Re: Pier design and sand


dmwmpd <westergren@...>
 

Hi Joe,

I come from the school of thought in astrophotography that there is
no substitude for stiffness, like in auto racing there's no
substitute for cubic inches. A lot of ideas for astro mounts,
cameras, etc can work, but some of them take special care. We are
fortunate now that DSLR's don't need the long term precision guiding
that film required in order to get nice astro photos. I used to
guide film with a piggy back scope that I thought was a very stiff
mount, yet the change in gravity direction in 45 minutes of tracking
caused elongated stars due to the deflection of the structure. Any
imbalance of the scope/mount, or change in the direction of hanging
cables can cause elongated stars over longer exposure times.

Good luck if you decide to try the PVC pier. It's certainly cheaper
and much easier to handle and install than a steel pier. I know, my
steel pier weighed over 450 lbs and took a lot of planning and
manpower to get it into position. It's never going to move now. Let
us know what you try, and how well it works.

Regards,
Don

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

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!

Thanks,
Joe



----- 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.

Regards,
Don

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

Hi,

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"
diameter,
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
pier
material. Advantages include being easier to construct, (even
remove to adjust
for height), and can eventually be moved to a better spot, or
removed
entirely.

Opinions please.

Joe



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