Building Loading – Tornado Load

(Revised: 05/13/2024)

The Tornado Load tab defines design-related information regarding the building’s Tornado loads.

Only buildings in Risk Category III or IV and in a tornado-prone region require Tornado loadings.

Additionally, Tornado loading only applies to 2023 Florida code and 2024 IBC projects (i.e., codes that require ASCE 7-22).

Tornado loads need NOT be applied when the tornadic wind speed (VT) is less than 60 mph. If the tornadic wind speed is less than a given percentage of the design's ultimate wind speed (percentage varies with wind exposure), as listed in Chapter 26, then tornado loads can be ignored. All these checks are provided as a flowchart in ASCE 7-22, as shown in Figure 32.1-2.

When Tornado loading is applicable, the ‘All Heights’ wind loading method must be used and selected on the Wind Load tab.

Is Project located in a Tornado-prone region?

Select either Yes or No by referring to Figure 32.1-1 in ASCE 7-22 for the map showing where the tornado-prone regions are.

All other input on this screen will be inactive/greyed out until “Yes” is selected.

Effective Plan Area (EPA)

Allows users to input values between 1 and 4,000,000 square feet (whole numbers). Square feet will convert to square meters with metric input.

For Essential Facilities and other structures required to maintain the functionality of Essential Facilities, the Effective Plan Area shall be equal to the area of the smallest convex polygon enclosing both the Essential Facility and all the buildings.

EPA / VT table

The two user input values will allow interpolation of the Tornado Wind Speed VT value. The values shown for the Effective Plan Area should convert to square meters when using metric input. The "mph" speed will convert to "m/z" or "kph" depending on the Region Setting.

The table header "1700-year MRI" should show when the Risk Category is III. The table header should change to "3000-year MRI" when the Risk Category is IV. The user input VT values must be greater than 0.0 and less than 140.0.

Tornado Wind Speed VT

This field will display the interpolated value from the user input values in the table above. The "mph" units will convert to "m/s" or "kph" depending on the Region Setting.

The interpolated value will be greyed out unless the user selects the checkbox to the left, which allows the user to override the interpolated/calculated value. If the user overrides the VT values, it must be greater than 0.0 and less than 140.0.

The tornado speed at any given geographic location will range from approximately Enhanced Fujita Scale EF0 – EF2 intensity, depending on the building's Risk Category and Effective Plan Area.

The tornado speed, VT, used in the determination of tornado loads on buildings shall be determined from ASCE 7-22 Figures 32.5-1 and 32.5-2 as follows:

1. For Risk Category III buildings and structures, use Figures 32.5-1A through 32.5-1H. (Pick the speed from the map with the EPA just lower than the actual EPA and one speed from the map just higher than the actual EPA.)

2. For Risk Category IV buildings and structures, use Figures 32.5-2A through 32.5-2H. (Pick the speed from the map with the EPA just lower than the actual EPA and one speed from the map just higher than the actual EPA.)

Velocity Pressure qhT

This field will display the tornado Velocity Pressure calculation result. The "psf" value will convert to the metric value per the Region Setting. This value cannot be overridden; it is displayed for information only. The value will update as changes are made to the input.

Standard Controls:

§ OK, Cancel, Apply, Help

See also:

§ Building Loading - Building Codes

§ Building Loading - Live Load

§ Building Loading - Wind Load

§ Building Loading - Snow Load

§ Building Loading - Seismic Load

§ Building Loading - Rain Load

§ Building Loading - Deflection Conditions