[TowerTalk] Windload calculation and freestanding versus guyed

[Tower2sell@aol.com]

The method to calculate to wind area of a tower depends on the building code you are governed under. The British code is very different than the US ANSI/TIA/EIA-222-F. Even within the US there are locations that require ASCE-7 or their local variations. The general process is to detemine the area of one face, find the shape factors for the flats and round surfaces; apply a factor for three or four sided towers and the wind direction. Now you have the wind area you need the wind pressure  which varies with height and in some codes with terrain. Next You need to find the gust factor and wind speed. What you end up with is a wind load (force on the tower and appurtances (antennas). The free standing tower is normaly controled by bending or compression and tension in the legs. This can be checked by determining the section modulas of the section based on the Ad^2 of the legs. (Note you can find this value in your ROHN catalog for your favorite tower type) After you found the bendi!
ng moment from the wind load (I did not go into the determining the shear and moment diagrams - that is a stength of materals 101 class) determine the stress from M/S + P/A. Compare these values to the allowables from your local building code and you may be OK  unless the braces are too small so you need to check the shear capacity of the braces.

Now for the foundation, the answer will depend on the type of foundation - Pier or mat type. The pier will require the most complicated analysis and the US Federal government has published a good book and of couse you will need to purchase a say $500 computer program to solve the problem (unless you are a power utility and embed your poles say 10% +2'). with the mat type foundation the analysis is easier, you need to learn about "kerns" and bearing pressures. Also consult your building code for other limitation.
 
Compicated - yes, and its going to get even more complicated with rev G of the EIA next year. Now do you really want to know how to design a free standing tower? If the answer is yes, then proceed with your engineering degree and four years of internship and you to can easily design towers. Or you can hire an engineer for say $100 and save eight years of your life.

Tower2sell@aol.com



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