Exposure Category Selection Guide - How to Determine B, C, or D

Overview of Exposure Selection

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.

Reference Height

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).

3-Second Gust

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.

Exposure C Basis

Maps assume Exposure C terrain (open terrain with scattered obstructions). Sites with Exposure B, D, or transition zones require adjustments via velocity pressure coefficients.

Risk Category II

Basic wind speeds are for Risk Category II structures (standard occupancy). Use importance factors I_w to adjust for Risk Categories I (0.87), III (1.15), or IV (1.15).

Key Wind Speed Map Figures in ASCE 7-16

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

How to Read the Wind Speed Maps

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.

Step-by-Step Map Reading Process

1. Locate Your Site

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.

2. Identify Contour Lines

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).

3. Interpolate If Needed

If your site falls between contours, linearly interpolate based on distance. For example, halfway between 120 mph and 130 mph contours = 125 mph.

4. Check Special Regions

Verify if your site is in a "Special Wind Region" (marked on maps). These areas require site-specific wind analysis by a qualified engineer.

5. Consider Topography

Maps show general terrain. Local hills, ridges, escarpments, or gorges may require topographic factor (K_zt) adjustments per Section 26.8.

6. Coastal Considerations

Coastal sites may have higher wind speeds due to hurricane exposure. Ensure you're reading the correct contour in high-wind coastal zones.

Pro Tip: Online Wind Speed Tools

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.

How to Select the Correct Exposure Category

Determining the correct exposure category is one of the most critical steps in wind load analysis. The exposure category directly affects velocity pressure coefficients (K_h and K_z), which can change design wind pressures by 30-80%. Per ASCE 7-22 Section 26.7 (same as ASCE 7-16), there are four exposure categories: B, C, D, and the rarely-used A.

This guide provides a systematic approach to correctly determine exposure category based on site terrain, distance requirements, and wind direction analysis.

Four Exposures

B (suburban), C (open terrain - default), D (coastal), and A (large city centers - rarely applicable)

Distance-Based

All exposures have specific upwind distance requirements that must be verified

Directional

Exposure can vary by wind direction - check each direction independently

ASCE 7-22 / 7-16

Section 26.7 defines all exposure categories (same in both editions)

Step-by-Step Exposure Selection Process

Follow this systematic process to determine the correct exposure category for your project:

STEP 1: Check for Exposure D (Coastal)

Always check Exposure D first - it has the highest wind pressures and specific distance requirements.

Exposure D applies if:

Site is within 600 feet of ocean, bay, sound, or large lake shoreline, OR
Site is within 60 × building height of shoreline (whichever is GREATER)
AND there is ≥5,000 ft (or 20× building height) of open water upwind

Example: 30 ft tall building, 800 ft from ocean
• 60 × 30 ft = 1,800 ft exposure D distance
• Site is 800 ft from water → Within 1,800 ft → Use Exposure D

If Exposure D applies: Use Exposure D for wind directions from the water. For land-side winds, proceed to Step 2.

If Exposure D does NOT apply: Proceed to Step 2.

STEP 2: Evaluate for Exposure B (Suburban/Urban)

Check if site qualifies for Exposure B - lowest wind pressures, requires numerous closely-spaced obstructions.

Exposure B requires:

Numerous closely-spaced obstructions having the size of single-family dwellings or larger
Examples: suburban neighborhoods, urban areas, wooded areas with dense trees, industrial parks with large buildings
Terrain must extend upwind for distance ≥ 2,600 ft OR 20× building height (whichever is greater)
Exception for low-rise buildings (h ≤ 30 ft): Only requires ≥1,500 ft OR 10× height

Example 1: Single-family house (h=20 ft) in suburban subdivision
• Check 1,500 ft upwind (low-rise exception)
• Numerous houses on all sides → Use Exposure B

Example 2: Farmhouse surrounded by crop fields
• Isolated building, no closely-spaced obstructions
• Does NOT meet Exposure B criteria → Use Exposure C

If Exposure B applies: Use Exposure B.

If Exposure B does NOT apply: Use Exposure C (default).

STEP 3: Default to Exposure C (Open Terrain)

When in doubt, use Exposure C - it's the baseline that ASCE 7 wind speed maps assume.

Exposure C characteristics:

Open terrain with scattered obstructions generally < 30 ft tall
Examples: farmland, grasslands, airports, rural areas, recently cleared land
This is the safe default - moderately conservative for most sites
ASCE 7 wind speed maps are calibrated to Exposure C at 33 ft height

Key Point: If site doesn't clearly meet Exposure B or D criteria, use Exposure C. It provides appropriate wind loads for most open/rural sites.

Exposure A - Rarely Applicable

Exposure A is defined for large city centers with ≥50% buildings over 70 ft tall. In practice, Exposure A is almost never used in the United States because:

Very few areas meet the strict criteria (dense high-rises like Manhattan cores)
Most urban areas qualify as Exposure B, not A
ASCE 7-22 maintains Exposure A but expects minimal application

Recommendation: Unless in a major downtown core with numerous tall buildings, do not use Exposure A. Use Exposure B or C instead.

Exposure Category Quick Reference

This table summarizes the key characteristics and requirements for each exposure category:

Exposure	Terrain Description	Upwind Distance Required	Relative Pressure
D	Flat, unobstructed coastal areas; wind over open water	Inland: 600 ft or 60h Water: 5,000 ft or 20h	Highest
C	Open terrain with scattered obstructions < 30 ft tall	Default - no specific requirement	Moderate
B	Urban/suburban with numerous closely-spaced obstructions (house-sized or larger)	2,600 ft or 20h (1,500 ft or 10h if h≤30 ft)	Lowest
A	Large city centers, ≥50% of buildings > 70 ft tall	Entire upwind area must meet criteria	Rarely Used

Note: h = mean roof height of building being designed

Directional Exposure Analysis

Exposure category can vary by wind direction. You must evaluate terrain conditions for each cardinal direction (N, NE, E, SE, S, SW, W, NW) independently.

Why Direction Matters

Terrain upwind of the building determines the exposure. A coastal site may be Exposure D for ocean winds but Exposure C for land winds.

Evaluate Each Direction

Check terrain roughness in each of 8 cardinal directions. Different exposures may apply to different wind directions.

Conservative Approach

Common practice: use the most critical (highest pressure) exposure for ALL directions to simplify analysis.

Rigorous Approach

For economical design, calculate wind loads separately for each direction using the correct exposure for that direction.

Example: Mixed Exposure Site

Scenario: Hotel 1,000 ft from Atlantic Ocean shoreline (building height = 35 ft)

Directional Analysis:

Wind Direction	Upwind Terrain	Check Exposure D?	Exposure
East (from ocean)	Open Atlantic Ocean	60×35 ft = 2,100 ft; Site is 1,000 ft → Within D zone	D
West (from land)	Developed beach town with buildings/trees	No ocean upwind	B or C
North/South	Coastal development parallel to shore	Ocean is perpendicular, not upwind	C

Design Approaches:

Conservative: Use Exposure D for ALL wind directions (simple, safe, but potentially over-design for land-side winds)
Economical: Use Exposure D for E/SE/NE winds, Exposure C for W/SW/NW winds, Exposure B/C for N/S winds (more complex analysis)

Exposure Selection Decision Tree

Follow this flowchart logic to systematically determine exposure category:

                        START: Identify project site and building height
                    
↓

                        Q1: Within 600 ft OR 60h of ocean/large lake?

                        YES → Check if ≥5,000 ft or 20h of open water upwind

                        → If YES: EXPOSURE D

                        → If NO: Continue to Q2

                        NO → Continue to Q2
                    
↓

                        Q2: Numerous closely-spaced obstructions (house-sized+)?

                        YES → Check if ≥2,600 ft or 20h upwind (or ≥1,500 ft/10h if h≤30 ft)

                        → If YES: EXPOSURE B

                        → If NO: Continue to Q3

                        NO → Continue to Q3
                    
↓

                        DEFAULT: EXPOSURE C

                        Open terrain with scattered obstructions

Common Exposure Selection Mistakes

Mistake #1: Not Checking the 60× or 20× Height Multiplier

WRONG: "My building is 700 ft from ocean, Exposure D only applies within 600 ft, so use Exposure C"

RIGHT: For a 30 ft tall building, check 60×30 = 1,800 ft. Site at 700 ft IS within Exposure D zone!

Impact: 17-21% under-design of wind loads

Mistake #2: Assuming All Suburban Sites are Exposure B

WRONG: "It's in the suburbs, automatically Exposure B"

RIGHT: Must have numerous closely-spaced obstructions extending ≥2,600 ft upwind. New developments or edge-of-town sites may be Exposure C.

Impact: 30-40% under-design if Exposure C is correct

Mistake #3: Not Evaluating Each Wind Direction

WRONG: "The site is Exposure C" (without specifying which wind direction)

RIGHT: Evaluate terrain upwind for each major wind direction. Coastal sites especially may have different exposures by direction.

Impact: Non-conservative design for critical wind directions

Mistake #4: Using Exposure A Incorrectly

WRONG: "This is in a city with tall buildings, use Exposure A"

RIGHT: Exposure A requires ≥50% of buildings to be >70 ft tall - extremely rare. Most urban areas are Exposure B, not A.

Impact: Under-design (Exposure A has lower pressures than B or C at low heights)

Exposure Selection Best Practices

Always check Exposure D first for any site within ~1 mile of large water bodies

Use aerial imagery or site visits to verify actual terrain conditions

Measure upwind distances carefully - use 60h or 20h multipliers, not just minimums

Evaluate exposure for each cardinal wind direction independently

When in doubt, use Exposure C - it's the safe, conservative default

Document exposure determination with photos, maps, and distance calculations

For complex sites, consider using the most critical exposure for all directions (conservative simplification)

Exposure Category Built Into Wind Load Calculations

WindLoadCalc.com applies the correct K_h coefficients for your selected exposure category

Calculate Wind Loads FREE

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