Understanding the New Tornado Load Requirements in ASCE 7-22: What It Means for Cold-Formed Steel Wall Assemblies

The 2024 International Building Code (IBC) introduces a significant update through its reference to ASCE 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, through the addition of Tornado Loads. This new chapter brings tornado-resistant design into typical design requirements for certain building types – most notably, Risk Category III and IV structures. While this change affects all exterior building components, this article focuses on its implications for Cold-Formed Steel (CFS) assemblies.

Who Is Affected?

Under ASCE 7-22, all Risk Category III and IV buildings must now be designed to resist tornado loads. These categories include structures that:

• Pose a significant risk to human life in the event of failure
• Could cause mass disruption to daily life
• Are essential facilities or contain hazardous materials

Examples include:

• Schools
• Hospitals
• Emergency response centers
• Power plants
• Facilities storing toxic or explosive materials

What Is the Intent?

The goal of these provisions is to ensure that buildings can withstand EF0 to EF2 tornadoes without structural damage. This does not mean the building becomes a storm shelter. FEMA/ICC 500 compliant shelters are still required where applicable. Instead, the building envelope (walls, cladding, etc.) is required to remain intact, allowing the facility to continue functioning after a tornado.

Tornado Loads vs. Typical Wind Loads

Tornadoes are classified as extreme-level events. ASCE 7-22 requires that CFS walls, bracing, and connections be designed to resist these elevated pressures. However, deflection checks are not required for tornado loads, as deflection limits are there to protect finishes, not building operational ability. Because CFS studs are typically designed separately for strength and deflection, and with non-load bearing framing often governs by deflection, the actual material impact may be less than expected.

That being said, the engineering effort increases. Bracing, anchorage, and connection detailing must be carefully evaluated, making the design process more involved than in previous code cycles.

Where Does This Apply?

ASCE 7-22 includes a tornado hazard map in Chapter 32 that encompasses all areas east of the Rocky Mountains. Combined with the minimum tornado wind speed and simplifying a few other parameters, you can see the map below for the area of the United States that may now require tornado loading to be evaluated with any design on Risk Category III/IV structures.

Special Consideration: Windows and Enclosure Classification

Windows must be evaluated as “open” unless they are explicitly designated as impact-resistant glazing. This requires compliance with ASTM E1996 missile impact testing. If not impact-rated, the building need to be evaluated for enclosure with open windows, which may classify the structure as partially enclosed. Partially enclosed buildings traditionally have higher wind pressures on walls and roofs.

This adds another layer of coordination between architects, engineers, and contractors.

A Real-World Example

To illustrate the practical impact, consider a two-story high school in Kansas City, MO, with a 140,000 sq ft footprint:

• The school is Risk Category IV (voluntarily adopted by the district)
• Windows are not impact-rated, making the building partially enclosed
• Typical wind C&C pressure: 49.9 psf
• Tornado C&C pressure: 45.6 psf

In this case, the tornado pressure is lower than the standard wind pressure already being used.  This shows that the impact may be more about engineering coordination than construction cost. In many areas this will become another administrative task, but in other areas the wind pressure may increase causing implication to structure costs.

Final Thoughts

While this article simplifies certain parameters to help readers understand the broader implications of ASCE 7-22, the actual requirements are complex and project-specific.

Recommendations:

• Contractors and architects should be aware of these changes and their potential cost implications.
• Consult with a licensed design professional to evaluate how these provisions affect your specific building and project.