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How strong is that concrete pier?

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#1 iandodd

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Posted 01 April 2013 - 12:19 PM

While this YouTube video is not specifically astronomy related (except that it was posted by astronomy blogger Phil Plait, aka, the Bad Astronomer), it's an interesting demo of just how strong a concrete pier can be. This 18" diameter by 36" high cylinder withstood up to 1.6 million pounds of pressure before it failed. Now, obviously a pier isn't going to be subjected to crushing forces (of, if it is, your mount and scope will yield long before your pier). But this gives you an idea of just how strong a concrete cylinder can be and why it makes a good choice for a pier. http://www.youtube.c...h?v=cfVvzjVzeE0

#2 Lee Jay

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Posted 01 April 2013 - 12:55 PM

Concrete is a good choice for a pier, but crushing strength isn't why that's true.

It's a good choice because:

- It has high mass (due to it being cheap and thus solid)
- It has high stiffness (due to diameter, mostly)
- It has a high damping coefficient (5-10 times that of steel or aluminum)

#3 JMW

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Posted 01 April 2013 - 08:54 PM

It's a good choice because it is inexpensive if you do all the labor.
If you do the base right it shouldn't move. 12 inch diameter was
plenty beefy for my AP900.

#4 Kendahl

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Posted 01 April 2013 - 09:03 PM

A concrete pier needs a solid footing below the frost line so that it won't shift with the seasons. Some rebar inside won't hurt.

Kendahl

#5 Kraus

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Posted 01 April 2013 - 10:11 PM


My 12 inch diameter pier is literally rock solid. Vibration, what's that?

#6 wz2

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Posted 01 April 2013 - 10:27 PM

A concrete pier needs a solid footing below the frost line so that it won't shift with the seasons. Some rebar inside won't hurt.

Kendahl


What is the need for rebar? Will the pier be stressed in tension?

Chris

#7 Phil Sherman

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Posted 02 April 2013 - 12:18 PM

Compression failure of a concrete post is irrelevant for astronomical purposes. The pier is a cantilevered beam subject to bending stress as the load (mount and scope) are moved to different positions in the sky. The deflection of the beam is what will spoil your astrophotos. Wind striking the scope tube and mount provide the force that bends the beam, setting up vibrations.

Check with an engineering textbook for formulas to calculate the amount of bending. Steel rebar reinforcing of a concrete post should decrease the bending because the steel bars will need to expand and compress as the post bends. (This assumes proper placement of the rebar.)

Concrete beams that are supported on both ends and are expected to support a load in the middle (ie bridge supports) always have steel reinforcement. Concrete is much stronger when compressed which is why pretensioned steel cables are used in bridge construction.

Phil

#8 EdmontonAB

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Posted 02 April 2013 - 02:26 PM

I wish I had a scope that would bend my pier :jump:

Concrete will bend but unless you are mounting a monster scope, don't worry about it. Pour and enjoy.






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