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Properties of Tornado Wind Speed Profiles Used in the Development of the ASCE 7-22 Tornado Provisions

Conventional wind design per ASCE 7 tells designers that wind speeds are lowest near the ground (due to friction) and increase with height at a rate that depends on surface roughness.  However, the new Chapter 32 - Tornado Loads introduced in ASCE 7-22 are constant with height up to 200’, and this paper dives into why these loads depart from the old height correlation model.  Researchers analyzed data from mobile radar profiles of actual tornado events and found that wind speeds peaked much lower in the tornado wind column than expected, with maximum velocity occurring around 160’ above the ground surface.

With how localized most tornadoes are (barring some of the mile-wide monsters), surface roughness also ceases to dominate behavior.  The ASCE 7 Tornado Task Committee established a new velocity pressure exposure coefficient for tornadoes, K.zTor, independent of exposure category.  This coefficient is defined as 1.0 from ground level up to 200’, then tapers down to 0.90 at 328’, and remains constant at 0.90 above that height.  

Key Takeaway:  

New ASCE 7-22 tornado wind design pressures are independent of exposure category and constant up to 200’, giving the greatest increase over conventional wind loads on shorter buildings in lower exposure categories.

Tekla Structures 2026 Service Pack 2.1

Technical Investigation of the May 22, 2011, Tornado in Joplin, Missouri

Released in March 2014, this 490+ page mega-report serves as an extensive post-mortem survey of damage to buildings from the 2011 Joplin tornado.  Particular attention is given to the performance of building envelopes and high-occupancy commercial building load paths.  Investigators even went to the lengths of analyzing tree-fall patterns to try to reconstruct localized pressure spikes caused by building discontinuities on larger structures.

Two of the structures studied to the greatest extent in this report were classic Big Box designs: a Walmart and a Home Depot that both experienced roof failure and subsequent wall collapse.  Walmart lost its entrances first, which then allowed for a dramatic increase in internal pressure, which overwhelmed the weak puddle welds connecting the roof decking to the joists, stripping the roof and allowing the masonry walls to collapse.  Home Depot similarly experienced complete roof stripping, which then allowed the tilt-up concrete wall panels to collapse.

Key Takeaway:

Envelope integrity is absolutely critical for preventing large internal pressure buildups.  Loss of just a few entryways can cause a dramatic jump in internal pressure, which can compromise non-redundant load paths in other portions of the structure.

PROFIS Engineering Diaphragm Design Software Demo

Breaking Down the 2024 IRC Changes: Part 2

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Fault Detection And Diagnostics (FDD) For HVAC Systems And Building Controls

Mass Timber Strategies for High-Performance Affordable Housing

👋 From the Editor

I’m Eric, the engineer behind the StructEd Bulletin.  I dig through stacks of journal articles and software patch notes to find useful information for practicing engineers and keep an eye on the scattered code updates & errata for you. I’m just getting started, so if you find this useful, the best way to support the newsletter is to share it with a colleague or post it on LinkedIn. It helps more than you’d think!

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