Re: Pier design and sand


dmwmpd <westergren@...>
 

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@yahoogroups.com, "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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