Topographic wind speed-up & the Kzt factor
Wind accelerates over hills, ridges and escarpments — raising pressure on structures near the crest. ASCE 7 captures this with the topographic factor Kzt.
Why terrain raises wind load
As wind climbs an isolated feature it compresses and accelerates — fastest on the upwind slope and at the crest. Kzt scales the velocity pressure to match.
Most development sites — no amplification.
Highest at the crest, returning to 1.0 away from the feature.
Kzt = (1 + K₁ K₂ K₃)²
Three multipliers from ASCE 7 Figure 26.8-1 combine into one squared factor. Each captures a different part of the speed-up.
Feature Shape
Accounts for the type of feature — hill, ridge or escarpment — and its steepness.
Distance from Crest
Reduces the effect as the structure sits farther upwind or downwind of the crest.
Height Above Terrain
Reduces the effect with height above the local ground surface — strongest near grade.
When Kzt must be calculated
All four conditions must be met. If any one fails — use Kzt = 1.0.
Isolated
Feature is unobstructed by similar terrain upwind.
Height
Feature height H meets the minimum versus surrounding terrain.
Slope
Upwind slope is significant — below H/Lh = 0.2 the effect is negligible.
Location
Structure sits within the speed-up zone near the crest.
Feature shapes & their Kzt range
ASCE 7 defines the speed-up by feature geometry. A steep escarpment produces the strongest crest amplification; a 3D hill lets wind spill around it.
Flat Terrain
No speed-up. Uses base wind speed straight from the ASCE 7 maps.
Escarpment
Cliff or steep slope between two level areas. Wind accelerates over the edge.
2D Ridge
Elongated hill, uniform cross-section. Speed-up peaks at the crest.
3D Hill
Isolated hill — wind spills around all sides, with amplification strongest near the crest.
Maximum Kzt by feature
Same wind, very different loads — geometry decides how much the pressure climbs.
| Feature | Kzt Range | Speed-Up | Typical Site |
|---|---|---|---|
| Flat terrain | 1.0 | None | Most development sites |
| 2D ridge | 1.0 – 2.04 | Moderate | Long mountain ridge |
| 3D hill | 1.0 – 2.34 | High | Isolated knoll / mound |
| Escarpment | 1.0 – 2.96 | Maximum | Cliff edge, bluff |
ASCE 7 references
The topographic factor sits in Section 26.8 across every current edition, with the geometry in Figure 26.8-1.
Topographic Factor · latest edition
Topographic Factor
Topographic Factor
Geometry & K₁, K₂, K₃ values
Let the calculator find your Kzt
Our sister site WindLoadCalc.com determines the topographic factor from your site geometry — K₁, K₂, K₃ and the full speed-up — inside a signed, sealed calculation report.