[Top] [All Lists]

Re: [TowerTalk] (Repost) Engineering advice on above ground foundation

To: "'Kevin Normoyle'" <>, "'Tower Talk List'" <>
Subject: Re: [TowerTalk] (Repost) Engineering advice on above ground foundation
From: "Hank Lonberg" <>
Date: Fri, 05 Jun 2009 20:57:47 -0700
List-post: <">>

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.

Lonberg Design Group, Ltd.
H.S. Lonberg, P.E.,S.E. / KR7X
-----Original Message-----
[] On Behalf Of Kevin
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
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
resisted by the soil at the bottom,  not the sides, which is
from the normal deep, not so wide, tower foundations. So it
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
(soil under the gravel)

Packed gravel is a good bearing surface. The concrete bears on
gravel, then the gravel bears on the soil farther down. It's
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.
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 ).

My guess is that analysis could show it's fine, assuming the
rebar was 
similar to the A.N. Wireless suggestions here: (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
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
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
still be adjusted. Like why 90' as opposed to 80' or 70', for
the tower.



TowerTalk mailing list
No virus found in this incoming message.
Checked by AVG - 
Version: 8.5.339 / Virus Database: 270.12.54/2157 - Release
Date: 06/05/09 17:55:00


TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>