Nominal Wind Loads Explained - ASD Design Wind Pressures

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What are Nominal Wind Loads?

Nominal wind loads are wind pressures calculated using ASCE 7 equations and applied in Allowable Stress Design (ASD) methodology. The term "nominal" indicates these are unfactored, service-level pressures that will be used with the 0.6W load factor in ASD load combinations.

Nominal wind loads represent the actual wind pressure on building components and are the basis for architectural component ratings (Design Pressure or DP values) used throughout the fenestration and cladding industry.

ASCE 7 Calculation

Nominal loads use standard ASCE 7 wind pressure equations: p = q_h[(GC_p) - (GC_pi)] without additional strength factors.

ASD Methodology

Nominal pressures are used exclusively with ASD (Allowable Stress Design) and the 0.6W load factor in structural combinations.

Component Ratings

Windows, doors, skylights, and cladding are rated using nominal pressures. A DP-50 rating means the component withstands 50 psf nominal pressure.

Service Level

Nominal loads represent service-level wind pressures - the actual expected loading before applying safety factors in structural design.

Nominal Wind Load Calculation - ASCE 7

The fundamental equation for nominal wind pressure on components and cladding:

Components & Cladding - Equation 30.4-1

p = q_h [(GC_p) - (GC_pi)]

Where:

p = Nominal design wind pressure (psf)
q_h = Velocity pressure at mean roof height (psf)
GC_p = External pressure coefficient from ASCE 7 Figures 30.4-1 through 30.4-7
GC_pi = Internal pressure coefficient (±0.18 for enclosed buildings)

This calculated pressure (p) is the nominal wind load used for:

Component selection (matching to DP ratings)
ASD structural analysis with 0.6W load combinations
Anchorage and fastening design
Performance testing specifications

Nominal vs Ultimate - Same Calculation

Important: The pressure calculated above is also called "ultimate" when used with LRFD. Same equation, same number, different terminology based on design method:

If using ASD: Call it "nominal" and apply with 0.6W
If using LRFD: Call it "ultimate" and apply with 1.0W
The pressure value itself is identical

Step-by-Step Nominal Wind Load Example

Let's calculate the nominal wind load for a typical window in a residential building.

Project Parameters

Building

Single-family residence
Risk Category II
Enclosed building
Mean roof height: 20 feet

Location

Raleigh, North Carolina
Basic wind speed: 115 mph
Exposure Category: B
Flat terrain (K_zt = 1.0)

Component

Living room window
Zone 4 (field of wall)
Effective area: 15 sq ft
Wall height: 8 feet

Design Method

ASD - Nominal Loads
Components & Cladding
Directional procedure
K_d = 0.85

Step 1: Calculate Velocity Pressure

q_h = 0.00256 K_h K_zt K_d K_e V²

Given values:
K_h = 0.57 (Table 26.10-1, Exposure B, h=20 ft)
K_zt = 1.0 (flat terrain)
K_d = 0.85 (buildings, Table 26.6-1)
K_e = 1.0 (elevation < 3,000 ft)
V = 115 mph (Risk Category II from maps)

Calculation:
q_h = 0.00256 × 0.57 × 1.0 × 0.85 × 1.0 × (115)²
q_h = 0.00256 × 0.57 × 0.85 × 13,225
q_h = 16.5 psf

Step 2: Determine Pressure Coefficients

From ASCE 7 Figure 30.4-1 (Components & Cladding, Walls, Zone 4):

External pressure coefficient:
GC_p = +0.90 (positive pressure - wind pushing on window)
GC_p = -0.90 (negative pressure - suction pulling on window)

Internal pressure coefficient (enclosed):
GC_pi = ±0.18

Step 3: Calculate Nominal Wind Pressure

p = q_h [(GC_p) - (GC_pi)]

Case 1 - Maximum Positive Pressure:
p_pos = 16.5 × [(+0.90) - (-0.18)]
p_pos = 16.5 × 1.08
p_pos = +17.8 psf (inward pressure)

Case 2 - Maximum Negative Pressure:
p_neg = 16.5 × [(-0.90) - (+0.18)]
p_neg = 16.5 × (-1.08)
p_neg = -17.8 psf (outward suction)

Step 4: Select Component Based on Nominal Pressure

Required nominal pressure: 17.8 psf (governing value)

Component selection:
Round up to next standard DP rating
17.8 psf → Select DP-25 rated window minimum
(DP-25 = 25 psf > 17.8 psf required ✓)

Safety margin: (25 - 17.8) / 17.8 = 40% margin

No Further Factoring Needed

The nominal pressure (17.8 psf) is compared DIRECTLY to the component DP rating. Do NOT multiply by 0.6 - that factor is for structural load combinations, not component selection.

WRONG: 17.8 × 0.6 = 10.7 psf, then select DP-15
RIGHT: Compare 17.8 psf directly to DP rating, select DP-25

Component Design Pressure (DP) Ratings

Architectural components are tested and rated using nominal wind pressures. Understanding DP ratings is essential for proper component selection in ASD design.

Standard DP Rating Levels

DP Rating	Nominal Pressure	Typical Applications	Wind Speed Equivalent (Approx)
DP-15	±15 psf	Low-rise residential, low wind zones	90-100 mph areas
DP-25	±25 psf	Standard residential, light commercial	100-110 mph areas
DP-30	±30 psf	Moderate wind zones, protected locations	110-115 mph areas
DP-40	±40 psf	Higher wind zones, elevated locations	115-125 mph areas
DP-50	±50 psf	Coastal areas, hurricane-prone regions	130-145 mph areas
DP-60	±60 psf	High wind coastal, HVHZ	145-160 mph areas
DP-70 to DP-100	±70-100 psf	Extreme wind zones, high-rise coastal	160+ mph areas, HVHZ

Component Selection Guidelines

Calculate First

Always calculate the actual nominal wind pressure using ASCE 7 before selecting components. Never rely solely on wind speed zone maps.

Round Up

Always round UP to the next standard DP rating. If calculated pressure is 32 psf, select DP-40, NOT DP-30.

Both Directions

DP ratings apply to both positive and negative pressures. A DP-50 window resists ±50 psf (both inward and outward).

Verify Testing

Ensure components are tested per ASTM E1886/E1996 or equivalent standards. Testing validates the DP rating.

Installation Matters

DP rating assumes proper installation per manufacturer specs. Incorrect installation voids the rating.

Large Missile vs Standard

HVHZ and coastal areas may require impact-resistant components with large missile testing beyond standard DP ratings.

Practical DP Rating Selection

Example: Calculated nominal pressure = 37.5 psf

✗ WRONG: Select DP-30 because "it's close enough" → UNSAFE
✗ WRONG: Multiply by 0.6 first (22.5 psf), then select DP-25 → UNSAFE
✓ RIGHT: Compare 37.5 psf to ratings, select DP-40 or higher → SAFE

Never interpolate or reduce DP ratings. Always select the next higher standard rating.

Common Nominal Wind Load Mistakes

1. Applying 0.6 Factor to Component Selection

WRONG: Calculate p = 40 psf, multiply by 0.6 = 24 psf, select DP-25

RIGHT: Calculate p = 40 psf, compare directly to DP ratings, select DP-40

The 0.6 factor is for STRUCTURAL load combinations (D + 0.6W), NOT for component selection

2. Confusing Nominal with Reduced/Factored Loads

WRONG: Thinking "nominal" means reduced or factored down

RIGHT: "Nominal" means the calculated ASCE 7 pressure WITHOUT additional factors - it's the baseline, not a reduction

3. Using Only Positive Pressure

WRONG: Only calculating and checking positive (inward) pressure

RIGHT: Calculate BOTH positive and negative pressures. Negative (suction) often governs for corner/edge zones and anchorage

4. Ignoring Zone Effects

WRONG: Using same DP rating for all windows regardless of location

RIGHT: Corner and edge zones (Zones 4, 5) have higher pressures than interior zones. Calculate separately for each zone

                Nominal Wind Load Checklist
                ✓ Calculate velocity pressure qh using ASCE 7 Equation 26.10-1
✓ Determine correct pressure coefficients for component zone
✓ Calculate both positive AND negative nominal pressures
✓ Compare nominal pressure directly to DP ratings (no 0.6 factor)
✓ Round UP to next standard DP rating
✓ Verify component testing certification (ASTM E1886/E1996)
✓ Check installation requirements match manufacturer specs
✓ For HVHZ: Verify impact resistance if required

            

Nominal Wind Loads