Wind Load Design Workflow

Step-by-step ASCE 7-22 wind load procedure: velocity pressure, exposure categories, pressure coefficients, and MWFRS vs C&C.

Wind load design follows a well-defined code procedure, but the parameters interact multiplicatively — a 10% error in exposure category, velocity pressure, or pressure coefficient individually can compound to a 30%+ error in the final design pressure. This page walks through each step of the ASCE 7-22 analytical procedure for buildings, explains the key decisions at each stage, and works through a complete example.

This guide is the procedural companion to the calculator. It is written as an educational guide, not as a wind engineering procedure. For step-by-step worked examples with detailed calculations, see the Wind load worked example.

For the full general verification workflow (units, replication strategy, sensitivity testing, and archiving), see How to verify calculator results.

Before You Start

Gather these parameters before running the calculator:

Step-by-Step Design Process

Step 1 — Determine the basic wind speed. Locate the site on the ASCE 7-22 wind speed map (Figure 26.5-1A for Risk Category I, 26.5-1B for II, 26.5-1C for III-IV). Record the mapped value. If the site lies between contours, interpolate linearly.

Step 2 — Classify exposure. Walk through Section 26.7. Exposure B is the default for most suburban and urban sites. If the site has open fetch in the upwind direction for 1,500 ft or more, it likely qualifies as Exposure C. Exposure D requires flat, unobstructed ground (including water surfaces) for 5,000 ft or 20 times the building height, whichever is greater.

Step 3 — Compute velocity pressure (qh or qz). Per ASCE 7-22 Eq. 26.10-1:

Where:

For rigid buildings, qh (computed at mean roof height h) is typically used for all surfaces, which is conservative for lower portions of the building.

Step 4 — Determine external pressure coefficients (Cp or GCp). For MWFRS using the directional procedure (Chapter 27):

For C&C (Chapter 30), GCp values depend on the effective wind area (EWA) and the zone (1-5 for walls, 1-3 for roofs). Zone corner pressures (Zone 5 for walls, Zone 3 for roofs) are typically 2-3 times higher than field-of-wall or field-of-roof pressures.

Step 5 — Determine internal pressure coefficient (GCpi). Per ASCE 7-22 Table 26.13-1:

The enclosure classification can dominate the net pressure, especially for roof uplift. A partially enclosed building experiences much higher net uplift than an enclosed building with the same external geometry.

Step 6 — Compute design wind pressure (p). For MWFRS (directional procedure, Chapter 27):

Where G = 0.85 for rigid buildings (Section 26.11). Note that internal pressure qi is typically evaluated at mean roof height h for all surfaces, but some cases require qi at the actual height of the opening.

For C&C (Chapter 30):

where GCp values come from the C&C figures (Chapter 30) and are zone-specific.

Step 7 — Apply to structural members. Multiply the design pressure by the tributary area. For MWFRS, pressures are applied to the projected building surface. Combine wind with other loads per the governing load combination standard (ASCE 7-22 Section 2.3 for LRFD, Section 2.4 for ASD).

MWFRS vs Components & Cladding (C&C)

The distinction between MWFRS and C&C is one of the most important concepts in wind load design. Using the wrong procedure can underestimate local pressures by a factor of 2-3.

Aspect MWFRS (Ch. 27) C&C (Ch. 30)
Purpose Design primary structure Design cladding and fasteners
Effective wind area Large (full building face) Small (individual panel, 10-500 ft^2)
Zone variation Minimal (one Cp per surface) Five wall zones, three roof zones
Corner/edge amplification Averaged into global pressures Explicitly amplified (Zone 5 corners, Zone 3 roof edges)
Gust effect G = 0.85 (constant for rigid) Built into GCp values
Typical governing pressure 20-40 psf (net lateral) 50-100 psf (corner zones)

For a typical metal building:

Always run both the MWFRS and C&C procedures. The critical wind direction may differ between the two.

Worked Example

Given: A two-story office building, 60 ft x 90 ft plan, 30 ft mean roof height, flat roof (slope < 10 degrees), enclosed, no topographic effects, Exposure B, Risk Category II. Location: Chicago, IL area. Wind speed V = 115 mph (ASCE 7-22 Fig. 26.5-1B).

Step 1 — Velocity pressure at mean roof height (h = 30 ft):

Step 2 — External pressure coefficients (wind normal to 60-ft wall):

Step 3 — Internal pressure:

Step 4 — Design pressures (MWFRS, G = 0.85):

Step 5 — C&C check (corner zone, wall Zone 5, EWA = 50 ft^2):

Result: MWFRS design uses 26.6 psf net lateral pressure. C&C design for corner wall panels uses 40.0 psf outward. C&C zone pressures control the cladding and fastener design.

Common Pitfalls

  1. Mixing exposure categories with different wind directions. The exposure category applies to the upwind sector for each wind direction. A site may be Exposure B for wind from the north (suburban fetch) but Exposure C for wind from the south (open water). Use the most adverse category for each direction.

  2. Using MWFRS pressures for cladding. MWFRS pressures average local pressure spikes over large areas. A roof panel at the eave corner (C&C Zone 3) can experience 2-3 times the MWFRS roof pressure. Using MWFRS values for cladding design is unconservative and can lead to panel or fastener failures.

  3. Misclassifying enclosure. The difference between enclosed (GCpi = +/- 0.18) and partially enclosed (GCpi = +/- 0.55) can change the net uplift on a flat roof by 30-50%. A single large door left open or a broken window during a storm can change the enclosure classification.

  4. Neglecting Kd. The 0.85 directionality factor provides a 15% reduction in velocity pressure. Omitting it is conservative but wasteful — it results in designing for a wind speed 8% higher than required.

  5. Forgetting the ground elevation factor (Ke). At elevations above 1,000 ft, Ke drops below 1.0, reducing the design velocity pressure. This is particularly relevant for mountain sites. Check ASCE 7-22 Table 26.9-1.

  6. Not checking minimum pressure. ASCE 7-22 Section 27.1.5 requires a minimum MWFRS pressure of 16 psf for the walls and 8 psf for the roof projected onto a vertical plane. This can govern for low-wind-speed regions or short buildings.

Frequently Asked Questions

Why does the wind pressure change with height? Wind speed increases with height above ground because the ground surface creates friction that slows the wind. Exposure B (urban/suburban) has more ground friction, so wind speed increases more slowly with height. Exposure D (flat/water) has minimal friction, so wind speed is higher at all heights. The Kz coefficient captures this profile.

When should I use the simplified method vs the analytical method? ASCE 7-22 provides a simplified directional procedure (Chapter 27, Part 2) for low-rise (h <= 60 ft) enclosed buildings with regular shapes. The simplified procedure uses tabulated pressure values rather than computed Cp values. It is faster but slightly more conservative. Use it for preliminary design; switch to the analytical procedure for final design or for buildings outside the simplified limits.

How do wind loads affect cladding compared to the main structure? Cladding experiences higher local pressures because wind flow separates at building corners and eaves, creating suction zones where pressure is concentrated over small areas. The main structure sees an averaged pressure across the full building face, so local spikes are smoothed out. This is why C&C design pressures are typically 2-3x higher than MWFRS pressures for the same building.

Can I use the wind load calculator for open structures (canopies, carports)? The basic calculator supports enclosed and partially enclosed buildings. Open structures (canopies, freestanding walls, monopitch roofs without walls) require different pressure coefficient sets (ASCE 7-22 Chapter 29 for other structures). The wind pressure calculation (qz) is the same, but the Cp values differ significantly.

Is this guide engineering advice? No. It is an educational description of the wind load procedure. Wind loading determination for a real project must follow the governing standard and should be performed by a qualified engineer.

Run This Calculation

→ Wind Load Calculator — ASCE 7-22 and AS/NZS 1170.2 wind pressures from site parameters, exposure category, and building geometry.

→ Portal Frame Calculator — rafter and column design for portal frames under wind and gravity load combinations.

→ Load Combinations Calculator — combine wind with dead, live, and snow per ASCE 7-22 LRFD and ASD.

Related pages

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

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