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## [TowerTalk] Tower Bases

 To: [TowerTalk] Tower Bases kr7x@attbi.com (Hank Lonberg) Thu Feb 27 12:01:22 2003
 ```Hello Tim: Well you are close to what can be done. Take your cross arms and fill in the empty spaces in between each arm with concrete and reinforcing and you have what is called in various circles a spread footing, a mat footing or sometimes even a raft footing. It is a large square or even rectangle in plan. Square shape works better for freestanding or cantilevered towers. What happens is that the overturning moment is resisted by the strength of of the soil on the underside of the footing. The distribution of the soil pressure is determined by the geometric shape which defines the geometric moment of inertia of the cross section. The up shot is the larger the plan dimensions the larger the I of the section and the lower the soil pressure for a given overturning moment and vertical load. What you do is to increase the plan dimensions until the soil pressure is equal to or lower than the allowable soil strength. Since this discussion is about towers and wind load the wind can blow from any direction and there fore the foundation needs to accommodate that fact. A cross may be ok for four directions but not all directions. The other common cantilever tower foundation is the post type which in usually square but deep. I acts more like a embedded post to resist the overturning moment. Both of these depend on the characteristic allowable soil strength. Rock is a different matter and can be handled differently. >From memory a 80 foot freestanding tower with 30 square feet of antenna at the top typically, in very poor soil, would take say a 6'x6'x 8' deep post type foundation or a 8'x8' by 2.5' plus/minus spread footing to accommodate the same moment. It is a matter of what works better in a given situation. The spread footing requires more concrete but it is easier to dig 2.5' down than 8' down. On the other hand that is a big area for the spread and you may not have that available. Also the bottom of the spread must be below the frost line so it may be thicker just for that. Anyway you were close Tim and your idea is actually used quite commonly in designing foundations. 73 H.S. Lonberg,P.E.,S.E. / KR7X Lonberg Design Group, LTD. -----Original Message----- From: towertalk-bounces@contesting.com [mailto:towertalk-bounces@contesting.com] On Behalf Of Tim Makins, EI8IC Sent: Wednesday, February 26, 2003 11:44 PM To: TowerTalk Subject: [TowerTalk] Tower Bases Many years ago, at school, I remember seeing the illustration of how you could move the world if you had a long enough lever. It occured to me recently that the same idea could be used to construct self-supporting tower bases, but, as I'm not an engineer, I thought I'd come to Towertalk to be proved wrong. The idea is: Instead of the usual cube of concrete, how about using a horizontal cross. Maybe 2 feet deep, and 2-3 feet wide, with an internal rebar cage to prevent snapping. The length of each arm of the cross to be determined by whatever formulas you guys use - I was thinking along the 12 foot mark, as a discussion-opener. You will note that I haven't mentioned tower height, as obviously all the parameters inter-relate. If you must have a figure, lets say 40-60 feet. The reason for this ? Well, I was thinking 'outback', and 'developing world' here, where a mechanical digger isn't always available. Easier to dig slit trenches than a big pit. Less problems from cave-in too. Sit down, take one of these beers, and discuss. 73s, Tim EI8IC www.qsl.net/ei8ic _______________________________________________ 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. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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