Tornado Loads (ASCE 7-22)

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1. Determining Applicability

1

Risk Category

Ensure the Building Risk Category is set to III or IV. Tornado loads are generally not required for Risk Category I or II structures.

2

Tornado-Prone Region

Select Yes for “Tornado-Prone Region”. You can verify this using ASCE 7-22 Figure 32.1-1 or by checking the ASCE Hazard Tool.

3

Tornado Speed

Input the Tornado Speed ($V_T$). This is distinct from the Basic Wind Speed ($V$) and must be sourced specifically for tornado hazards.

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2. Tornado Loads Required Check

• No: Standard wind loads govern, and no further tornado analysis is needed.
• Yes: You must proceed to calculate the specific tornado pressures.

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3. Calculation Type Selection

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4. Worked Example: Community School (Risk Category III)

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Procedure

1

Obtain Site Data via ASCE Hazard Tool

Navigate to the ASCE Hazard Tool to retrieve the precise wind parameters.

1. Search by Location: Enter “Moore, Oklahoma”.
2. Select Standard: Choose ASCE/SEI 7-22 from the “Standard Version” dropdown.
3. Risk Category: Select III.
4. Select Load Types: Under the “Load Types” menu, ensure both the Wind and Tornado checkboxes are selected.

5. Click the VIEW RESULTS button.
6. Review Location Results: Note the Basic Wind Speed ($V$) is 115 mph.

7. Select Tornado Speed ($V_T$): Click Details next to the Tornado section to view the table. Select the speed corresponding to your building’s Effective Plan Area (footprint).

• Example: For a school with a 40,000 $ft^2$ footprint, the table indicates $V_T$ = 89 mph.

2

Configure Project Defaults

In Calcs.com, navigate to the Project Defaults tab on the sidebar to apply these environmental parameters globally.

• Code: Select International Building Code (IBC) 2024 (references ASCE 7-22).
• Building Risk Category: Select III (Substantial Risk).
• Exposure Category: Select C (Standard for open terrain).
• Basic Wind Speed ($V$): Enter 115 mph.
• Tornado Speed ($V_T$): Enter 89 mph (Based on 40,000 $ft^2$ plan area).

3

Create Calculation

You can add the calculator instantly using the direct link or select it manually.Option A: Quick Start (Direct Link)

Instantly create a new Wind Load calculation by clicking here:Create Wind Load Calculator(This link automatically opens a new project and sheet in Calcs.com.)

Option B: Manual Selection

1. Click Add New Calculation in the sidebar.
2. Select Wind Loads (ASCE 7-22).
3. Once inside the calculator, locate the Roof Type dropdown and select Flat.

4

Verification

Scroll down to the Tornado Load Parameters (ASCE 7-22, Ch 32) section within the calculator.

• The Tornado Loads Required field should read “Yes”.
• Reasoning: $V_T$ (89 mph) $\ge$ 60% of $V$ (which is 69 mph for 115 mph wind speed).
• The calculator determines the Tornado Velocity Pressure ($q_{h,T}$), accounting for the Tornado Pressure Exposure Coefficient ($K_{h,Tor}$).

5

Reviewing Pressures

Navigate to the Summary section on the right side of the calculator and look for the Design Tornado Pressures table.The calculator automatically applies the specific Directionality Factor ($K_{d,T}$) and Internal Pressure Coefficients ($GC_{pi,T}$) required for tornado loading to output the final design values:

• Tornado Pressure Factor for Vertical Winds ($K_{vT}^+$ and $K_{vT}^-$)
• Positive Tornado Pressure ($p_T^+$) in psf
• Negative Tornado Pressure ($p_T^-$) in psf