Data Source and Methodology
Authoritative Source: ASCE/SEI 7-22, Minimum Design Loads and Associated Criteria for Buildings and Other Structures — Tornado Loads (2022). Publisher: American Society of Civil Engineers (ASCE).
Reference reading: ASCE 7-22 standard adds a tornado chapter (ASCE), NIST Design Guide.
Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte. This tool applies a simplified pressure calculation consistent with ASCE wind methodology using the tornado design wind speed. Final design must follow the full standard, including surface-specific coefficients, torsional/surface combinations, and load combinations.
The Formula Explained
For velocity pressure at height z (psf):
\\( q_z \;=\; 0.00256 \; K_z \; K_{zt} \; K_{d,t} \; V_t^{2} \\)
Simplified net design pressure on a surface:
\\( p_{net} \;=\; q_z \; G \; (C_p \; \text{or} \; GC_p) \;-\; q_z \; (GC_{pi}) \\)
Optional resultant force on tributary area A (lb):
\\( F \;=\; \lvert p_{net} \rvert \; \times \; A \\)
Glossary of Variables
- Vt (mph): Tornado design wind speed from ASCE 7-22 maps (3-sec gust).
- Kz (—): Velocity pressure coefficient at height z (exposure and height effects).
- Kzt (—): Topographic factor (≥ 1.0 when applicable).
- Kd_t (—): Directionality factor for tornado loading (often 1.0; confirm in ASCE 7-22).
- G (—): Gust effect factor (≈ 0.85 for rigid structures unless otherwise determined).
- Cp or GCp (—): Exterior pressure coefficient (MWFRS) or net coefficient (C&C) as per ASCE 7-22 tables.
- GCpi (—): Internal pressure coefficient (use both signs to envelope).
- qz (psf): Velocity pressure based on Vt and coefficients.
- p_net (psf): Net design pressure on the surface (positive = pressure; negative = suction).
- A (ft²): Tributary area for force calculation.
- F (lb): Resultant force magnitude on area A.
How It Works: A Step-by-Step Example
Inputs: Vt = 160 mph, Kz = 1.00, Kzt = 1.00, Kd_t = 1.00, G = 0.85, Cp = −0.90 (MWFRS roof zone), |GCpi| = 0.18, A = 50 ft².
- Compute qz: qz = 0.00256 × 1.00 × 1.00 × 1.00 × (160)² = 0.00256 × 25600 = 65.536 psf.
- Exterior term: qz × G × Cp = 65.536 × 0.85 × (−0.90) = −50.176 psf.
- Internal term (±|GCpi|): qz × (±0.18) = ±11.796 psf.
- Envelope net pressures:
- GCpi = +0.18 → p_net = −50.176 − 11.796 = −61.972 psf (suction).
- GCpi = −0.18 → p_net = −50.176 + 11.796 = −38.380 psf (suction).
- Resultant force (max |p_net|): F = 61.972 × 50 = 3,098.6 lb.
This demonstrates using both signs of GCpi to envelope internal pressure effects per the standard approach.
Frequently Asked Questions (FAQ)
Where do I get the tornado design wind speed Vt?
Use the ASCE Hazard Tool (ASCE 7-22 edition) to obtain the tornado design wind speed for your site and risk category.
Does this calculator consider torsional or localized effects?
No. This simplified calculation focuses on planar pressures. Torsional, localized peak suctions, and detailed zoning per ASCE tables are not explicitly included.
Should I use MWFRS or C&C?
Use MWFRS for global system effects and C&C for individual elements like cladding, fasteners, and small components. Select the appropriate coefficient tables from ASCE 7-22.
What Kd_t value should I select?
Many applications of ASCE 7-22 tornado provisions use Kd_t = 1.0 (no reduction). Confirm the correct value in the standard and project specifications.
How should I select Kz?
Compute Kz per ASCE 7-22 based on exposure category and height z. This tool expects you to input the resulting Kz directly.
Can I rely on this tool for final design?
No. This tool aids preliminary estimation. Use the full ASCE 7-22 standard (and governing codes) for final design, detailing, and documentation.