What is the most common exposure category used in wind load calculations?

Exposure C (open terrain) is the default and is used when the site does not meet the criteria for Exposure B (suburban/urban) or Exposure D (ocean/shoreline). Many suburban sites that engineers casually classify as Exposure B actually require a more careful assessment — if the suburban terrain does not extend sufficiently far upwind, the site may need to be classified as Exposure C.

What is new in ASCE 7-22 compared to ASCE 7-16 for wind loads?

The most significant change is the introduction of the Ground Elevation Factor Ke in Section 26.9, which reduces design wind pressures for buildings at high altitudes (above sea level) to account for lower air density. Sites at 5,000 feet see approximately a 15–16% reduction in velocity pressure compared to ASCE 7-16 calculations. The wind speed maps were also updated with refined data.

When do you use MWFRS wind loads vs Components and Cladding (C&C) loads?

Use MWFRS loads for designing the primary lateral force resisting system — shear walls, moment frames, diaphragms, and overall building drift. Use C&C loads for designing individual elements that directly receive wind loads — wall studs, roof trusses, glazing, cladding panels, and fastener connections. C&C pressure coefficients are higher because they capture localized peak pressures at building edges and corners.

Which Risk Category should I use for a hospital?

Hospitals are classified as Risk Category IV (essential facilities) under IBC Table 1604.5 and ASCE 7-22 Table 1.5-1. Use the Risk Category IV wind speed map (Figure 26.5-1C), which gives higher design wind speeds than the Risk Category II map. This increases design wind pressures for the building.

How do I determine exposure category when the terrain is different in different wind directions?

ASCE 7-22 requires that the exposure category be assessed for each wind direction considered in the design. A site might be Exposure B for winds from the north (sheltered by a suburban neighborhood) and Exposure C for winds from the south (open agricultural land). Engineers must evaluate upwind terrain for each critical direction and use the appropriate Kz values for each. When in doubt, the more conservative (more severe) exposure should be used.

ASCE 7-22 Chapter 26 Wind Loads: How to Determine Design Wind Speeds and Pressures

A practical explanation of ASCE 7-22 Chapter 26 wind load fundamentals — covering basic wind speed maps, Risk Categories, exposure categories, topographic and directionality factors, velocity pressure formula, MWFRS vs C&C, and key changes from ASCE 7-16.

Introduction: Why Wind Loads Matter

Wind loads govern the design of lateral force resisting systems, exterior cladding, roofing systems, glazing, and signage for most buildings outside high seismic zones. Getting wind loads right — and understanding what the code actually requires — is fundamental to structural engineering practice. ASCE 7-22 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures), published by the American Society of Civil Engineers, is the reference standard adopted by the International Building Code (IBC 2024) for wind design in the United States.

This article walks through Chapter 26 of ASCE 7-22 — the general wind load requirements that apply before you can use any of the wind design procedures in Chapters 27 through 31.

The ASCE 7-22 Wind Load Framework

ASCE 7-22 organizes wind design into a series of chapters:

Chapter 26: General requirements — wind speed, exposure categories, pressure coefficients applicable to all procedures
Chapter 27: Main Wind Force Resisting System (MWFRS) — analytical procedure for enclosed and partially enclosed buildings
Chapter 28: MWFRS — simplified procedure for low-rise buildings (Envelope Procedure)
Chapter 29: Wind loads on building appurtenances and other structures (roof-mounted equipment, parapets, signs)
Chapter 30: Components and Cladding (C&C) — wind loads on individual elements (windows, wall panels, roof cladding)
Chapter 31: Wind tunnel procedure

Chapter 26 provides the fundamental parameters that all of the above procedures depend on. Every wind load calculation begins here.

Section 26.5 — Basic Wind Speed (V)

The basic wind speed V is the 3-second gust wind speed at 33 feet (10 m) above ground in open terrain (Exposure Category C) that has a specified probability of being exceeded in any given year. The term "basic wind speed" may feel misleadingly simple — V is actually a risk-calibrated, site-specific value derived from statistical analysis of historical wind data.

ASCE 7-22 Section 26.5 provides V through wind speed maps (Figure 26.5-1), which are different maps for each Risk Category:

Figure 26.5-1A: Risk Category II (the most commonly used map — standard commercial and residential occupancies)
Figure 26.5-1B: Risk Category III (assembly buildings, schools, power stations)
Figure 26.5-1C: Risk Category IV (essential facilities — hospitals, fire stations, emergency operations centers)
Figure 26.5-1D: Risk Category I (agricultural, minor storage, low-hazard facilities)

The wind speed maps were substantially revised in ASCE 7-10 when ASCE moved from a single wind speed map (with load factors applied) to separate maps for each Risk Category with strength-level wind speeds. This change was maintained in ASCE 7-16 and 7-22. The Risk Category II map for most of the continental United States shows V between 85 and 130 mph, with higher values in hurricane-prone coastal regions (Southeast coast, Gulf Coast) exceeding 180 mph.

For locations not in the contiguous United States, Section 26.5.2 provides requirements for special wind regions and Hawaii. The International Building Code also permits use of regional wind speed records when approved by the authority having jurisdiction.

Section 26.6 — Wind Directionality Factor (Kd)

The wind directionality factor Kd accounts for the fact that the maximum wind-induced pressure on a building rarely comes from the direction that simultaneously creates the maximum pressure coefficient. The values are tabulated in ASCE 7-22 Table 26.6-1:

Buildings (MWFRS): Kd = 0.85
Buildings (C&C): Kd = 0.85
Arched roofs: Kd = 0.85
Chimneys, tanks, and similar structures (circular): Kd = 0.95
Solid signs: Kd = 0.85
Open signs and lattice frameworks: Kd = 0.85
Trussed towers (equilateral triangle): Kd = 0.85
Trussed towers (square): Kd = 0.85

Kd is applied in the velocity pressure equation as a multiplier, not as a separate load factor. It should not be confused with a factor of safety.

Section 26.7 — Exposure Categories

The exposure category describes the surface roughness of the terrain upwind of the site. It significantly affects wind pressure because buildings in open terrain experience higher wind speeds than buildings sheltered by urban surroundings at the same site wind speed V.

ASCE 7-22 defines three exposure categories:

Exposure B: Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. This is the most sheltered exposure and produces the lowest wind pressures. Applies to sites where Exposure B terrain extends at least 1,500 feet (or 20 times the building height, whichever is greater) upwind in the wind direction considered.
Exposure C: Open terrain with scattered obstructions having heights generally less than 30 feet (e.g., flat open country, grasslands, shoreline areas not classified as Exposure D). This is the default exposure for sites that don't qualify for B or D. Open flat terrain produces higher wind speeds at lower heights than Exposure B.
Exposure D: Flat, unobstructed areas exposed to wind flowing over large bodies of water (ocean, bay, lake). This is the most severe exposure and applies within 1,500 feet of the shoreline or within 600 feet of the mean high water mark, whichever is greater. Exposure D generates the highest wind pressures at low building heights.

Determining exposure category is one of the most judgment-dependent steps in wind load calculation. The exposure category can change based on wind direction — the engineer must assess upwind terrain for all critical wind directions. When terrain is mixed, conservative judgment toward a more severe exposure is appropriate.

Section 26.8 — Topographic Factor (Kzt)

Wind speed accelerates over isolated hills, ridges, and escarpments. The topographic factor Kzt accounts for this speed-up effect when a building is located on or near such topographic features.

Kzt is calculated from the formula:

Kzt = (1 + K₁K₂K₃)²

Where K₁, K₂, and K₃ are coefficients from Figure 26.8-1 that depend on the type of topographic feature (2D ridge, 2D escarpment, or 3D axisymmetric hill), the height and half-width of the feature, the location of the building relative to the crest, and the height of the building above the ground.

For sites on flat terrain with no nearby hills or escarpments, Kzt = 1.0 (no topographic speed-up). For sites on exposed hillcrests, Kzt can reach 1.5 or higher, significantly increasing design wind pressures.

Section 26.9 — Ground Elevation Factor (Ke)

ASCE 7-22 introduced the ground elevation factor Ke in the 2022 edition (it did not exist in ASCE 7-16). This factor accounts for the fact that air density decreases with elevation above sea level, reducing wind pressure at high-altitude sites.

Ke is determined from Table 26.9-1 as a function of ground elevation (feet or meters above sea level). At sea level Ke = 1.0. At 5,000 feet elevation Ke ≈ 0.85, and at 10,000 feet Ke ≈ 0.73. For projects in mile-high Denver or high-altitude mountain sites, Ke provides a meaningful reduction in calculated wind pressures — this is one of the substantive technical changes between ASCE 7-16 and 7-22.

Section 26.10 — Velocity Pressure Exposure Coefficient (Kz and Kh)

The velocity pressure exposure coefficient Kz accounts for the variation of wind speed with height above ground and with exposure category. Wind speed increases with height and increases more rapidly in open exposures than in sheltered exposures.

Kz values are tabulated in ASCE 7-22 Table 26.10-1 for heights from 0–15 feet up to 500 feet for each exposure category. Key observations:

At 33 feet (10 m) in Exposure C, Kz = 1.0 by definition (this is the reference condition for wind speed maps)
At 33 feet in Exposure B, Kz = 0.70 (lower wind speeds in sheltered terrain)
At 33 feet in Exposure D, Kz = 1.03 (slightly higher than C at this reference height)
At 100 feet in Exposure C, Kz ≈ 1.20
At 200 feet in Exposure B, Kz ≈ 1.20 (sheltered terrain "catches up" at greater heights)

For the mean roof height, use Kh (same table, evaluated at height h).

The Velocity Pressure Formula

ASCE 7-22 Section 26.10.2 gives the velocity pressure at height z:

qz = 0.00256 × Kz × Kzt × Ke × Kd × V²

Where:

qz = velocity pressure at height z (psf)
Kz = velocity pressure exposure coefficient at height z
Kzt = topographic factor
Ke = ground elevation factor (NEW in ASCE 7-22)
Kd = wind directionality factor
V = basic wind speed (mph)
0.00256 = constant derived from air density at sea level and unit conversions

This velocity pressure (psf) is then multiplied by pressure coefficients (Cp, Cpi, GCp, GCpf) from subsequent chapters to determine actual design pressures on the building surfaces.

MWFRS vs Components and Cladding (C&C)

A critical distinction in ASCE 7 wind design is between loads on the Main Wind Force Resisting System and loads on Components and Cladding.

Main Wind Force Resisting System (MWFRS): The structural system that resists overall wind forces on the building — moment frames, shear walls, diaphragms, lateral bracing. MWFRS loads represent wind loads integrated over large tributary areas. Pressure coefficients for MWFRS are lower than for C&C because large areas average out peak local pressures.

Components and Cladding (C&C): Individual structural elements (wall studs, roof trusses, purlins, glazing, roof membrane anchorage, metal wall panels) that receive wind loads directly and transfer them to the MWFRS. C&C design uses higher pressure coefficients to account for local pressure peaks (particularly at corners, edges, and eaves where flow separation creates intense suction) that would be averaged away in MWFRS calculations.

The same building requires both analyses. Use MWFRS loads for sizing lateral systems; use C&C loads for sizing cladding elements, connections, and any element whose tributary area is small enough to experience peak local pressures.

Key Changes from ASCE 7-16 to ASCE 7-22

Ground Elevation Factor Ke (Section 26.9): The most significant procedural change. Ke provides wind pressure reductions for high-altitude sites. Sites at 5,000+ feet elevation see meaningful reductions in design wind pressure compared to 7-16 calculations.
Updated wind speed maps: The 7-22 wind speed maps incorporate additional historical wind data and refined hurricane modeling, resulting in some changes to design wind speeds in specific regions, particularly in the Gulf Coast and portions of the Northeast.
Enclosed simple diaphragm building procedures: Refinements to the simplified analytical procedure in Chapter 27 Part 2.
Topographic factor updates: Minor refinements to Kzt calculation parameters.

Practical Example: Velocity Pressure Calculation

A five-story office building (65 feet to mean roof height) in Denver, Colorado (ground elevation approximately 5,280 feet):

Risk Category II → Use Figure 26.5-1A → V = 115 mph (from map)
Terrain: suburban → Exposure Category B
No nearby hills → Kzt = 1.0
Ground elevation 5,280 ft → Ke = 0.84 (Table 26.9-1)
Building (MWFRS) → Kd = 0.85 (Table 26.6-1)
At mean roof height 65 ft, Exposure B → Kh = 0.85 (Table 26.10-1)
qh = 0.00256 × 0.85 × 1.0 × 0.84 × 0.85 × (115)² = 0.00256 × 0.85 × 0.84 × 0.85 × 13,225 = 20.6 psf

Without Ke (as in ASCE 7-16): qh = 0.00256 × 0.85 × 1.0 × 0.85 × (115)² = 24.5 psf. The Ke factor reduces velocity pressure by approximately 16% for this Denver site.

Conclusion

ASCE 7-22 Chapter 26 provides the foundation for all wind load calculations in U.S. practice. The basic wind speed, exposure category, and velocity pressure equation determine the magnitude of wind forces before any shape-specific pressure coefficients are applied. The addition of Ke in 7-22 is the most important technical change for high-altitude projects. Master these fundamentals and the subsequent MWFRS and C&C procedures become straightforward applications of the velocity pressure baseline.

🌬️ ASCE 7-22 Chapter 26 Wind Loads: How to Determine Design Wind Speeds and Pressures