ASCE 7-22 · CHAPTER 32 · TORNADO LOADS
The First Codified Tornado Load Provisions in ASCE 7
Chapter 32 is brand new in ASCE 7-22 — a dedicated, mandatory tornado-load procedure for Risk Category III and IV structures inside the tornado-prone region.
THE LOAD CONCEPT
Rotating Flow Plus a Pressure Drop
A tornado wraps a building in a narrow, spinning column of air while the vortex core pulls atmospheric pressure down — two effects ordinary straight-line wind never combines.
The Chapter 32 check captures spin, suction, and the narrow swept path qualitatively — site-specific values come from the ASCE 7-22 tornado provisions and maps.
APPLICABILITY
Who Must Run the Tornado Check
Two conditions must both be true. Risk Category I and II buildings stay on the standard wind provisions only.
Risk Category III
Substantial-hazard occupancies — schools, large assembly, jails, power and water facilities.
RC III · 1,700-YR MRIRisk Category IV
Essential facilities — hospitals, fire and police stations, emergency operations centers, shelters.
RC IV · 3,000-YR MRIThe second trigger: the structure must sit inside the tornado-prone region — the defined map area covering much of the central and eastern United States. Outside that region, Chapter 32 does not apply. The tornado map is separate from the basic wind speed maps, and Chapter 32 is in addition to, never a replacement for, standard Chapter 26–30 wind design.
THE PROCEDURE
How the Tornado Check Works
It parallels the wind procedure: read a tornado speed from a dedicated map, build the tornado load case, then let the greater of tornado or wind govern.
1 · Find the region
Confirm the site falls inside the defined tornado-prone region for RC III or IV.
SCOPE2 · Tornado speed
Read the tornado design speed from the separate tornado hazard map for the building's risk category and MRI.
MAP3 · Tornado load
Build the tornado load case with the chapter's tornado-specific factors for rotation, narrow flow, and pressure drop.
LOAD CASE4 · Compare to wind
Run the tornado load case alongside the standard Chapter 26–30 wind case for the same structure.
COMPARE5 · Greater governs
The larger of the tornado and wind demands sets the design — the structure must satisfy both.
GOVERNINGTORNADO vs STRAIGHT-LINE WIND
Why Tornadoes Get Their Own Chapter
Four behaviors separate a tornado from the boundary-layer wind that Chapter 26 models.
Rotation
Flow spins around a vertical core, so wind direction changes across the footprint rather than blowing one way.
VORTEXPressure drop
The vortex core lowers atmospheric pressure, adding outward suction that ordinary wind does not produce.
APCNarrow path
A tornado strikes a slim swept corridor instead of a wide front, changing how the load is applied to the structure.
SWEPT WIDTHDirectionality
Because the wind can arrive from any bearing within the vortex, tornado directionality is handled differently than straight-line wind.
ANY BEARINGPUT IT TO WORK
Design the Governing Case with Confidence
Run your standard ASCE 7-22 wind design, then layer the Chapter 32 tornado check for essential and substantial-hazard buildings.