Strength-based design wind pressures for LRFD methodology
The ASCE 7-16 standard includes comprehensive wind speed maps (Figures 26.5-1A through 26.5-1C) that provide basic design wind speeds for use in structural calculations throughout the United States and its territories. These maps are fundamental to determining wind loads for buildings and structures per the 2018 and 2021 International Building Code (IBC).
The wind speeds shown on ASCE 7-16 maps are 3-second gust speeds at 33 feet (10 meters) above ground in Exposure C (open terrain), based on Risk Category II buildings (standard occupancy). These values represent the basis for all wind load calculations and must be adjusted using the appropriate importance factors for Risk Categories I, III, and IV.
All wind speeds are referenced at 33 feet (10 meters) above ground level. Actual pressure calculations account for height effects using velocity pressure exposure coefficients (Kh or Kz).
Wind speeds represent the fastest 3-second average wind speed, which corresponds to peak gust effects on structures. This differs from 1-minute or hourly average wind speeds.
Maps assume Exposure C terrain (open terrain with scattered obstructions). Sites with Exposure B, D, or transition zones require adjustments via velocity pressure coefficients.
Basic wind speeds are for Risk Category II structures (standard occupancy). Use importance factors Iw to adjust for Risk Categories I (0.87), III (1.15), or IV (1.15).
Ultimate wind loads are the unfactored wind pressures calculated according to ASCE 7 used in LRFD (Load and Resistance Factor Design) structural analysis. Despite the name, ultimate wind loads are identical to nominal wind loads—both use the same ASCE 7 equations and produce the same pressure values.
Ultimate wind loads = Nominal wind loads
The only difference is context:
The term "ultimate" refers to ultimate limit state design philosophy, not to a maximum or factored load. The confusion arises because LRFD applies load factors (1.0W) to these ultimate/nominal loads, but the loads themselves remain unfactored ASCE 7 pressures.
Ultimate wind loads are calculated using the standard ASCE 7 velocity pressure and external/internal pressure coefficients:
p = qh[(GCp) - (GCpi)]
Where:
qh = 0.00256 Kh Kzt Kd Ke V²
| Parameter | Description | Typical Values |
|---|---|---|
| Kh | Velocity pressure exposure coefficient | 0.85-1.03 (Exp C, h=15-40ft) |
| Kzt | Topographic factor | 1.0 (flat terrain) |
| Kd | Wind directionality factor | 0.85 (buildings) |
| Ke | Ground elevation factor (ASCE 7-22) | 1.0 (elevation ≤ 1000 ft) |
| V | Basic wind speed | 115-200 mph |
Office building - Tampa, FL
Risk Category II
Enclosed building
Height: 35 ft
V = 170 mph (ASCE 7-22)
Exposure C
Kzt = 1.0 (flat)
Kd = 0.85
Metal wall panel
Effective area = 15 ft²
Zone 4 (wall)
GCp = ±0.90
Enclosed
GCpi = ±0.18
(ASCE 7 Fig. 26.13-1)
1. Calculate velocity pressure exposure coefficient Kh:
For Exposure C, h = 35 ft:
Kh = 2.01(35/33)^(2/9.5) = 1.01
2. Calculate velocity pressure qh:
qh = 0.00256 × 1.01 × 1.0 × 0.85 × 1.0 × 170²
qh = 0.00256 × 1.01 × 0.85 × 28,900
qh = 63.5 psf
3. Calculate ultimate wind pressure (negative):
p = qh[(GCp) - (GCpi)]
p = 63.5[(-0.90) - (+0.18)]
p = 63.5 × (-1.08)
p = -68.6 psf (suction)
4. Calculate ultimate wind pressure (positive internal):
p = 63.5[(-0.90) - (-0.18)]
p = 63.5 × (-0.72)
p = -45.7 psf (suction)
5. Design for governing case:
Ultimate wind load = -68.6 psf (controls)
This is the unfactored pressure for use with LRFD load combinations (1.0W)
Ultimate wind loads are compared directly to component design pressure (DP) ratings when using LRFD methodology. Unlike ASD which uses 0.6W factored loads, LRFD uses 1.0W loads requiring different DP rating interpretation.
| DP Rating | Ultimate Load Capacity (psf) | ASD Equivalent (psf) | Typical Applications |
|---|---|---|---|
| DP-15 | 15.0 | 9.0 | Very low wind zones only |
| DP-20 | 20.0 | 12.0 | Low wind, sheltered locations |
| DP-25 | 25.0 | 15.0 | Standard residential, V ≤ 115 mph |
| DP-30 | 30.0 | 18.0 | Moderate wind, V = 115-125 mph |
| DP-40 | 40.0 | 24.0 | Higher wind, V = 130-140 mph |
| DP-50 | 50.0 | 30.0 | High wind, V = 145-160 mph |
| DP-60 | 60.0 | 36.0 | Coastal/hurricane, V = 165-175 mph |
| DP-70 | 70.0 | 42.0 | High hurricane zones, V ≥ 180 mph |
| DP-80 | 80.0 | 48.0 | Extreme wind zones, corners/edges |
| DP-100 | 100.0 | 60.0 | HVHZ, extreme exposures |
When using ultimate wind loads with LRFD (1.0W):
Component DP rating ≥ Ultimate wind pressure (p)
Example: p = -68.6 psf → Select DP-70 or higher
If manufacturer provides ASD ratings only:
Convert: DPASD ÷ 0.6 = DPLRFD
Example: DP-40 (ASD) = 40 ÷ 0.6 = 66.7 psf LRFD capacity
Calculate ultimate wind loads per ASCE 7, then apply LRFD load combinations (1.0W) per AISC 360. Design members for required strength using φRn.
Use ultimate wind loads with ACI 318 strength design. Load factors from ASCE 7: 1.2D + 1.0W or 0.9D + 1.0W for uplift.
Apply ultimate loads to NDS LRFD provisions. Use load duration factor CD = 1.6 for wind, format factor KF = 2.16.
Compare ultimate pressure directly to DP rating (LRFD basis). No additional factoring needed if DP rating is LRFD-based.
Design fasteners and connectors using LRFD provisions with ultimate wind loads. Check both strength and serviceability.
Main Wind Force Resisting Systems designed with ultimate loads and LRFD combinations. Check all load cases per ASCE 7.
When ultimate wind loads are calculated, apply these ASCE 7 LRFD combinations:
| Combination | Load Case | When to Use |
|---|---|---|
| 3 | 1.2D + 1.6(Lr or S or R) | Gravity loads (baseline) |
| 4 | 1.2D + 1.0W + L + 0.5(Lr or S or R) | Wind + gravity (typical) |
| 5 | 1.2D + 1.0E + L + 0.2S | Seismic loads |
| 6 | 0.9D + 1.0W | Wind uplift (critical!) |
| 7 | 0.9D + 1.0E | Seismic uplift |
Note: LRFD uses 1.0W (not 0.6W like ASD). Combination 4 governs for downward loads, Combination 6 governs for uplift. Always check both!
WRONG: "Ultimate loads are 1.6× nominal loads"
RIGHT: Ultimate loads ARE nominal loads (same ASCE 7 calculation). Load factors (1.0W) are applied separately in load combinations.
Impact: Over-design, wasted material, incorrect structural analysis
WRONG: Ultimate load = 40 psf, so DP-40 (ASD) is adequate
RIGHT: DP-40 (ASD) = 24 psf LRFD capacity (40 × 0.6). Need DP-50 or higher for 40 psf ultimate load.
Impact: Under-designed components, potential failures
WRONG: Only checking Combination 4 (1.2D + 1.0W)
RIGHT: Must also check Combination 6 (0.9D + 1.0W) for uplift. Often governs for roof and light components.
Impact: Roof uplift failures, inadequate fastening
WRONG: Ultimate pressure = 40 psf, then applying 1.6 safety factor → 64 psf design load
RIGHT: Ultimate pressure = 40 psf is used directly with 1.0W in LRFD combinations. Resistance factors (φ) are applied to capacity, not loads.
Impact: Massive over-design, economic waste
Calculate ultimate wind pressure using ASCE 7 (same as nominal)
Apply LRFD load combinations (1.0W load factor)
Check BOTH Combination 4 (1.2D + 1.0W) AND Combination 6 (0.9D + 1.0W)
Verify component DP ratings are LRFD-based (or convert from ASD)
Use resistance factors (φ) on capacity side, not load factors on loads
Check all wind directions and pressure zones
Verify enclosure classification (enclosed vs. partially enclosed)
Document ultimate wind load calculations in project files
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