The shape of a tower base and not just the weight of it has a lot to do with
how well you tower will stay where it is supposed to. This is far more
important for a freestanding tower than for a guyed tower.
The base on a guyed tower is mostly to keep the tower from sinking into the
ground and from moving horizontally. It also helps keep the first couple of
sections vertical before the real guys are on it.
On an unguyed tower, the ONLY thing that keeps it standing vertically is the
base in the ground. The base actually coming up and out of the ground is
not likely. What IS likely is that the base can ROTATE in the ground. By
"rotating in the ground", I mean turning underground as the tower lays over
sideways. I don't mean rotating around the long axis of the tower. If you
consider the worst case shape for a base, it would be spherical. There is
little other than friction to keep a spherical base from rotating in the
ground. A cube or a rectangular solid is MUCH better since a lot of earth
has to actually MOVE for that base to rotate in the ground. A cube or
reactangular solid with a wide bottom is EVEN BETTER since even MORE earth
has to move for it to rotate. As K7LXC pointed out, making the base wide at
the bottom is what Rohn does for large towers. It's in the book.
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