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Velocity Pressure Coefficient

Height Factor (Kz)

Wind hits harder at the top of a building than at the bottom. The Kz factor captures this - higher floors need stronger construction.

100 ft - Kz = 1.18
60 ft - Kz = 1.08
40 ft - Kz = 1.02
20 ft - Kz = 0.90
0-15 ft - Kz = 0.85
Height (ft) Exposure B Exposure C Exposure D
0-150.570.851.03
200.620.901.08
300.700.981.16
400.761.041.22
600.851.131.31
1000.991.261.43
0.85
15 ft
Ground
0.98
30 ft
2-Story
1.13
60 ft
5-Story
1.26
100 ft
8-Story

Higher = Faster Wind

Imagine standing in a forest. The trees block the wind and you feel barely a breeze. Now climb to the top of a tall tree - suddenly the wind is much stronger!

Buildings work the same way. Near the ground, trees, houses, and other buildings create friction that slows the wind. As you go higher, there are fewer obstacles, so the wind moves faster.

At 100 feet, wind can be 40% faster than at ground level. Since wind pressure increases with the square of velocity, that means almost double the pressure!

Exposure Changes Everything

The Kz factor depends heavily on what is around your building. Engineers call this the "Exposure Category."

Exposure B: Urban areas with lots of buildings, trees, and obstacles. Wind slows down a lot near the ground.

Exposure C: Open terrain with scattered obstacles. Most suburban areas and the default for Miami-Dade.

Exposure D: Flat, unobstructed areas like beaches and waterfronts. Wind has nothing to slow it down.

A building on the beach (Exposure D) experiences much higher wind pressure than the same building downtown (Exposure B).

B
Urban
0.70
@ 30 ft
C
Suburban
0.98
@ 30 ft
D
Coastal
1.16
@ 30 ft
qz = 0.00256 x Kz x Kzt x Kd x Ke x V2
Kz = 0.98
30 ft, Exposure C
=
qz = 72 psf
at 180 mph

Kz in the Formula

The Kz factor plugs directly into the velocity pressure formula. It multiplies with all the other factors to give you the wind pressure at a specific height.

For a typical Miami-Dade building at 30 feet in Exposure C:

qz = 0.00256 x 0.98 x 1.0 x 0.85 x 1.0 x 1802 = approximately 72 psf

If that same building was 100 feet tall, the top floor would see about 93 psf - nearly 30% higher pressure just from the height difference!

Frequently Asked Questions

What is the Kz factor in wind load calculations? +
Kz is the velocity pressure exposure coefficient that adjusts wind loads for height above ground. It accounts for the fact that wind speed increases with height due to reduced friction from surface obstacles. Kz ranges from about 0.57 near ground level to over 1.5 at very tall heights.
How do I calculate Kz for my building height? +
Kz is calculated using the formula: Kz = 2.01 x (z/zg)^(2/alpha), where z is height, zg is gradient height, and alpha is a terrain exponent. For Exposure C at 30 feet, Kz equals approximately 0.98. Most engineers use tables from ASCE 7-16 Table 26.10-1.
Why does wind pressure increase with building height? +
Near the ground, wind slows down due to friction from trees, buildings, and terrain. As you go higher, there are fewer obstacles to slow the wind. At 500 feet, wind can be 30-50% faster than at ground level, creating significantly higher pressures on tall buildings.
What Kz value should I use for a 30-foot building in Miami-Dade? +
For a 30-foot building in Exposure C (typical suburban/urban), Kz equals approximately 0.98. For Exposure B (urban with obstructions), Kz is about 0.70. For Exposure D (flat coastal), Kz is about 1.16. The exposure category significantly affects the Kz value.

Get Kz Values for Your Building

Our calculator automatically determines Kz based on your building height and exposure category. Accurate wind pressures for every floor.

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