Those are all very good comments, which is why I think it is unreliable for
anyone here on the reflector to encourage him to do much of anything specific,
short of having an engineer check out his proposed installation. If the rock
isn't really "solid" with a compression strength somewhere in the vicinity of
3,000 PSI (and AN Wireless actually specs 4,000 PSI for their freestanding
towers), then the rock isn't really equivalent to concrete and the required
foundation width to resist an overturning moment is rather indeterminate.
The alternative, of course, is to simply overkill everything and put in lots of
concrete (particularly in width) .... but at $100+ per cubic yard it wouldn't
take much extra of it to offset the "savings" of not hiring an engineer.
I'll shut up now ...
From: "Kevin Normoyle" <firstname.lastname@example.org>
To: "Tower and HF antenna construction topics." <email@example.com>
Sent: Fri, 12 Feb 2010 22:46:57 -0800
Subject: Re: [TowerTalk] tower installation HG52SS
my thought was that Kevin might not be "3 1/2 feet into solid rock on
all four sides" as it would have been difficult to do that just with a
jackhammer. (unless it was soft rock?)
What kind of rock is this? it must be different on bottom vs sides.
these two pages should give you some insight into "pad" style (pad and
pier) foundations for a what AN wireless would call a 50ft tower's "D"
foundation type. the horizontal dimension for these is more interesting
than the vertical. You can see 6x6 spec'ed on the bottom with this style.
(with rebar as specified). They are designing for "normal soil" and
I only mention this, because it might be more better to just go wider
(6x6) with whatever depth you hit "solid rock" (3 1/2'?) then worrying
about tying into stuff. (of unknown quality). If you really have better
than normal soil, you might be able to just go 5x5.
You're worried about an overturning moment, not just vertical and
lateral forces (it's not like analyzing a house foundatoin).
Maybe someone with more relevant engineering expertise might comment,
but it doesn't sound to me like you should just do 42"x42" by 3.5' deep
(it sounds like that's what you've got right now?..I guess because I
don't believe it's solid rock?
when you say "with a pad one to two feet tall around it above
ground"...I think the usual practice is to say that doesn't add any
value (since the "weight" of the thing isn't much of a contribution to
resisting overturning, in typical tower bases.
Kevin said "With a sufficiently tied-in top pad, the tower cannot move
in any direction as the above ground part would try to push down onto
the rock around it, which it cannot do (as the rock 1" and less
underground is solid
I'm no engineer, just my thoughts."
Yeah, but it has to be wide enough so it doesn't tip over. (obviously 2'
wide is insufficient. The calcs are all about "how wide is wide
enough?"). Making it "thicker" above ground does something, but not as
much as wider (with appropriate rebar).
If you're a worrier, 6x6 to a depth of 3-1/2' should work for a 52'
crankup (especially since you'll crank it down in big wind) More
analysis should probably get you something smaller.
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