[TowerTalk] TowerTalk Digest, Vol 86, Issue 53

[Wilson Lamb]

In answer to Kipton, below.

The slug of concrete sitting in the rock bowl can't be expected to hold 
anything, unless the sides of the hole are near vertical.  The point of the 
rods drilled into the rock is to take all the load in tension, like the 
nails someone mentioned and as they will do in all the base designs with 
concrete.  He would have been as well off to just drill into the top of the 
rock.

The unknown, of course, is the quality of the rock, which he can determine 
with deeper borings and horizontal excavation to see if there are cracks 
coming to the surface.  In other words, he doesn't want to bore into a 
separated block of rock that can move.

Do not depend upon letting the tower down in strong winds!  Sure, you can do 
it sometimes, but what about the time you're away, or asleep?

Deep expansion anchors, with their threaded rods grouted or epoxied, seem a 
good way to go.
It would be a good idea to have an engineer sign off on it though, at least 
under the assumption that the rock is good.  He would not have to come out 
to do it.

A good first cut would be to use threaded rod as large as will go through 
the holes in the tower base and set the anchors to develop the working 
strength of the rod.  No design can do better than that, if you use the 
right rod material.

If we had a drawing of the base and the expected moment, we could actually 
talk about the loads with some accuracy.

Of course one set of "insurance" guywires at the top would make things 
really nice!

Wilson



Not to hijack the thread or anything, but I have been following this
discussion, and the comments suggest drilling holes into the rock base
and attaching the rebar into the holes with something like epoxy. That I
can understand.

He is 3 1/2 feet down. Would it be a good idea to also drill
horizontally into the sides and epoxy rebar into the sides also? It
seems to me (uneducated in his stuff) that would have a big effect
preventing the forces from moving the hunk of concrete. With all the
rebar cemented vertically you are depending on the strength of the epoxy
(plus the weight of the concrete) to prevent the tower from falling
over.  But if you also went in the sides, then you would also have to
shear the rebar to get it to tip over.

Of course you would have to be low so you do not have a chance of
breaking out the top of the stone. Am I thinking correctly or is there
something I am missing?

-- 
Kipton Moravec AE5IB .- . ..... .. -...