[TowerTalk] Wind Speed by Location

[JVarney]

Gary K9RX wrote:

>> is the ...tower manufacturers ratings based on
>> that value at 30' knowing that the height
>> adjusted wind will be higher?

With the TIA-222-G the wind force on a tower
increases with height, but it's not as severe as
you suggest.

The wind pressure on a tower at height z above
ground is

qz = 0.00256 Kz Kzt Kd (V**2) I    lbs/sq ft

where

Kz = height coefficient
Kzt = topographic factor
Kd = wind direction probability factor
I = importance category
V = basic wind speed

Kz, for typical suburban areas with trees, is 0.7 at
or near the ground.  As you go higher it increases and
hits 1.0 roughly around 100 feet.

I varies between 0.87 and 1.50 depending on risk to
human life and public need for tower service.

For towers below 450 ft in height, a gust effect
factor of 0.85 is applied to qz.

Decades ago ham towers were rated simply as "15 square
feet of antenna load at 60 mph."  With the current
standards that type of simple rating no longer makes
sense. Tower load ratings now are highly location
specific.

73 Jim K6OK