MWFRS - Main Wind Force Resisting System

What is MWFRS?

The Main Wind Force Resisting System (MWFRS) is the structural assembly of load-bearing elements that provides support and stability to the overall building or structure. The MWFRS transfers wind loads from the point of application through the structural system down to the foundation. This includes the primary structural frame, bracing systems, shear walls, diaphragms, and connections that work together to resist lateral wind forces.

Unlike Components and Cladding (C&C) which addresses localized pressures on individual building elements, MWFRS design focuses on the overall structural stability and lateral load path of the entire building system. MWFRS pressures are generally lower than C&C pressures because they're averaged over larger tributary areas.

MWFRS vs. Components & Cladding

Understanding the fundamental difference between MWFRS and C&C is critical for proper wind load analysis:

ASCE 7 Calculation Methods for MWFRS

ASCE 7-16 and ASCE 7-22 provide multiple methods for calculating MWFRS wind loads, each with specific applicability criteria:

1. Directional Procedure (Chapter 27)

The most widely used method for buildings of all heights. This procedure accounts for wind directionality and applies different pressure coefficients based on building geometry and wind direction.

2. Envelope Procedure (Chapter 28)

A simplified method that provides wind loads without requiring separate calculations for each wind direction. The envelope procedure generates conservative pressures that envelope the results from all directions.

3. Wind Tunnel Procedure (Chapter 31)

Required for complex buildings, flexible structures, and buildings where ASCE 7 analytical methods don't apply. Wind tunnel testing provides the most accurate results but requires specialized facilities and expertise.

MWFRS Load Components

MWFRS design pressures in ASCE 7 are calculated using the following formula:

Velocity Pressure (qz)

Velocity pressure varies with height and is calculated as:

qz = 0.00256 × Kz × Kzt × Kd × V²

• Kz = Velocity pressure exposure coefficient (varies with height and exposure)
• Kzt = Topographic factor (1.0 for flat terrain, up to 1.5+ for hills/escarpments)
• Kd = Wind directionality factor (0.85 for buildings)
• V = Basic wind speed (mph) from ASCE 7 wind maps

Gust Effect Factor (G)

For rigid buildings (fundamental frequency ≥ 1 Hz), G = 0.85. For flexible buildings, a detailed calculation per ASCE 7 Section 26.11 is required, accounting for dynamic amplification and resonance effects.

External Pressure Coefficients (Cp)

Pressure coefficients depend on building geometry and wind direction. Common values include:

• Windward wall: Cp = +0.8 (positive/inward pressure)
• Leeward wall: Cp = -0.5 to -0.3 (suction)
• Side walls: Cp = -0.7 (suction)
• Roof: Varies significantly based on slope and location (-0.9 to -0.18 for typical roofs)

Structural Systems That Comprise MWFRS

Lateral Force Resisting Systems

The MWFRS includes various structural systems designed to resist lateral wind forces:

• Moment-Resisting Frames: Beam-column connections resist lateral loads through frame bending action
• Braced Frames: Diagonal bracing elements carry lateral loads through axial forces (tension and compression)
• Shear Walls: Solid or perforated walls resist lateral loads through in-plane shear and bending
• Dual Systems: Combination of moment frames and shear walls or braced frames working together
• Diaphragms: Horizontal elements (floor/roof systems) that distribute lateral loads to vertical resisting elements

Load Path Considerations

A complete and continuous load path is essential for MWFRS performance:

1. Roof Level: Wind pressure on roof transfers to roof diaphragm
2. Roof Diaphragm: Distributes loads to shear walls, braced frames, or moment frames
3. Vertical Elements: Shear walls or frames carry loads down through each floor level
4. Floor Diaphragms: Collect wall loads and redistribute to vertical elements
5. Foundation: Transfers all loads to the ground through footings or piles
6. Connections: All connections must be designed to transfer loads without failure

MWFRS Design Considerations

Risk Category and Importance

Buildings are assigned Risk Categories (I, II, III, IV) based on use and occupancy. Higher risk categories have increased wind speed requirements and lower allowable drift limits:

• Risk Category I: Low hazard to human life (agricultural facilities, minor storage)
• Risk Category II: Standard occupancy (most buildings including residential, office, retail)
• Risk Category III: Substantial public assembly or essential facilities (schools, jails, power plants)
• Risk Category IV: Essential facilities (hospitals, fire stations, emergency operations centers)

Serviceability and Drift

Beyond strength requirements, MWFRS must also satisfy serviceability criteria. Lateral drift under wind loads should be limited to prevent:

• Cracking of architectural finishes
• Damage to non-structural components
• Occupant discomfort
• P-delta effects in tall buildings

Typical drift limits range from H/400 to H/600 (where H = building height), but specific project requirements may vary.

Load Combinations

MWFRS must be designed using appropriate load combinations per ASCE 7 Chapter 2. Key combinations involving wind include:

Strength Design (LRFD):

• 1.2D + 1.0W + L + 0.5(Lr or S or R)
• 0.9D + 1.0W

Allowable Stress Design (ASD):

• D + 0.6W
• 0.6D + 0.6W

Where: D = Dead load, W = Wind load, L = Live load, Lr = Roof live load, S = Snow load, R = Rain load

Special Considerations for Different Building Types

Low-Rise Buildings (Mean roof height ≤ 60 ft)

Low-rise buildings can use simplified procedures and often have more prescriptive design options. Wind pressures on low-rise buildings are significantly affected by roof geometry and local pressure zones near corners and edges.

High-Rise Buildings (Mean roof height > 60 ft)

Tall buildings require consideration of:

• Variation of wind pressure with height
• Dynamic effects and potential for wind-induced vibrations
• Along-wind and across-wind forces
• Torsional effects from eccentric loading
• Acceleration limits for occupant comfort

Open and Partially Enclosed Buildings

Buildings with large openings require special analysis for internal pressure. The building enclosure classification affects internal pressure coefficients:

• Enclosed: GCpi = ±0.18
• Partially Enclosed: GCpi = ±0.55 (significantly increases net design pressures)
• Open: Special provisions apply per ASCE 7 Section 26.4

MWFRS Analysis Software

Modern MWFRS wind load calculations are typically performed using specialized software that automates the complex procedures outlined in ASCE 7. Professional wind load calculator software provides:

• Automatic determination of all ASCE 7 parameters
• Multiple analysis methods (Directional, Envelope)
• Pressure diagrams and load application points
• Load combinations per ASCE 7 Chapter 2
• Detailed calculation reports for permit submittal
• Compliance with state-specific code editions

Conclusion

The Main Wind Force Resisting System is fundamental to building safety and structural performance under wind loads. Proper MWFRS design requires thorough understanding of ASCE 7 procedures, structural load paths, lateral force resisting systems, and load combinations. Engineers must carefully distinguish between MWFRS and Components & Cladding design, as each addresses different failure modes and uses different pressure calculations.

Whether designing a simple low-rise building using the Envelope Procedure or a complex high-rise requiring wind tunnel testing, the goal remains the same: ensure complete structural stability and load transfer from the point of wind application to the foundation. Professional wind load calculator software streamlines this process while maintaining accuracy and code compliance.