[TowerTalk] Self-supportig tower base size

[Red <RedHaines@centurytel.net>]

Hello to all;

Be cautious about applying one manufacturer's recommendations to another 
manufacturer's tower in deciding on dimensions for the base of a self 
supporting tower.

Total weight of base, tower, and everything on the tower is one 
variable.  Minimum radius of the bottom of the base from its center is 
another.  The sum of the wind induced moments is a third major 
variable.  Soil characteristics are a fourth variable.

One fairly simple test for adequate base dimensions it that the total 
weight of everything times the minimum radius from center to edge of the 
bottom of the base must be greater than the maximum sum of all the 
moments.  If this test is met, the tower will not blow over unless the 
soil under it moves.

It is common to use only the weight of the base or just the combined 
weight of tower and base, and to ignore the weight of antennas, masts, 
rotators, and wires, because base and tower weights are the greatest 
part of the total weight. 

This is an easy test to apply. 

The hardest part is estimating the wind induced moment.  That is 
simplified by using a simplified wind force calculation; 30, 40, or 50 
pounds per square foot, for all components of antenna, mast, tower, and 
transmission lines, each multiplied by the center of the item's vertical 
cross section area.  These figures are valid for small cylindrical 
components, few inches or less, having Reynolds numbers less than I 
don't recall the exact value.  It isn't valid for shapes other than 
small cylindrical ones.

This simple approach does not take into account soil characteristics.  
In most circumstances, it errs on the safe side, provided the base is 
buried in the soil. 

Soil, when not at bedrock, acts somewhat as a viscous liquid.  Push on 
it and it slowly yields.  A post can be rocked loose because the soil 
around it yields and rocking it creates a loose hole around the post.  
However, pressure of the 'liquid' soil at the bottom of the base slowly 
pushes the soil back into contact when the rocking forces quit.

As a result, a base sitting on top of the ground would resist blowing 
the tower over in the short term, but it might gradually work the soil 
out from under itself as wind blows from first one direction and then 
another, and the displaced soil would not return as fast as it is pushed 
away.  The same base buried in the soil would also rock in response to 
the wind, but the soil will flow back under it when the wind changes 
direction.

As Steve recommends, install your tower in accordance with its 
manufacturer's directions.  Alternatively, pay a professional engineer 
to certify your plan.  If you don't want to do this, put your tower 
where it doesn't hurt anyone or anything when it falls over.

73 de WOØW

_______________________________________________

See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

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