[TowerTalk] Self-supportig tower base size

[Alan C. Zack]

I fully agree with all Red has written below.  One more point, don't 
order your tower until AFTER you have your Bldg Permit.  Many 
states/counties/cities bldg codes have gone from UBC-97 70 MPH wind 
calc specs to the newer IBC-2000 90 MPH wind calc specs.  U.S. Towers, 
for example, uses the UBC-97 codes and does not have a licensed PE on 
their staff to do the newer IBC-2000 calcs, and could not provide them 
for me.  I was forced to cancel my pre-order quote with them and go to 
another manufacturer.
Some bldg depts may ask for a soil sample analysis, which can be 
expensive.  If asked for one, it may be cheaper for you to just ask to 
meet their requirement for the size hole needed for their most 
stringent soil sample test requirement rather than actually doing the 
test.

Red wrote:

> 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


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__________________________________________________________________________ 

Alan Zack
Amateur Radio Station K7ACZ
Las Vegas, Nevada, USA
Quality Engineer, The Boeing Company, Retired
Aviation Chief Warrant Officer, U.S. Coast Guard, Retired
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