Master the critical differences between C&C and MWFRS wind load calculations. Learn when to use each method, how pressures differ, and why getting this right matters for building safety and code compliance.
Calculate Both C&C and MWFRSC&C (Components & Cladding) = Wind pressures for windows, doors, wall panels, roof panels, and their connections. Higher pressures due to localized effects.
MWFRS (Main Wind Force Resisting System) = Wind forces for structural frame, columns, beams, shear walls, and foundations. Lower pressures averaged over larger areas.
Key Rule: Always calculate BOTH. C&C for envelope design, MWFRS for structural design. They serve different purposes and are not interchangeable.
ASCE 7 requires two separate wind load calculations because different building systems respond differently to wind. The building envelope (exterior components) experiences concentrated localized pressures, while the structural frame responds to overall building forces. Using the wrong pressures can result in inadequate designs or unnecessary over-engineering.
What it covers:
What it covers:
One of the most common questions is: "Why do C&C calculations produce higher pressures than MWFRS?" The answer lies in how wind affects buildings at different scales:
C&C pressures depend on effective wind area - smaller areas experience higher peak pressures because localized gusts have more impact. A 3-foot window experiences higher peak pressure than a 50-foot wall section. MWFRS pressures are averaged over the entire building surface because the structural system responds to overall forces, not localized peaks.
C&C calculations divide the building into zones with different pressure coefficients:
Corner zones (Zone 3) experience the highest pressures due to wind flow separation and vortex shedding.
C&C pressures are based on peak instantaneous pressures that must be resisted by individual components. MWFRS pressures are based on the sustained loads that the overall structure must resist, allowing for load sharing among structural elements.
| Characteristic | C&C | MWFRS |
|---|---|---|
| ASCE 7 Chapter | Chapter 30 | Chapters 27, 28 |
| Pressure Magnitude | Higher (peak localized) | Lower (averaged) |
| Tributary Area | Individual components (1-500 sq ft typical) | Entire building face |
| Zone Dependence | Yes (Interior, Edge, Corner) | No (uniform distribution) |
| Used For | Product selection, connection design | Structural frame design |
| Both + and - Pressures | Yes | Yes |
| Internal Pressure | Included | Included |
• Selecting windows, doors, or storefronts (comparing product DP ratings to required pressures)
• Specifying wall cladding systems (metal panels, EIFS, stucco)
• Designing roof panel fastening patterns
• Determining hurricane shutter requirements
• Designing connections between envelope components and structure
• Specifying skylights, louvers, or other penetrations
• Designing the structural frame (steel, concrete, wood, masonry)
• Sizing lateral force resisting elements (braces, shear walls, moment frames)
• Designing foundations for overturning and sliding
• Checking overall building drift and stability
• Designing diaphragms (roof, floor decks)
• Determining base shear for lateral analysis
Never use MWFRS pressures to select C&C products! MWFRS pressures are lower and will result in under-designed envelope components. A window selected based on MWFRS pressures may fail during a high-wind event, causing envelope breach, water intrusion, and potential structural damage from internal pressure increase.
A critical concept in C&C calculations is effective wind area. This is NOT simply the area of the component being designed. For rectangular products:
Effective Wind Area = Span × Span/3
However, effective wind area need not be less than the actual tributary area. This calculation accounts for the correlation of wind gusts across a surface - larger spans "see" more averaged wind pressures while smaller spans experience more concentrated peaks.
Most engineers use Chapter 30 Part 1 or Part 3 for C&C and Chapter 27 Part 1 for MWFRS, though simplified methods are available for certain building types.
Consider a 3-story commercial building in Miami, FL (175 mph design wind speed, Exposure C):
| Location | C&C Pressure | MWFRS Pressure |
|---|---|---|
| Wall - Interior Zone | ±72 psf | ±48 psf |
| Wall - Corner Zone | ±108 psf | ±48 psf (same) |
| Roof - Interior Zone | -85 psf / +32 psf | -62 psf / +28 psf |
| Roof - Corner Zone | -142 psf / +32 psf | -62 psf / +28 psf |
Notice how C&C corner zone pressures can be nearly 3× higher than MWFRS pressures for the same location. This is why using the wrong pressure type can be dangerous.
The WindLoadCalc.com calculator automatically generates both C&C and MWFRS calculations for your project:
WindLoadCalc.com calculates both Components & Cladding and MWFRS pressures per ASCE 7-22 and ASCE 7-16. Enter your location and building dimensions to get comprehensive wind load calculations for envelope and structural design.
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