Polk County, IA | 105-115 mph Design Wind Speed | Iowa State Building Code (IBC) | Tornado Alley & Derecho Risk | Exposure B/C
Calculate Des Moines Wind Loads Now →Des Moines, the capital and most populous city of Iowa located in Polk County, has moderate to high wind load requirements that account for its location in Tornado Alley and exposure to severe convective weather systems including derechos. Iowa has adopted the Iowa State Building Code based on the International Building Code (IBC), which Des Moines implements with local amendments. Design wind speeds for Risk Category II structures typically range from 105-115 mph (3-second gust) based on ASCE 7-22 wind speed maps.
These requirements exist because Des Moines experiences frequent severe weather including tornadoes, severe thunderstorms with straight-line winds, devastating derecho events, and hail storms. Located in the heart of Tornado Alley and the Corn Belt, Des Moines is particularly vulnerable to organized derecho systems that produce widespread, catastrophic wind damage across central Iowa. The August 10, 2020 derecho caused over $11 billion in damage across Iowa, with Des Moines experiencing sustained hurricane-force winds and extensive structural damage.
Design Wind Speed (Risk Category II): 105-115 mph (3-second gust, varies by location)
Design Wind Speed (Risk Category III): ~120-130 mph
Design Wind Speed (Risk Category IV): ~130-140 mph
Exposure Category: B (urban areas), C (plains/developing areas)
Building Code: Iowa State Building Code (adopts IBC)
Wind Load Standard: ASCE 7-22 (current edition)
County: Polk County
Weather Risks: Derechos (catastrophic), tornadoes (Tornado Alley), severe thunderstorms, hail, straight-line winds
Des Moines' design wind speed of 105-115 mph for Risk Category II structures reflects the city's location in Tornado Alley and extreme vulnerability to catastrophic derecho events. Des Moines wind speed requirements account for:
The catastrophic August 10, 2020 derecho remains the defining severe weather event for Des Moines building standards. This organized complex of severe thunderstorms traveled from South Dakota to Ohio, producing sustained hurricane-force winds across central Iowa. Des Moines experienced winds of 100-110 mph with gusts exceeding 140 mph in some locations, causing catastrophic structural damage, roof failures, window breakage, and total destruction of tens of thousands of trees. The event demonstrated the critical importance of robust wind load design for derecho-prone regions.
Calculating wind loads for Des Moines projects requires following ASCE 7-22 methodology as adopted by Iowa's State Building Code. The fundamental velocity pressure equation is:
qz = 0.00256 Kz Kzt Kd Ke V²
For Des Moines with V = 110 mph (typical central Des Moines) and standard conditions, the resulting pressures are moderate to high. A Des Moines project with:
Results in a velocity pressure of approximately qz = 22.4 psf—moderate to high compared to most inland regions, reflecting Des Moines' significant derecho and severe weather exposure.
For open plains areas using Exposure C (suburban Des Moines and developing areas), the pressures increase due to the higher velocity pressure coefficient for open terrain exposure typical of Iowa's landscape.
Iowa has adopted the Iowa State Building Code based on the International Building Code (IBC), which Des Moines implements with local amendments for its insurance capital and severe weather environment.
Key Des Moines building code considerations:
You can access Des Moines building permits and requirements through the City of Des Moines Development Services Department.
Des Moines projects must be classified into Risk Categories per ASCE 7-22 Table 1.5-1. Higher risk categories require increased design wind speeds:
| Risk Category | Des Moines Design Wind Speed | Building Types |
|---|---|---|
| Risk Category I | ~100-105 mph | Agricultural facilities, temporary structures, minor storage |
| Risk Category II | 105-115 mph | Residential, commercial, most standard occupancies |
| Risk Category III | ~120-130 mph | Schools, assembly >300, substantial hazardous materials |
| Risk Category IV | ~130-140 mph | Hospitals, fire stations, emergency shelters, EOCs |
Des Moines projects require careful Exposure Category determination based on surrounding terrain and development density:
Exposure Category B (Urban): Most established Des Moines areas qualify as Exposure B due to dense urban development with numerous buildings, trees, and other obstructions. Exposure B assumes urban and suburban areas with buildings having heights generally less than 30 feet extending more than 800 feet upwind. This applies to downtown Des Moines, East Village, Sherman Hill, Beaverdale, and mature suburbs.
Exposure Category C (Plains/Developing Areas): Suburban Des Moines, newer developments, and areas adjacent to Iowa's agricultural plains may qualify as Exposure C. Open terrain with scattered obstructions having heights generally less than 30 feet. Exposure C produces significantly higher wind pressures than Exposure B and is particularly relevant for Des Moines given the surrounding plains landscape.
Engineering Judgment Required: The transition between Exposure B and C in Des Moines' developing suburban areas requires professional engineering judgment. When in doubt, the more conservative Exposure C should be used, especially given Des Moines' derecho vulnerability.
Des Moines' wind load requirements are significantly influenced by the city's extreme vulnerability to catastrophic derecho events and location in Tornado Alley:
August 2020 Derecho - The Defining Event: The August 10, 2020 derecho remains the most catastrophic wind event in modern Iowa history. This organized complex of severe thunderstorms produced sustained hurricane-force winds of 100-140 mph across central Iowa, with Des Moines experiencing widespread catastrophic damage. Over 43 million acres of Iowa crops were damaged or destroyed, with total economic losses exceeding $11 billion. Des Moines saw thousands of structures damaged, extensive roof failures, total tree canopy devastation, and prolonged power outages. This event fundamentally reshaped Iowa's understanding of wind load design requirements and demonstrated that derecho winds can match or exceed design wind speeds for extended periods.
Derecho vs. Tornado Wind Loads: While ASCE 7 wind loads are based on synoptic (large-scale) wind events rather than tornadoes, derechos represent a unique hybrid threat. Unlike tornadoes (highly localized, brief duration), derechos produce widespread, sustained high winds affecting entire metropolitan areas simultaneously. The 2020 derecho maintained hurricane-force winds for hours across hundreds of miles, causing widespread structural failures, roof system collapses, and window breakage across Des Moines. This sustained wind loading represents a critical design consideration for central Iowa structures.
Tornado Alley Location: Beyond derecho risk, Des Moines is situated in Tornado Alley. Polk County averages 2-4 tornadoes annually, with violent EF3+ tornadoes occurring regularly throughout central Iowa. While ASCE 7 doesn't specifically design for tornadoes, the elevated base wind speed accounts for Des Moines' severe weather exposure.
Hail Damage Considerations: Central Iowa ranks among the nation's highest hail-prone regions. Des Moines frequently experiences baseball-sized hail and larger. Roof systems must be designed to resist both wind uplift and hail impact, requiring coordinated design approaches that account for combined loading scenarios.
Des Moines zip codes span Polk County with relatively consistent wind speed requirements. Common Des Moines zip codes include:
The WindLoadCalc.com wind load calculator automatically applies the appropriate Des Moines wind speed based on your specific zip code or street address, accounting for local terrain conditions and exposure.
Wind load calculations for Des Moines building permits have varying PE requirements depending on building type and complexity:
Iowa Professional Engineers must be licensed through the Iowa Engineering and Land Surveying Examining Board.
Engineers, architects, and contractors should reference these official resources for Des Moines wind load compliance:
The wind load calculator at WindLoadCalc.com automatically applies all Des Moines-specific requirements:
WindLoadCalc.com automatically handles all Des Moines-specific requirements including the 105-115 mph design velocity, appropriate Exposure Category selection, Risk Category adjustments, and component pressure coefficients. Simply enter your Des Moines project address or zip code for instant, accurate calculations.
Calculate Des Moines Wind Loads Now →Des Moines' wind load requirements reflect its extreme derecho vulnerability and Tornado Alley location compared to other major Midwest cities:
| City | Design Wind Speed (Risk Cat II) | Primary Weather Risk |
|---|---|---|
| Des Moines, IA | 105-115 mph | Derechos (catastrophic), tornadoes (Tornado Alley), severe thunderstorms |
| Kansas City, MO | 105-115 mph | Tornadoes (Tornado Alley core), severe thunderstorms, derechos |
| Omaha, NE | 105-115 mph | Tornadoes, severe thunderstorms |
| Minneapolis, MN | 100-110 mph | Severe thunderstorms, derechos |
| Chicago, IL | 100-110 mph | Severe thunderstorms, lake-effect winds |
| Cedar Rapids, IA | 105-115 mph | Derechos (2020 event epicenter), tornadoes |
Des Moines' wind load requirements are among the highest in the Midwest, reflecting its extreme vulnerability to catastrophic derecho events and Tornado Alley severe weather frequency.
Des Moines serves as Iowa's state capital and the insurance capital of the United States, with major insurance and financial services companies headquartered in the metro area. This unique economic role has implications for wind load design:
The Des Moines metropolitan area spans multiple jurisdictions in Polk County and surrounding counties, requiring attention to jurisdiction-specific requirements:
Engineers working across the Des Moines metro should verify specific wind speeds, exposure categories, and permit requirements for each project location based on the governing jurisdiction.
Des Moines sits at the heart of Iowa's Corn Belt, surrounded by vast agricultural plains. This landscape has critical implications for wind load design:
Designers should account for Des Moines' position at the interface between urban development and Iowa's agricultural plains when determining exposure categories and wind load requirements.
WindLoadCalc.com provides instant, accurate wind load calculations for Des Moines projects. Our software automatically handles the 105-115 mph velocity range, Exposure B/C determination, and generates PE-ready reports for building permit submission.
Try Des Moines Wind Load Calculator →