Understanding the Allowable Stress Design method for wind load calculations
Allowable Stress Design (ASD), also called Working Stress Design (WSD), is a traditional structural design methodology where structural members are proportioned so that stresses (forces divided by area) remain below allowable limits under design loads. For wind loads, ASD uses a load factor of 0.6 applied to the wind load (W) in governing load combinations per ASCE 7 Section 2.4.
In ASD, the design philosophy is straightforward: Actual Stress ≤ Allowable Stress. Wind loads calculated using ASD methodology are often called nominal wind loads because they represent service-level pressures without the strength reduction factors used in Load and Resistance Factor Design (LRFD).
ASD compares actual stresses against allowable stresses based on material yield strength divided by a safety factor. Structural members must satisfy: σactual ≤ σallowable
Wind loads in ASD are factored by 0.6 in governing load combinations. The 0.6W factor represents the probability of maximum wind occurring simultaneously with other loads.
ASD uses allowable stresses defined as Fy/FS where Fy is yield strength and FS is factor of safety (typically 1.5-2.0 depending on material and application).
ASD is widely used for metal building design, post-frame construction, residential structures, and component-level analysis including windows, doors, and cladding systems.
| Load Combination | Formula | When It Governs |
|---|---|---|
| Combination 5 | D + L + 0.6W | Structures with significant dead and live loads where wind uplift is not critical |
| Combination 6 | D + 0.6W + 0.75(L + S) | Structures with roof snow load in combination with wind pressure |
| Combination 7 | D + 0.6W + 0.75(L + R) | Structures where roof live load and wind act simultaneously |
| Combination 8 | 0.6D + 0.6W | Wind uplift cases - governs for lightweight structures, canopies, overhangs, and roof components |
Wind pressures for ASD are calculated using the same fundamental ASCE 7 equations as LRFD, but the resulting pressures are nominal (unfactored) values. These nominal pressures are then used with the 0.6W load factor in structural analysis.
p = qh [(GCp) - (GCpi)]
Where:
qh = 0.00256 Kh Kzt Kd Ke V²
Where:
The wind pressures calculated above are identical for both ASD and LRFD. The difference is in how they're used:
To avoid confusion: Some references call ASD pressures "nominal" and LRFD pressures "ultimate," but they're calculated the same way. The load factor (0.6 vs 1.0) makes the difference.
Let's walk through a complete example of calculating ASD wind loads for a typical building component.
Commercial retail building
Risk Category II
Enclosed building
Mean roof height: 25 feet
Charlotte, North Carolina
Basic wind speed (V): 115 mph
Exposure Category: B (suburban)
Flat terrain (Kzt = 1.0)
Wall window
Zone 4 (field of wall)
Effective wind area: 20 sq ft
Components & Cladding analysis
Allowable Stress Design (ASD)
Using 0.6W load factor
Directional procedure (Kd = 0.85)
Step 1: Calculate Velocity Pressure (qh)
qh = 0.00256 Kh Kzt Kd Ke V²
Given:
Kh = 0.70 (Table 26.10-1, Exposure B, h=25 ft)
Kzt = 1.0 (flat terrain)
Kd = 0.85 (Table 26.6-1, buildings)
Ke = 1.0 (elevation < 3,000 ft)
V = 115 mph
Calculation:
qh = 0.00256 × 0.70 × 1.0 × 0.85 × 1.0 × (115)²
qh = 0.00256 × 0.595 × 13,225
qh = 20.1 psf
Step 2: Determine Pressure Coefficients
From ASCE 7 Figure 30.4-1 (Components & Cladding, Wall, Zone 4):
GCp = +0.90 (positive pressure, wind toward wall)
GCp = -0.90 (negative pressure, wind away from wall)
Internal pressure coefficient (enclosed building):
GCpi = ±0.18
Step 3: Calculate Design Pressures
p = qh [(GCp) - (GCpi)]
Case 1 - Maximum Positive Pressure:
ppos = 20.1 × [(+0.90) - (-0.18)]
ppos = 20.1 × 1.08
ppos = +21.7 psf (wind pushing on window)
Case 2 - Maximum Negative Pressure:
pneg = 20.1 × [(-0.90) - (+0.18)]
pneg = 20.1 × (-1.08)
pneg = -21.7 psf (suction pulling on window)
Step 4: Apply to ASD Load Combinations
These nominal pressures are used with 0.6W in load combinations:
Governing ASD combination: D + 0.6W
Design pressure for window = 0.6 × 21.7 psf = 13.0 psf
The window must be rated to withstand allowable stress under 13.0 psf design pressure (accounting for the 0.6 load factor already applied).
Both positive and negative pressures must be checked. For this window:
While ASD and LRFD use the same fundamental wind pressure calculations, they differ significantly in how those pressures are applied in structural design. Understanding these differences is critical for proper component selection and structural analysis.
| Design Aspect | ASD (Allowable Stress Design) | LRFD (Load & Resistance Factor Design) |
|---|---|---|
| Wind Load Factor | 0.6W in load combinations | 1.0W in load combinations |
| Design Philosophy | Actual Stress ≤ Allowable Stress | Factored Load ≤ Factored Resistance |
| Material Strength | Allowable stress = Fy / FS (FS = 1.5-2.0) | Design strength = φ × Fy (φ = 0.75-0.90) |
| Terminology | "Nominal" wind loads or "ASD" pressures | "Ultimate" wind loads or "LRFD" pressures |
| Typical Use Cases | Metal buildings, post-frame, residential, components & cladding | Commercial steel/concrete, high-rise, main structural systems |
| Component Ratings | Products rated in ASD pressures (DP-40, DP-50, etc.) | Must convert component ASD ratings for use in LRFD analysis |
Given: Wind pressure p = 25 psf, Dead load D = 15 psf downward
ASD Uplift Check (Combination 8):
Load = 0.6D + 0.6W
Load = 0.6(-15) + 0.6(-25)
Load = -24.0 psf net uplift
Component must resist 24.0 psf using allowable stress design
LRFD Uplift Check (Combination 4):
Load = 0.9D + 1.0W
Load = 0.9(-15) + 1.0(-25)
Load = -38.5 psf net uplift
Component must resist 38.5 psf using factored resistance (φRn)
The 0.6 factor reflects the low probability that maximum wind will occur simultaneously with other design loads (dead, live, snow). It's NOT a safety reduction - the safety is built into the allowable stresses (which include factors of safety of 1.5-2.0).
By contrast, LRFD uses 1.0W but compensates with resistance factors (φ) on the strength side. Both methods achieve similar overall reliability when properly applied.
ASD remains the preferred or required design method for several building types and industries, despite the increasing adoption of LRFD in commercial construction.
Pre-engineered metal buildings (PEMBs) traditionally use ASD. The Metal Building Manufacturers Association (MBMA) design manuals are based on ASD methodology, and most metal building software uses nominal (ASD) pressures.
Post-frame construction (pole barns, agricultural buildings) almost exclusively uses ASD. The National Frame Building Association (NFBA) standards are ASD-based, making it the industry standard.
Windows, doors, curtain walls, and cladding systems are rated using ASD pressures (Design Pressure or DP ratings). Product testing standards (ASTM E 1886, E1996) use ASD methodology.
Single-family and low-rise residential construction commonly uses ASD, particularly for wood-framed structures. Prescriptive building codes often reference ASD values.
When analyzing existing structures designed with ASD, it's often simpler to continue using ASD for additions and modifications to maintain consistency.
Some building departments, inspectors, and plan reviewers prefer ASD because of familiarity. Check local preferences before selecting design method.
Most architectural components (windows, doors, skylights, curtain wall panels) are tested and rated using Design Pressure (DP) values, which are ASD nominal pressures. Understanding these ratings is critical for proper component selection.
| DP Rating | Nominal Pressure (psf) | Typical Applications |
|---|---|---|
| DP-15 | ±15 psf | Low-rise residential in low wind zones (90-100 mph areas) |
| DP-25 | ±25 psf | Standard residential, low-rise commercial (100-110 mph) |
| DP-30 | ±30 psf | Moderate wind zones, protected building locations |
| DP-40 | ±40 psf | Higher wind zones (115-130 mph), elevated locations |
| DP-50 | ±50 psf | Coastal areas, hurricane zones (130-150 mph) |
| DP-60 to DP-80 | ±60-80 psf | High wind coastal, HVHZ (High Velocity Hurricane Zones) |
Step 1: Calculate ASD nominal wind pressure using ASCE 7 equations
Step 2: Select component with DP rating ≥ calculated pressure
Step 3: Verify component meets required load combinations (D + 0.6W)
Step 4: Check installation requirements and fastening schedules
Example: If calculated pressure = 32.5 psf, select minimum DP-40 rated product (do NOT interpolate or use DP-30, which would be unsafe)
Several common errors occur when applying ASD methodology to wind loads. Avoiding these mistakes ensures safe, code-compliant designs.
WRONG: Calculating wind pressure p = 30 psf, then multiplying by 0.6 before comparing to component DP rating
RIGHT: Comparing the full calculated pressure (30 psf) to the DP rating. The 0.6 factor is for structural load combinations, not component selection.
WRONG: Using ASD wind loads (0.6W) with LRFD resistance factors (φ)
RIGHT: Stay consistent - use ASD loads with allowable stresses OR LRFD loads with factored resistances. Never mix methodologies.
WRONG: Only checking positive (inward) pressure on components
RIGHT: Always check BOTH positive and negative pressures. Negative (suction) pressures often govern for roof components, overhangs, and edge/corner zones.
WRONG: Assuming Exposure C for all sites
RIGHT: Properly determine Exposure B, C, or D based on upwind terrain for all wind directions. Exposure B is common in suburban/residential areas but produces lower pressures than Exposure C.
WRONG: Only checking combinations with full dead load and live load
RIGHT: Combination 8 (0.6D + 0.6W) often governs for lightweight structures, roof components, and uplift conditions. This is the critical check for many components.
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