Open country and grasslands with scattered obstructions
The ASCE 7-16 standard includes comprehensive wind speed maps (Figures 26.5-1A through 26.5-1C) that provide basic design wind speeds for use in structural calculations throughout the United States and its territories. These maps are fundamental to determining wind loads for buildings and structures per the 2018 and 2021 International Building Code (IBC).
The wind speeds shown on ASCE 7-16 maps are 3-second gust speeds at 33 feet (10 meters) above ground in Exposure C (open terrain), based on Risk Category II buildings (standard occupancy). These values represent the basis for all wind load calculations and must be adjusted using the appropriate importance factors for Risk Categories I, III, and IV.
All wind speeds are referenced at 33 feet (10 meters) above ground level. Actual pressure calculations account for height effects using velocity pressure exposure coefficients (Kh or Kz).
Wind speeds represent the fastest 3-second average wind speed, which corresponds to peak gust effects on structures. This differs from 1-minute or hourly average wind speeds.
Maps assume Exposure C terrain (open terrain with scattered obstructions). Sites with Exposure B, D, or transition zones require adjustments via velocity pressure coefficients.
Basic wind speeds are for Risk Category II structures (standard occupancy). Use importance factors Iw to adjust for Risk Categories I (0.87), III (1.15), or IV (1.15).
| Figure Number | Geographic Coverage | Typical Wind Speed Range | Key Features |
|---|---|---|---|
| Figure 26.5-1A | Contiguous United States | 90-170 mph | Covers 48 states, shows hurricane zones, special wind regions, topographic effects |
| Figure 26.5-1B | Alaska | 85-130 mph | Accounts for extreme northern exposures, seasonal effects |
| Figure 26.5-1C | Hawaii & Territories | 105-170 mph | Covers Hawaii, Puerto Rico, Guam, Virgin Islands, American Samoa |
| Figure 26.5-1D | Puerto Rico (Detail) | 145-170 mph | High-resolution map showing variation across Puerto Rico |
Reading ASCE 7-16 wind speed maps correctly is critical for accurate wind load calculations. The maps use contour lines (isopleths) to show areas of equal wind speed, similar to elevation contours on a topographic map.
Identify your project location on the appropriate map (Figure 26.5-1A for most US sites). Use latitude/longitude coordinates or city/county references for precision.
Find the nearest wind speed contour lines above and below your site. Contours are labeled with wind speeds in mph (e.g., 110, 115, 120, 130, 140, 150 mph).
If your site falls between contours, linearly interpolate based on distance. For example, halfway between 120 mph and 130 mph contours = 125 mph.
Verify if your site is in a "Special Wind Region" (marked on maps). These areas require site-specific wind analysis by a qualified engineer.
Maps show general terrain. Local hills, ridges, escarpments, or gorges may require topographic factor (Kzt) adjustments per Section 26.8.
Coastal sites may have higher wind speeds due to hurricane exposure. Ensure you're reading the correct contour in high-wind coastal zones.
For precise wind speed determination without manual map reading, use WindLoadCalc.com's automated wind speed lookup tool. It provides ASCE 7-16 and 7-22 wind speeds by zip code or GPS coordinates, eliminating interpolation errors and saving time.
Exposure C is the most commonly used exposure category in wind load design, representing open terrain with scattered obstructions. It is defined in ASCE 7-22 (and ASCE 7-16) Section 26.7 as the default exposure when site conditions don't clearly meet Exposure B or D criteria.
Exposure C produces moderate wind pressures - higher than Exposure B (suburban) but lower than Exposure D (coastal). It is considered the conservative default for most projects.
Flat, open country with scattered obstructions generally less than 30 ft tall.
ASCE 7 wind speed maps assume Exposure C - use this when in doubt.
30-33% higher wind pressures than Exposure B at typical building heights.
Defined in Section 26.7 of both ASCE 7-22 (latest) and ASCE 7-16.
Exposure C represents open terrain with scattered obstructions having heights generally less than 30 feet. This includes flat, open country and grasslands with few trees or structures.
Exposure C: Open terrain with scattered obstructions having heights generally less than 30 ft (9.1 m). This category includes flat, open country and grasslands.
Key Characteristics:
Exposure C is considered the baseline for wind load calculations. The basic wind speeds shown on ASCE 7 maps represent wind speeds at 33 feet above ground in Exposure C terrain.
Crop fields, pastures, farmland with scattered farm buildings, orchards with low trees, grazing land with fences.
Prairie, meadows, rangeland, open fields with native grasses, scattered shrubs and bushes.
Airport grounds, runways, taxiways, open areas around terminals (away from building clusters).
Isolated houses on large lots (> 2 acres), farmhouses surrounded by fields, widely spaced homes.
Cleared land for future development, open lots, recently cleared construction sites, vacant land.
Isolated warehouses or factories in open areas, widely-spaced industrial buildings, edge of industrial zones.
| Location Type | Exposure | Example Locations |
|---|---|---|
| Farmland | C | Iowa cornfields, Kansas wheat fields, Nebraska farmland |
| Rural house (isolated) | C | Single farmhouse surrounded by fields |
| Airport facility | C | Hangars, terminals in open airport areas |
| New subdivision (cleared) | C | Recently cleared development land without mature landscaping |
| Open grassland | C | Great Plains, prairie, rangeland with scattered shrubs |
| Mature suburban neighborhood | B | NOT Exposure C - numerous closely-spaced houses = Exposure B |
| Coastal shoreline | D | NOT Exposure C - within 600 ft of ocean = Exposure D |
When in Doubt: If you cannot clearly determine whether a site is Exposure B or C, use Exposure C. It's the conservative default and what ASCE 7 wind speed maps assume.
Exposure C uses moderate velocity pressure exposure coefficient (Kh) values - the baseline that ASCE 7 wind speed maps are calibrated to.
| Height (ft) | Exposure B Kz | Exposure C Kz | Exposure D Kz | C vs B | C vs D |
|---|---|---|---|---|---|
| 0-15 | 0.57 | 0.85 | 1.03 | +49% | -17% |
| 20 | 0.62 | 0.90 | 1.08 | +45% | -17% |
| 25 | 0.66 | 0.94 | 1.12 | +42% | -16% |
| 30 | 0.70 | 0.98 | 1.16 | +40% | -16% |
| 40 | 0.76 | 1.04 | 1.22 | +37% | -15% |
| 60 | 0.86 | 1.13 | 1.31 | +31% | -14% |
Critical Comparison: At 15 ft height, Exposure C produces 49% higher wind pressures than Exposure B, but 17% lower than Exposure D. Choosing the wrong exposure has major impacts on component sizing!
Agricultural storage building
Rural farmland (crop fields)
Mean roof height: 25 ft
Risk Category II
V = 115 mph (ASCE 7-22)
Exposure C (farmland)
Kzt = 1.0, Kd = 0.85
Window - Zone 5
Effective area = 10 ft²
GCp = -1.00
Enclosed: GCpi = ±0.18
p = -29.6 psf
Required: DP-30
(vs DP-25 for Exposure B)
1. Kh for Exposure C, h=25 ft:
Kh = 0.94 (from ASCE 7 Table 26.10-1)
2. Velocity pressure qh:
qh = 0.00256 × 0.94 × 1.0 × 0.85 × 1.0 × 115²
qh = 25.1 psf
3. Wind pressure (governing case):
p = qh[(GCp) - (GCpi)]
p = 25.1[(-1.00) - (+0.18)] = -29.6 psf
4. Component selection (ASD with 0.6W):
ASD pressure = 0.6 × 29.6 = 17.8 psf
Select DP-30 (30/0.6 = 50 psf > 29.6 psf ✓)
Comparison - Same building in Exposure B:
Kh = 0.66 → qh = 17.6 psf → p = -20.8 psf
Would only need DP-25 (vs DP-30 for Exposure C)
WRONG: "This subdivision has some open areas, so use Exposure C"
RIGHT: Suburban neighborhoods with closely-spaced houses are Exposure B, even if some lots are vacant. Check ASCE 7 Section 26.7 requirements.
Impact: Over-design by 30-40% (wasted material cost)
WRONG: Rural coastal site → "It's farmland, so Exposure C"
RIGHT: If within 600 ft of ocean or within distance 60× building height, may be Exposure D. Check ASCE 7 Section 26.7.4.
Impact: Significant under-design (17%+ lower pressures than required)
WRONG: "It's a farm, automatically Exposure C"
RIGHT: Farms with dense tree groves, closely-spaced barns/silos, or in wooded valleys may qualify for Exposure B. Evaluate actual terrain roughness.
Impact: Potential over-design if B is appropriate
WRONG: "Future homes will be built nearby, so I'll use Exposure B now"
RIGHT: Determine exposure based on CURRENT terrain conditions. Use Exposure C until future development actually creates Exposure B conditions.
Impact: Under-designed structure for current actual conditions
Verify terrain is open with scattered obstructions < 30 ft tall
Confirm site is NOT in suburban/urban area with closely-spaced houses (if so → Exposure B)
Confirm site is NOT within 600 ft of ocean/large water body (if so → check Exposure D)
Check all wind directions - use most critical exposure if different upwind
Document exposure determination with aerial photos or site survey
When uncertain between B and C, use C (conservative default)
Remember: ASCE 7 wind speed maps ASSUME Exposure C as baseline
WindLoadCalc.com automatically applies correct Kh coefficients for Exposure C open terrain
Calculate Exposure C Loads FREE✓ ASCE 7-22 & 7-16 compliant ✓ All exposure categories ✓ Component DP ratings ✓ Instant results
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