Iowa Wind Load Overview
The 2020 Derecho: A Wake-Up Call
On August 10, 2020, Iowa experienced one of the most powerful derechos in U.S. history. With wind speeds reaching 140 mph (equivalent to a Category 4 hurricane), the storm caused $11 billion in damage, destroyed 14 million acres of crops, and damaged or destroyed over 10,000 structures. This event highlighted the critical importance of wind-resistant design in Iowa.
Iowa is a voluntary wind load state, meaning PE-sealed wind load calculations are not legally required statewide. However, the devastating 2020 derecho has increased awareness of wind load design importance, and many cities have adopted building codes with wind provisions.
Quick Facts
- State Requirement: Voluntary (not legally required statewide)
- Adopted Code: 2018/2021 IBC with ASCE 7-16 (varies by jurisdiction)
- Basic Wind Speed Range: 105-115 mph (Risk Category II)
- Tornado Risk: High - averages 48 tornadoes annually
- Derecho Risk: Very High - Midwest derecho corridor
- Local Amendments: Des Moines, Cedar Rapids, Iowa City have codes
Derecho Wind Events in Iowa
Iowa sits in the "Midwest derecho corridor" - a region particularly prone to these devastating straight-line wind events. Unlike tornadoes, derechos affect much larger areas and can produce sustained hurricane-force winds across hundreds of miles.
Derecho vs. Tornado Design
While ASCE 7 basic wind speeds are designed for straight-line winds (including derecho-type events), the duration and extent of derecho winds differ from typical design assumptions. The 2020 derecho's 140 mph winds exceeded the ASCE 7-16 Risk Category II design wind speed of 115 mph for most of Iowa.
Historic Iowa Wind Events
| Event | Date | Max Winds | Impact |
|---|---|---|---|
| 2020 Iowa Derecho | August 10, 2020 | 140 mph | $11B damage, 14M acres crops destroyed |
| Parkersburg Tornado | May 25, 2008 | EF5 (205 mph) | 9 fatalities, town devastated |
| 2011 Derecho | July 11, 2011 | 100+ mph | Significant crop and structure damage |
| Mapleton Tornado | April 9, 2011 | EF3 (165 mph) | 60% of town damaged |
Iowa Building Code Framework
Iowa has a voluntary statewide building code, with the state adopting the IBC as a baseline that municipalities can choose to enforce. Larger cities generally have code enforcement, while rural areas often have minimal or no building codes.
Major Jurisdiction Code Adoption
| Jurisdiction | Adopted Code | Wind Standard | Special Requirements |
|---|---|---|---|
| Des Moines | 2018 IBC | ASCE 7-16 | State capital requirements |
| Cedar Rapids | 2018 IBC | ASCE 7-16 | Post-derecho enhanced review |
| Iowa City | 2018 IBC | ASCE 7-16 | University requirements |
| Davenport | 2015 IBC | ASCE 7-10 | Quad Cities coordination |
| Sioux City | 2018 IBC | ASCE 7-16 | Tri-state metro area |
Post-Derecho Code Review
Following the 2020 derecho, several Iowa jurisdictions have begun reviewing their building code requirements. Cedar Rapids in particular has enhanced plan review processes and is considering additional wind-resistant construction requirements for new buildings.
Iowa Wind Speed Zones
Iowa basic wind speeds per ASCE 7-16/7-22 range from 105 mph to 115 mph for Risk Category II structures. Note that these design values were exceeded during the 2020 derecho.
| Region | Risk Cat II | Risk Cat III | Risk Cat IV |
|---|---|---|---|
| Des Moines Metro | 110 mph | 115 mph | 120 mph |
| Cedar Rapids Area | 115 mph | 120 mph | 125 mph |
| Northwest Iowa | 115 mph | 120 mph | 125 mph |
| Southeast Iowa | 105 mph | 110 mph | 115 mph |
ASCE 7 Wind Load Formula
Velocity Pressure Equation
When Wind Load Calculations Are Needed
Although Iowa is a voluntary state, wind load calculations are recommended or required in several situations:
Recommended/Required Situations
- Commercial buildings in Des Moines, Cedar Rapids, Iowa City and other cities
- Risk Category III/IV structures (schools, hospitals, emergency facilities)
- Post-derecho reconstruction - enhanced requirements in affected areas
- Insurance requirements - many insurers now require engineering after 2020
- Agricultural structures - grain elevators, equipment buildings
- Wind turbines - Iowa is a major wind energy producer (#2 in nation)
- Solar installations - ground-mount and rooftop systems
Related State Guides
Explore wind load requirements for neighboring and similar states
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