Understanding how terrain features amplify wind speeds and increase design pressures per ASCE 7
The topographic factor, Kzt, is a multiplier in ASCE 7 wind load calculations that accounts for wind speed-up effects caused by terrain features such as hills, ridges, and escarpments. When wind flows over these features, it accelerates on the upwind slope and near the crest, resulting in higher wind speeds and increased pressures on structures.
Where:
Hill, ridge, or escarpment is unobstructed by other similar topographic features upwind
The topographic feature height (H) meets minimum criteria relative to surrounding terrain
The upwind slope exceeds 10% (H/Lh > 0.10)
Structure is within the speed-up zone (specific distances from crest)
Kzt = 1.0
Most development sites. No wind speed-up effects. Uses base wind speed from ASCE 7 maps.
Learn More →Kzt: 1.0 to 1.45
Cliff or steep slope separating two level areas. Wind accelerates over the edge.
Learn More →Kzt: 1.0 to 1.55
Elongated hill with uniform cross-section. Maximum speed-up at crest.
Learn More →Kzt: 1.0 to 1.35
Isolated hill with wind flow around all sides. Lower speed-up than 2D ridges.
Learn More →Our wind load calculator automatically determines Kzt based on your site's topography
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