhang pressure coefficients are addressed in Section 30.7. Some of the gable and hip figures in the 2016 edition provided coefficients for effective wind areas less than 10 square feet. In the 2022 edition, all the figures truncate the effective wind area at 10 square feet. The Commentary advises practitioners may need to determine the approximate pressure coefficient for roof systems that have effective wind areas less than 10 square feet (such as asphalt shingles and tile). The Commentary provides guidance for doing so.
• Other roof shapes: Neither the pressure coefficients nor zones changed for multispan gable, monoslope, sawtooth and dome roofs.
• Buildings with attached canopies: The 2016 edition only provided pressure coefficients for canopies on buildings up to 60 feet high. Figure 30.9-2A was added in 2022 to provide coefficients for canopies on buildings higher than 60 feet.
• Roof pavers: Section 30.12, which is new, provides an equation for calculating uplift pressures on roof pavers. The Commentary references the 2013 edition of ANSI/SPRI RP-4, “Wind Design Standard For Ballasted Single Ply Roofing Systems,” for guidance on roof heights up to 150 feet. The Commentary of the 2019 edition of ANSI/SPRI RP-4 provides guidance for heights greater than 150 feet.

Tornado loads

The new Chapter 32 only applies to Risk Category III and IV buildings in the tornado-prone region. Category III buildings include schools and theaters. Category IV buildings include hospitals, police stations and fire stations.

For sites located in the tornado-prone region, the ASCE 7 Hazard Tool provides tornado speeds for use with the new load criteria. The tornado return periods for each risk category are the same as the mean recurrence intervals for straight-line winds given in Chapter 26. Design tornado speeds approximately correspond to tornado intensities of EF0-EF2 on the Enhanced Fujita Scale.

Design tornado speeds also are a function of a building’s plan size and shape. A larger building has a greater design tornado speed than a smaller facility at the same geographic location. Not all Category III and IV buildings located in the tornado-prone region are required to be designed for tornadoes.

Registered design professionals must determine whether tornado loads are required. Depending on tornado speed and the ratio of it to the basic wind speed given in Chapter 26, tornado loads may or may not control over wind loads. A flow chart in Chapter 32 shows the process for determining when design for tornado loads is required. By using the chart, a designer can quickly determine when design for tornadoes is not required.

The procedures and equations for determining tornado loads are similar to the ones for determining wind loads. However, parameters used in tornado design are modified to account for unique characteristics of tornadoes and their interaction with the considered building.

The Federal Emergency Management Agency and National Institute of Standards and Technology have published a design guide regarding the new tornado load requirements. The guide is intended to help design professionals and building officials determine when a building or other structure is required to be designed for tornadoes and how to calculate design tornado loads. The guide includes a design example to illustrate application of the new tornado criteria in Chapter 32.

It is important to note neither Chapter 32 nor its Commentary address existing buildings. In my opinion, when reroofing a Category IV building in the tornado-prone region is planned, it would be prudent for the building owner to consider having a design professional design the new roof system and rooftop equipment to meet the Chapter 32 loads. It also would be prudent to consider strengthening the roof deck attachment to meet tornado loads.

The only requirement regarding wind-borne debris pertains to glazing. However, the Commentary notes wind-borne debris can penetrate other portions of a building enclosure and create a pathway for rain to enter. The Commentary references a FEMA publication for design guidance to minimize debris and/or rain penetration through roof and wall assemblies.

NIST published case studies comparing tornado and wind loads and cost implications of tornado loads on several building types (elementary school, high school, fire station, hospital) in nine cities in the tornado-prone region. The study includes discussion of potential effects on roof systems. In some cases, the tornado loads governed; in others, the increased load did not affect the number of required insulation or membrane fasteners or spacing of foam ribbon adhesive. Sometimes, the tornado loads require additional foam ribbon adhesive and/or additional fasteners. In no case did the tornado designs require the use of a different roof system or roof system components.

Get the standard

A subscription to an online version of the publication is available at asce7.online and provides a side-by-side display of the standard’s provisions and explanatory information in the Commentary. The online version also offers real-time updates when supplements or errata are issued.

In the print version, vertical lines in the margins indicate changes from 2016. However, the online version explicitly shows the changes, so it is much easier to see what changes were made. Neither the printed nor online version show the Commentary changes.

THOMAS L. SMITH, AIA, RRC, F.SEI, is president of TLSmith Consulting Inc., Rockton, Ill., and a member of the ASCE 7 Task Committee on Wind Loads.

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