Re: Pier design and sand

Joe Zeglinski

Hi Don,

My engineering background reminds me of these pitfalls as well, and it is
a concern at the back of my mind, but I still wonder how much stiffness in
this application, is really enough. If I were building an "observatory pier",
fully committed to location, I would go for maximum strength of materials, but
this pier is a test system, if I can call it that. If it works sufficiently
well, (and I don't kick it, intentionally out of frustration, or
unintentionally), then I will consider it a viable success. Since this is my
backyard, I don't expect blustery winds (certainly not if I have to be
outside - in which case the seeing will be a deciding factor, not the pier).
There are enough buildings, shrubs, fences, and low trees, etc. to provide
some wind break. Heaving of the ground from year to year, might eventually
require a reset, or redesign with heftier materials, but a 4 foot depth and
packed with yard soil (not necessarily sand). I don't expect much long term
motion, especially since there is no "footing" to push up - it's just a
relatively thin 1/2" cylinder wall rather than a flat bottom. As for short
term stability, I am trusting that the wall thickness will suffice for the
load being carried. But by then I will know if this is the best spot -
certainly can't be worse than using my 6" diameter Losmandy G11 aluminum
tripod supporting my AP900 and Questar-7. If there is any shift caused by
seasonal heaving, I can realign the mount, but hopefully it won't be required,
or at least not often. I like the clean lines of green PVC, no painting
required, no pitting or rusting below ground, and it won't ring like a bell.

As for pulling cables, or unbalance - if cables are pulled, I have some
serious soldering repairs ahead of me. The mount should be perfectly
balanced - after all it's one of the best, and if anything, we like our setups
perfectly balanced.

I'm still investigating the feasibility, but appreciate any heads up and
warnings I can get from you and others. For now, I am considering it a
"temporary pier" - with no early plans to require dismantling it (once
planted). I had hoped someone had already tried this approach, but I might
have to be the first to do so.

What I really would like is to turn it into a Greek Column during the day,
as I described - that WOULD be the topper!


----- Original Message -----
From: "dmwmpd" <westergren@...>
To: <ap-gto@...>
Sent: Wednesday, October 31, 2007 8:51 PM
Subject: [ap-gto] Re: Pier design and sand

Hi Joe,

I really don't think PVC pipe buried in the soil is going to be a
very good pier. The ringing that you mention is a high frequency
tone, that the PVC pipe wouldn't have compared to a metal (steel)
pipe. But the real concern for pier design is how stiff it is
against bumping (not hitting), wind, weight shift (as the mount
moves with any unbalance), pulling cables, etc. These are all long
time effects, like many seconds to hours. The stiffness of the pier
pipe is a function of E (modulus of elasticity of the material) and
the Section Moment of Inertia (depends on diameter and wall
thickness. The stiffness is E*I. It then determines how much
delction of a canteleverd pier has at the top, when one or more of
the steady forces or moments I mentioneed above are applied.

The modulus for steel is 30 million. The modulus for PVC is between
380,000 and 540,000. In other words, the PVC pipe would flex more
than 55 times as far compared to the the same size steel pipe.

The paper on pier design by Dennis Persyk mentioned a few days ago
on this forum is a very good guide to best pier design, although it
doesn't address the dynamics that I mentioned in my posts.


--- In ap-gto@..., "Joseph Zeglinski" <J.Zeglinski@...>


I have been thinking about putting a temporary pier in the
ground, to see
how the back yard observing spot works out. I don't want to commit
to heavy
steel posts and concrete bunker footing.

I was thinking of using one of those very thick walled, 10"
street water main/sanitation pipes, and simply bury it 3 or 4 feet
deep (3
feet above ground), and fill the inside back in with earth
(perhaps only to
ground level). I am hoping that the perhaps 1/2", (or thicker),
PVC walls
won't ring as much as steel, and will be solid enough for an AP900
system. I
think these street pipes should be temperature stable and should
not warp or
vibrate, in normal city street use, so it might have advantages as
material. Advantages include being easier to construct, (even
remove to adjust
for height), and can eventually be moved to a better spot, or

Opinions please.


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