Understanding the two fundamental wind load types in ASCE 7
One of the most confusing aspects of ASCE 7 wind load design is the terminology around "nominal" and "ultimate" wind loads. This confusion stems from the fact that the same wind pressure value is called by different names depending on the design method being used.
Key Concept: Nominal and ultimate wind loads are calculated using identical ASCE 7 equations. The wind pressure (p) in psf is the same number. What differs is how that pressure is labeled and applied in structural design.
Both nominal and ultimate wind loads use p = qh[(GCp) - (GCpi)] from ASCE 7. The pressure value is identical.
"Nominal" is the ASD term, "Ultimate" is the LRFD term. Same wind pressure, different terminology based on design philosophy.
ASD uses nominal loads with 0.6W factor. LRFD uses ultimate loads with 1.0W factor. Application differs, not the pressure itself.
Both methods achieve similar overall structural reliability through different combinations of load factors and resistance factors.
| Term | Used With | Meaning | Example |
|---|---|---|---|
| Nominal Wind Load | ASD (Allowable Stress Design) | Wind pressure calculated per ASCE 7, used with 0.6W load factor | p = 30 psf → "nominal 30 psf" |
| Ultimate Wind Load | LRFD (Load & Resistance Factor Design) | Same wind pressure, used with 1.0W load factor | p = 30 psf → "ultimate 30 psf" |
| Service Wind Load | Deflection/drift checks | Unfactored wind pressure for serviceability limits | p = 30 psf → check deflections |
| Factored Wind Load | LRFD after load combinations | After applying 1.0W (or 0.6W for ASD) | 1.0W = 1.0 × 30 psf = 30 psf |
Many engineers incorrectly believe nominal and ultimate are different pressure values. In reality:
Let's calculate wind pressure for a wall window and see how it's used in both ASD and LRFD:
Step 1: Calculate Pressure
qh = 20.5 psf (from ASCE 7 Eq 26.10-1)
GCp = +0.90, GCpi = -0.18
p = 20.5[(0.90)-(-0.18)] = 22.1 psf
Step 2: Label as "Nominal"
Nominal wind load = 22.1 psf
Step 3: Apply ASD Load Factor
Design load = D + 0.6W
Design pressure = 0.6 × 22.1 = 13.3 psf
Step 4: Compare to Allowable
Component must withstand 13.3 psf using
allowable stress design methodology
Step 1: Calculate Pressure
qh = 20.5 psf (from ASCE 7 Eq 26.10-1)
GCp = +0.90, GCpi = -0.18
p = 20.5[(0.90)-(-0.18)] = 22.1 psf
Step 2: Label as "Ultimate"
Ultimate wind load = 22.1 psf
Step 3: Apply LRFD Load Factor
Design load = 1.2D + 1.0W
Design pressure = 1.0 × 22.1 = 22.1 psf
Step 4: Compare to φRn
Component must withstand 22.1 psf using
factored resistance (φRn) methodology
Key Observation: Both methods calculated p = 22.1 psf. ASD calls it "nominal," LRFD calls it "ultimate," but it's the SAME number from the SAME ASCE 7 equation.
The terminology reflects the underlying design philosophy:
| Aspect | Nominal (ASD) | Ultimate (LRFD) |
|---|---|---|
| Philosophy | Working stress - service level loads | Strength design - failure level loads |
| Load Factor | 0.6W (reduced from nominal) | 1.0W (full ultimate load) |
| Safety Location | In allowable stresses (Fy/FS) | In resistance factors (φRn) |
| Interpretation | "Nominal" = not yet factored for combinations | "Ultimate" = at strength limit state |
MYTH: "Ultimate wind loads are 1.6 times nominal wind loads"
REALITY: Nominal and ultimate wind PRESSURES are identical (same ASCE 7 calculation). The 1.6 factor relates to load COMBINATIONS, not base pressures.
MYTH: "There are different ASCE 7 equations for nominal vs ultimate"
REALITY: ASCE 7 Chapter 27-30 equations are the SAME for both. The pressure (p) you calculate is used as-is for both ASD and LRFD.
MYTH: "A DP-40 window is only for ASD, not LRFD"
REALITY: DP ratings are nominal (ASD) pressures. For LRFD use, convert the LRFD load to ASD equivalent (divide by ≈1.6) and compare to DP rating.
MYTH: "If I calculated wind pressures for ASD, I need to recalculate for LRFD"
REALITY: Wind pressures stay the same. You just apply them differently in load combinations and resistance checks.
What's the Same:
What's Different:
Calculate wind pressure using ASCE 7. This value serves as both nominal (for ASD) and ultimate (for LRFD).
Decide whether project requires ASD or LRFD based on material, code, or client requirements.
Use 0.6W with allowable stresses (ASD) OR 1.0W with φRn (LRFD). Don't mix methods.
Clearly state which method used and apply terminology consistently throughout design documents.
Visit WindLoadCalc.com for professional wind load calculation software that automatically provides both nominal (ASD) and ultimate (LRFD) pressures from a single calculation. Switch between design methods instantly.
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