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

To: Tower Talk List <>
Subject: Re: [TowerTalk] (Repost) Engineering advice on above ground foundation
From: Kevin Normoyle <>
Date: Fri, 05 Jun 2009 20:16:24 -0700
List-post: <">>
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 ).

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 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.



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