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Pier pipe size defection table

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#1 outofdark

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Posted 22 September 2019 - 10:27 PM

Not to long ago I saw a posting with a table showing the arc second defections compared to pipe size, wall thickness, and height.  I'm unable to find that posting.  Can anyone point me in the right direction to locate that information?

 

thanks in advance.

 

len



#2 macdonjh

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Posted 23 September 2019 - 10:16 AM

Look for posts by "speedster" in the Observatories forum.

#3 speedster

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Posted 23 September 2019 - 01:44 PM

Here you go: 

 

0001.jpg

 

I think the threads you are looking for are:

 

https://www.cloudyni...ng#entry9621820

https://www.cloudyni...ng#entry9561416

 

Sounds like you might like these.



#4 outofdark

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Posted 23 September 2019 - 09:40 PM

Hi Speedster,

 

yes, that was the table. I read through the posting, all quite helpful. 

 

So question.  I am setting up for imaging. I will be using a dome.  Are the loading considerations the same for a dome building, no wind I assume.

 

My pier base is 38 x 38 x 36 deep.  I estimate that I need around an 8' pier.  So do I need a 10' schedule 80 pipe to  keep the flex within "spec".  Or would an 8' pipe work?

 

len



#5 speedster

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Posted 23 September 2019 - 10:01 PM

Howdy Len!

 

The five pound horizontal load at the top of the pier is just a number pulled out of the air to simulate a good bump and give us a value to calculate deflections of various piers under the same load.  The "bump" load applies whether in a dome, or not. 

 

I'd stay away from schedule 80.  For the added weight and cost over schedule 40, it doesn't get you very much.  The big gain is going up in diameter.

 

To size your pipe, first decide how much deflection you want to have in the pipe, remembering that bolts, cages, etc. also add deflection.  Let's say you want no more than 0.25 arc-sec in the pipe.  Let's say the pipe is 48" tall (don't count the concrete if it is poured against soil and not formed below grade).  Then, enter the table at 48" in the left column and read across until you are less than 0.25 arc-sec.  Stop at the 8" column which is 0.167 arc-sec.

 

I don't know your length of pipe but you can interpolate in the table and be close.  Deflection is not a linear equation but you'll be fine.

 

Just for grins, read on across the table to the 12" concrete column.  Much less deflection and also much less cost.  Like $100 for concrete vs. $500 for steel. 

 

Best of luck with it!  Feel free to ask any questions.



#6 outofdark

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Posted 23 September 2019 - 10:21 PM

Just a rough guess is 8 feet of pipe at this point.  I get my dome in a week or so.  Once I get it mounted I will have a better   Feel for the height and view I need.



#7 outofdark

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Posted 24 September 2019 - 08:56 PM

Hi Speedster,

 

Another question.  How do I use a calculator like this:

 

http://www.meracalcu...-tube-beams.php

 

to find the same values in the table you have above?

 

When I put in

length = 96

diameter 10.75  (OD for a 10" pipe"

Wall THickness .594  (Sched 80, I assume your heavy duty pipe?)

and a force of 5 lbs

 

I get Deflection as 6014.021 inches

Bending stress of 10.523

 

Not being an ME, or playing one on television, I assume that I have no idea what I am looking at.

 

How do you take the output and get to arc-seconds?

 

len



#8 speedster

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Posted 25 September 2019 - 04:50 AM

Their equation is wacko.  They have something messed up. 

 

Is your mount really going to be 8' above the ground?  The length you need to use is the actual length of pipe which is typically about 4' or less.  If you really are 8', or any length not in the table, you can get real close to the deflection by picking a length and diameter out of the table, then:

 

1.     cube the table length

 

2.     cube your actual length

 

3.     divide the table deflection by the cube of the length (#1) and then multiply by the cube of your actual length (#2).

 

The deflection equations give you deflection in inches so the angle is the arc-tan of (deflection / height)  which gives you degrees and then divide by 3600 to get arc-sec.



#9 outofdark

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Posted 25 September 2019 - 09:09 PM

Hi Speedster,

 

My building has eight foot walls, plus a 4" rise for the roof skirt, and then a 12" base ring under the dome.  9'4" to the base of the dome.  I have a CGX-L mount.  My horizon isn't straight horizontal.  So I expect I can go a bit lower, maybe 7'.  I expect to get my dome in a week or so.  Once it's mounted I can throw up a ladder and see where I come out with a good view of the sky.

 

Oh, and think you for the explanation of how to get to arc-seconds.  That is REALLY helpful. 

 

After I get my dome mounted things should settle down a bit and I can do a write up for my observatory.  I'm been working every day though the summer 12+ hours a day to get it dried in before winter.  (Regular work and then hobby time.  Weekends too.) 

 

Cheers,

 

len



#10 outofdark

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Posted 25 September 2019 - 10:33 PM

Hi Speedster,

I'm trying to calculate the arc seconds to match the above table with out any luck. 

 

I used the following site to calculate the deflection:

 

https://www.easycalc...-tube-beams.php

From the inputs

60
8.625
0.322
5

I get 0.00016554180869117683 , which seems reasonable.

Then

Arc-tan((0.00016554180869117683 / 60)) / 3600 = .000000439111

I'm not even close.  I'd like to be able to work back to the table.  I'm feeling quit dumb at this point...



#11 kiwiguy

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Posted 26 September 2019 - 12:08 AM

Hi Speedster,

I'm trying to calculate the arc seconds to match the above table with out any luck. 

 

I used the following site to calculate the deflection:

 

https://www.easycalc...-tube-beams.php

From the inputs

60
8.625
0.322
5

I get 0.00016554180869117683 , which seems reasonable.

Then

Arc-tan((0.00016554180869117683 / 60)) / 3600 = .000000439111

I'm not even close.  I'd like to be able to work back to the table.  I'm feeling quit dumb at this point...

 

You need to multiply by 3600, not divide.  3600 arc-secs per degree.



#12 outofdark

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Posted 26 September 2019 - 09:32 PM

You need to multiply by 3600, not divide.  3600 arc-secs per degree.

That did it, thank you



#13 555aaa

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Posted 06 October 2019 - 09:22 AM

It would be nice to include torsional stiffness since that is the force that the pier is reacting to on normal use.

#14 speedster

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Posted 07 October 2019 - 02:23 PM

Good thought.  The bump to the mount we are about that gives a 5# horizontal force will most likely not be perp to the axis of the pier so let's take a look.  Assume an 8" steel pipe pier 48" high and, instead of bumping the pier, we bump the end of the counterweight shaft.  We'll assume that is 16" from the pier center line and give it a 5# bump.  Angular deflection is 0.25 arc-sec.  So, we have a rotation of 0.25 arc-sec AND a vertical deflection of something less than 0.167 arc-sec.  Still very acceptable.  If we get picky, the values in the table are minimum deflection with the 5# force acting perp to the pier.

 

For a fixed height pier, the easiest way to reduce rotation is likely by increasing pier diameter.

 

I wondered who would be concerned about torsion but then I saw your Zerxes mounts and see why you would be getting that detailed.  Excellent work!

 

For peeps with less than your mount, we can live with the simplified deflections in the table.  There are so many variables on a specific pier, like how the connections are detailed, where you hit it, at what angle, is it really a 5# force, rat cage or not, etc. that the specific math can get way beyond the margin of error for a backyard pier.  The idea is that, if we design for minimum deflection and use the table to get under 0.5 arc-sec, other forces, like torsional rotation, won't be a problem.




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