Kevin:
You are absolutely correct on this and I have been in e-mail
contact with Rudy concerning his foundation and was cc'ing the
list but it seems that those didn't get through. Richards post
just a bit ago has part of my response to him also. Apparently
the hole was excavated 3.5' deep and water started to seep
into the excavation. His contractor opted to place about 2' of
compacted gravel in the excavation as a base. This is just
fine and done quite often. They then poured the foundation on
this gravel and exposed 3.5' of the foundation above the
ground. The foundation is not floating, nor would it, on some
soup of soil it is in the dry and on compacted gravel which
for frost heave is another common way to handle the issue. It
should have been deeper as far as the gravel bed is concerned
but if the soil is fairly sandy then heave will not be much of
an issue. The bearing capacity of the gravel base is the same
as the existing soil. Any settlement will occur quite quickly
as the foundation weighs way more the tower and its antennas.
Any plumb adjustments will be easy to make utilizing the stub
legs and the stick built nature of the tower erection process.
Regards
Lonberg Design Group, Ltd.
H.S. Lonberg, P.E.,S.E. / KR7X
President
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Kevin
Normoyle
Sent: Friday, June 05, 2009 8:16 PM
To: Tower Talk List
Subject: Re: [TowerTalk] (Repost) Engineering advice on above
ground foundation
I think David's lack of additional detail, plus focus on
little stuff
that doesn't matter so much, makes it sound dicier
than it probably is. For instance, he says only 1.5' of the
foundation
is below ground, but what was done with the gravel and hole is
a key
part of the foundation. He doesn't specify that part though.
For these large pad foundations, the overturning moment is
mostly
resisted by the soil at the bottom, not the sides, which is
opposite
from the normal deep, not so wide, tower foundations. So it
doesn't
matter so much whether all the concrete is above or below
ground. What
matters is the bearing of the stuff resisting it all at the
bottom.
(soil under the gravel)
Packed gravel is a good bearing surface. The concrete bears on
the
gravel, then the gravel bears on the soil farther down. It's
important
that the gravel can't wash out/move.
It wasn't said how far down a hole was dug, or how much gravel
used as
fill before the concrete. I suspect the hole was dug below
frost line?
That would seem to be necessary.
Rebar schedule wasn't mentioned.
10x10x5 is bigger than the normal 9x9x5 solid block A.N.
wireless
specifies. So assuming good bearing at the bottom, it does
sound like it
could be in the right ballbark for a HD-90. (it must be a
70mph or so
wind area, looking at
http://www.anwireless.com/tablelg.html ).
My guess is that analysis could show it's fine, assuming the
rebar was
similar to the A.N. Wireless suggestions here:
http://www.anwireless.com/alt.html (or better..some engineers
seem to
like #9 rebar rather than lots of #5 rebar).
In fact it could be better than what a typical ham might have
done..i.e.
ignore the water table issues and just install a foundation
per drawing
that specifies "normal soil" when you don't have normal soil.
The unknowns are the bearing capacity of the soil at the
bottom of the
hole, the gravel size/compaction, and the rebar schedule.
Yeah, you lose some from being essentially above ground so the
sides
don't count for resisting the overturning moment. But I think
it might
be, that on these type of foundations, the sides aren't
included in the
calcs anyhow?
My totally unqualified two cents.
oh p.s. In terms of "too late", there's plenty of things that
could
still be adjusted. Like why 90' as opposed to 80' or 70', for
the tower.
-kevin
ke6rad
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