The first-ever tornado load provisions in ASCE 7 history. Learn about the new Chapter 32 requirements for Risk Category III and IV buildings in tornado-prone regions.
Calculate Wind LoadsChapter 32 is brand new. For the first time in ASCE 7 history, tornado loads have their own dedicated chapter with specific design requirements.
Who it affects: Risk Category III buildings (schools, assembly) and Risk Category IV buildings (hospitals, emergency facilities) in tornado-prone regions.
Key change: These buildings must now be designed for tornado wind speeds in addition to standard wind loads, using tornado-specific maps and load factors.
Sections 32.1-32.6 | First introduced in ASCE 7-22 | No equivalent in ASCE 7-16 or earlier
Prior to ASCE 7-22, tornado loads were not explicitly addressed in the standard. Engineers either relied on basic wind load provisions (which don't capture tornado behavior), referenced FEMA guidelines, or used ICC 500 for storm shelter design. This left a gap in the design of critical facilities in tornado-prone regions.
The addition of Chapter 32 was driven by several factors:
Chapter 32 does NOT replace Chapter 26-30 wind load requirements. Buildings must still be designed for conventional wind loads. Chapter 32 provisions are in addition to standard wind design for applicable Risk Category III and IV buildings.
ASCE 7-22 Section 32.1 defines the scope of tornado load provisions. They apply when both conditions are met:
Tornado provisions apply to:
Tornado provisions apply to:
Standard residential and commercial buildings (Risk Category I and II) are not required to be designed for tornado loads under ASCE 7-22. They continue to use only Chapter 26-30 wind provisions.
However, voluntary tornado shelter areas within these buildings may be designed per Chapter 32 or ICC 500.
ASCE 7-22 Figure 32.4-1 defines tornado-prone regions through contour maps showing tornado design wind speeds. The highest risk areas correspond to "Tornado Alley" and surrounding regions:
Note: Actual tornado design wind speeds must be determined from ASCE 7-22 Figure 32.4-1. Contact a licensed engineer for site-specific values.
Based on ASCE 7-22 Figure 32.4-1, the following states have significant portions within tornado-prone regions requiring Chapter 32 design:
Chapter 32 establishes tornado design wind speeds that differ from standard basic wind speeds in Chapter 26. Key aspects:
| Parameter | Chapter 26 (Standard Wind) | Chapter 32 (Tornado) |
|---|---|---|
| Speed Basis | 3-second gust at 33 ft | 3-second gust tornado wind speed |
| Map Reference | Figures 26.5-1 through 26.5-2 | Figure 32.4-1 |
| Speed Range | 95-180 mph typical | 60-250 mph (varies by location) |
| Risk Category Dependence | Yes (different maps) | Yes (different contours for III vs IV) |
| Exposure Category | B, C, or D per site | Exposure C assumed per Section 32.5 |
Section 32.5 specifies that Exposure C shall be used for tornado load calculations regardless of actual site conditions. This conservative approach accounts for the fact that tornadoes can level surrounding terrain, eliminating surface roughness effects.
ASCE 7-22 Section 32.5 outlines the tornado load calculation procedure. Key provisions include:
The velocity pressure equation for tornado loads follows the same form as Equation 26.10-1, but uses tornado-specific parameters:
qT = 0.00256 Kz Kzt Ke VT2
Where VT is the tornado design wind speed from Figure 32.4-1.
Tornado loads require consideration of Atmospheric Pressure Change (APC) effects unique to tornado vortices. Section 32.5.3 provides requirements for internal pressure that account for the rapid pressure drop as a tornado passes.
Unlike standard wind, tornadoes create a significant atmospheric pressure drop within their vortex. ASCE 7-22 addresses this through APC provisions that must be combined with wind-induced internal pressures for enclosed buildings.
This is a major difference from standard wind design and can significantly affect roof and wall pressures.
Section 32.6 addresses tornado shelters and safe rooms. Key provisions:
ASCE 7-22 itself does not mandate tornado shelters. However, the following may require them:
For detailed tornado shelter design requirements including debris impact resistance, door specifications, ventilation, and occupancy calculations, refer to ICC 500-2020 Standard for the Design and Construction of Storm Shelters.
For engineers implementing ASCE 7-22 Chapter 32, consider the following workflow:
| Aspect | ASCE 7-16 and Earlier | ASCE 7-22 |
|---|---|---|
| Tornado provisions | None - not addressed | Full Chapter 32 dedicated provisions |
| Tornado design wind speeds | Not provided | Figure 32.4-1 maps |
| Critical facility tornado design | Optional/by jurisdiction | Required for RC III/IV in tornado-prone regions |
| Atmospheric pressure change | Not addressed | Section 32.5.3 provisions |
| Tornado shelter reference | Separate (FEMA, ICC 500) | Integrated with ICC 500 reference |
While ASCE 7-22 provides the technical requirements, state adoption varies:
Chapter 32 is one of several significant changes in ASCE 7-22. Related updates include:
For a comprehensive overview of all ASCE 7-22 wind changes, see our ASCE 7 Standards Guide.
WindLoadCalc.com provides ASCE 7-22 compliant wind load calculations for standard wind design. Enter your location and building parameters to get professional-grade calculations instantly.
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