LRFD Wind Loads

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What is LRFD (Load and Resistance Factor Design)?

Load and Resistance Factor Design (LRFD), also called Strength Design, is a modern structural design methodology that uses probabilistic analysis to achieve consistent reliability across different failure modes and materials. For wind loads, LRFD uses a load factor of 1.0 applied to the wind load (W) in governing load combinations per ASCE 7 Section 2.3.

LRFD separates load factors (applied to loads) from resistance factors φ (applied to member strengths). The design philosophy is: Factored Load ≤ Factored Resistance, or more formally: ∑γ_iQ_i ≤ φR_n, where γ represents load factors, Q represents loads, φ is the resistance factor, and R_n is nominal strength.

Design Philosophy

LRFD uses probability-based load and resistance factors to achieve target reliability index (β ≈ 3.0 for 50-year service life), providing consistent safety across structural systems.

Wind Load Factor

Wind loads in LRFD are factored by 1.0 in governing strength combinations. The full calculated wind pressure is used without reduction, with safety on the resistance side via φ factors.

Resistance Factors

LRFD applies resistance factors φ to nominal strengths: φ = 0.90 (tension), 0.90 (flexure steel), 0.75 (compression), 0.65-0.90 (concrete flexure), accounting for uncertainty in material properties.

Common Applications

LRFD is the standard method for commercial steel and concrete construction, high-rise buildings, engineered wood structures, and Main Wind Force Resisting System (MWFRS) design.

LRFD Load Combinations for Wind (ASCE 7 Section 2.3)

Load Combination	Formula	When It Governs
Combination 3	1.2D + 1.6(L_r or S or R) + (L or 0.5W)	Gravity-dominated structures where wind is secondary
Combination 4	1.2D + 1.0W + L + 0.5(L_r or S or R)	Most wind load cases - full wind with moderate gravity loads
Combination 5	1.2D + 1.0E + L + 0.2S	Seismic (not wind, but shown for completeness)
Combination 6	0.9D + 1.0W	Wind uplift - critical for lightweight structures, roofs, overhangs, and negative pressures

Calculating LRFD Wind Pressures

Wind pressures for LRFD use the same ASCE 7 equations as ASD. The key difference is how these pressures are applied in load combinations - LRFD uses the full calculated pressure (1.0W) rather than a reduced value (0.6W), with safety addressed through resistance factors φ.

Main Wind Force Resisting System (MWFRS) - LRFD

ASCE 7 Equation 27.4-1 (Directional Procedure)

p = q K_z K_zt K_d C_p (leeward and windward walls)

Where:

p = Design wind pressure (psf) - ultimate/strength level for LRFD
q = Velocity pressure (psf) calculated per Equation 26.10-1
K_z = Velocity pressure exposure coefficient (varies with height)
K_zt = Topographic factor (≥1.0)
K_d = Wind directionality factor (0.85 for buildings)
C_p = External pressure coefficient (Figure 27.4-1 for enclosed buildings)

Resistance Factors for Wind Load Design

Material/Member Type	Resistance Factor (φ)	Application
Steel - Tension	φ = 0.90	Yielding of gross section, fracture of net section (AISC 360)
Steel - Flexure	φ = 0.90	Lateral-torsional buckling, local buckling, yielding
Steel - Compression	φ = 0.90	Column buckling, local buckling (AISC 360-16)
Concrete - Flexure	φ = 0.65-0.90	Depends on tension vs compression-controlled (ACI 318)
Concrete - Shear	φ = 0.75	Shear and torsion in beams and slabs
Wood - All Limit States	φ = 0.85	Bending, compression, tension, shear (NDS for Wood)

LRFD Philosophy: 1.0W with φR_n

LRFD applies the full wind load (1.0W) to the structure, then ensures safety by reducing the resistance side using φ factors:

Load Side: 1.2D + 1.0W = Maximum expected combined loads
Resistance Side: φR_n = Reduced strength accounting for material variability
Design Check: 1.2D + 1.0W ≤ φR_n (factored load ≤ factored resistance)

This provides consistent reliability across all structural materials and failure modes.

Step-by-Step LRFD Wind Load Example

Let's walk through a complete LRFD design example for a steel beam subjected to wind loads.

Example Project Parameters

Structure

Office building
Risk Category II
Enclosed, h=40 feet
Houston, Texas

Site Conditions

V = 130 mph
Exposure Category C
Flat terrain (K_zt = 1.0)
Elev < 3,000 ft

Member

Steel roof beam W16×26
Span: 25 feet
Tributary width: 12 ft
Roof slope: 4:12 (18.4°)

Design Method

LRFD per AISC 360-16
1.0W load factor
φ = 0.90 (flexure)
Strength design

Step 1: Calculate Velocity Pressure

q_h = 0.00256 K_h K_zt K_d K_e V²

K_h = 0.85 (Table 26.10-1, Exposure C, h=40 ft)
K_zt = 1.0, K_d = 0.85, K_e = 1.0, V = 130 mph

q_h = 0.00256 × 0.85 × 1.0 × 0.85 × 1.0 × (130)²
q_h = 30.9 psf

Step 2: Calculate Wind Pressure

For MWFRS, windward roof (18.4° slope):
C_p = -0.9 (suction, Figure 27.4-1)
GC_pi = ±0.18

p = q_h [(C_p) - (GC_pi)]
p = 30.9 × [(-0.9) - (+0.18)]
p = -33.4 psf (net uplift)

Step 3: Apply LRFD Load Combination

Wind load W = -33.4 psf × 12 ft = -400.8 lb/ft
Dead load D = -25 psf × 12 ft = -300 lb/ft

LRFD Combination 6: 0.9D + 1.0W (uplift)
w_u = 0.9(-300) + 1.0(-400.8)
w_u = -671 lb/ft (factored uplift)

Step 4: Check Beam Strength

Required Strength:
M_u = w_uL²/8 = 671 × (25)² / 8
M_u = 52.5 kip-ft

Design Strength (W16×26, AISC 360-16):
M_n = 118 kip-ft (nominal capacity)
φM_n = 0.90 × 118
φM_n = 106 kip-ft

Check: M_u ≤ φM_n
52.5 kip-ft < 106 kip-ft ✓ OK
Utilization = 52.5/106 = 50%

LRFD Design Process Summary

1. Calculate Loads: Wind pressure p using ASCE 7 equations
2. Apply Load Factors: 1.2D + 1.0W (or 0.9D + 1.0W for uplift)
3. Determine Required Strength: M_u, V_u, P_u from structural analysis
4. Calculate Design Strength: φR_n per material specifications
5. Verify: Required Strength ≤ Design Strength

When to Use LRFD for Wind Loads

LRFD is the modern standard for commercial construction, providing consistent reliability across materials and structural systems.

Commercial Steel

AISC 360 steel design specifications are primarily LRFD-based. Modern commercial steel structures, office buildings, and industrial facilities use LRFD as standard.

Concrete Structures

ACI 318 concrete code uses "Strength Design" (equivalent to LRFD). All modern concrete buildings use factored loads with φ reduction factors.

High-Rise Buildings

Tall buildings and complex structures exclusively use LRFD for consistent reliability analysis across all structural elements and load paths.

MWFRS Design

Main Wind Force Resisting Systems (lateral systems) typically designed using LRFD for frames, shear walls, and braced frames.

Engineered Wood

Modern engineered wood products (glulam, LVL, CLT) increasingly use LRFD per NDS, though ASD remains common for traditional lumber.

Code Preference

Many jurisdictions and design standards now require or strongly prefer LRFD for commercial construction due to probabilistic basis.

Material Design Standards - LRFD Adoption

Material	Design Standard	Primary Method	Status
Steel	AISC 360-16	LRFD	Industry standard since 1986
Concrete	ACI 318-19	Strength Design (LRFD)	Required - ASD not available
Wood	NDS 2018	ASD/LRFD both	Transition ongoing, LRFD growing
Masonry	TMS 402/602	Strength Design/ASD	Both methods available

Common LRFD Wind Load Mistakes

1. Using ASD Component Ratings with LRFD Loads

WRONG: Applying 1.0W LRFD loads directly to DP-rated components (DP ratings are ASD)

RIGHT: Convert LRFD loads to ASD equivalent by dividing by 1.6, OR verify component has LRFD-specific rating

2. Forgetting Combination 6 (0.9D + 1.0W) for Uplift

WRONG: Only checking Combination 4 (1.2D + 1.0W) for all wind cases

RIGHT: ALWAYS check BOTH Combo 4 AND Combo 6. The 0.9D factor often governs for lightweight roofs and uplift

3. Mixing Load Factors from Different Code Editions

WRONG: Using ASCE 7 load factors with outdated material resistance factors from older codes

RIGHT: Ensure material code edition matches IBC/ASCE 7 edition for consistent reliability (e.g., ASCE 7-16 with AISC 360-16)

4. Incorrect Resistance Factor (φ) Selection

WRONG: Using φ = 0.90 for all steel members regardless of limit state

RIGHT: Select φ based on specific failure mode: 0.90 (flexure), 0.90 (tension), 0.90 (compression), 0.75 (shear/bearing)

                LRFD Code Compliance Checklist
                ✓ Calculate wind pressures using ASCE 7 equations (same methodology as ASD)
✓ Apply correct load combinations - check BOTH Combo 4 (1.2D + 1.0W) AND Combo 6 (0.9D + 1.0W)
✓ Use 1.0W load factor (not 0.6W from ASD)
✓ Select appropriate resistance factors φ for each limit state per material code
✓ Verify: Factored Load ≤ φRn for all members
✓ Check serviceability under unfactored loads (deflections, drift)
✓ Ensure material design code edition matches ASCE 7 edition
✓ Document all assumptions and calculations for code official review

            