Watch how wind accelerates over terrain features - and why your building on a hill needs stronger design. The Kzt factor explained!
Get accurate ASCE 7-22 wind load calculations including topographic factors for your Broward County project.
Calculate Wind LoadsMost of Broward County is flat coastal terrain, so Kzt = 1.0 for the vast majority of projects. You only need to consider Kzt > 1.0 if building near significant hills, ridges, or escarpments - which are rare in South Florida!
Imagine a river flowing toward a large rock. The water has to go over or around the obstacle, and it speeds up as it squeezes past. Wind does the exact same thing over hills - it compresses and accelerates at the crest.
The maximum speed-up happens at the crest of the hill and extends downwind for a distance. A building right on top of a steep hill might experience 80% faster wind than one on flat ground nearby!
ASCE 7 requires Kzt > 1.0 only when specific conditions are met. The terrain feature must be significant enough to cause measurable speed-up:
Since pressure is proportional to velocity squared, even small speed-ups have big effects:
A 20% wind speed increase (Kzt=1.44) means 44% more pressure on your building!
ASCE 7 categorizes terrain features into:
Kzt is a wind speed-up factor that accounts for increased wind speeds over hills, ridges, and escarpments. When wind flows over elevated terrain features, it accelerates similar to water speeding up over rocks in a stream. Kzt can increase wind pressure by up to 80% (Kzt up to 1.8) at the crest of significant terrain features.
For most of Broward County, Kzt = 1.0 because the terrain is relatively flat. The topographic speed-up factor only applies when buildings are located on or near hills, ridges, or escarpments that meet specific height and slope requirements defined in ASCE 7. Florida's generally flat coastal terrain rarely triggers Kzt greater than 1.0.
Kzt greater than 1.0 applies to: (1) 2D ridges - long elevated features, (2) 2D escarpments - cliff-like edges, and (3) 3D axisymmetric hills - isolated elevated features. The feature must have H/Lh >= 0.2 (where H is height and Lh is horizontal distance to half-height), and the building must be in the upper half of the hill or within a certain distance downwind.
Kzt can range from 1.0 (no increase) to about 1.8 (80% increase in velocity pressure) depending on terrain feature geometry and building location. Since pressure is proportional to velocity squared, even a Kzt of 1.3 (speed-up factor) results in 69% higher velocity pressure than flat terrain.
Kzt is part of the velocity pressure formula: qz = 0.00256 x Kz x Kzt x Kd x Ke x V squared. When Kzt = 1.0 (flat terrain), it has no effect. When Kzt greater than 1.0, it directly multiplies the velocity pressure, significantly increasing wind loads for buildings on elevated terrain.
Our calculator accounts for topographic conditions when needed. Enter your project location and get ASCE 7-22 compliant wind loads for Broward County.
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