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Trefoil pier

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#1 WalterG

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Posted 05 September 2021 - 04:04 PM

I would appreciate your thoughts on building a "trefoil" style pier instead of using a very large diameter single pipe? This would be used to mount a Planewave L350 mount and it would be in an observatory. The height of the total pier would be about 50" tall. I was thinking of having someone weld three smaller diameter pipes in a triangle form with maybe 1/4-inch plate steel on the top and bottom that of-course would be drilled and tapped so I could bolt to concrete and also bolt the L350 to the top plate. See image... not to scale just to give you an idea of what I was thinking about.

 

Walter 

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#2 Garthid

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Posted 05 September 2021 - 04:21 PM

But why?



#3 WalterG

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Posted 05 September 2021 - 05:01 PM

I have not been able to find a large diameter pipe (12-14”) for any reasonable price near me. I have found very long segments of used pipe from Texas but I have no way of transporting it to Michigan were I live. I wanted some input on what others thought of the stability of such a design.

#4 macdonjh

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Posted 05 September 2021 - 08:11 PM

In post #1 in "pier engineering" CN member speedster posted a spreadsheet showing the calculations for the stiffness of several diameters of pipe for use as piers.  Even though that L350 mount is big, you might be surprised at how small a pipe will make a rigid pier.

 

Another thing to consider is using concrete for your pier. I did that because I don't know how to weld.


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#5 Kunama

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Posted 05 September 2021 - 10:39 PM

The idea is sound though I would suggest top plate to be 1/2" and the bottom plate at least 1/2" but preferably even thicker....

If you need to use the 1/4" plate for some reason then I suggest welding 3 brace pieces at half height to join the smaller tubes together...


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#6 mmalik

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Posted 06 September 2021 - 12:17 AM

Trefoil pier...

There are two approaches:

 

 

1. Beating around the bush approach

 

2. Cut to the chase approach

 

 

For a long time I was in #1 myself, until I tried #2 and found it to be the way to go. All of the posturing and dilemma will go away if you commit yourself to a pouring (concrete that is). I've broken it down to few basics any 'one' person can do by themselves. More here.... Regards


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#7 WalterG

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Posted 06 September 2021 - 06:21 AM

There are two approaches:

 

 

1. Beating around the bush approach

 

2. Cut to the chase approach

 

 

For a long time I was in #1 myself, until I tried #2 and found it to be the way to go. All of the posturing and dilemma will go away if you commit yourself to a pouring (concrete that is). I've broken it down to few basics any 'one' person can do by themselves. More here.... Regards

Do you know if you can fill a sonotube in multiple pours? I would add rebar to each section embedding it half the way in the wet pour and leaving the other half of the rebar exposed for the new pour.  I'm building what amounts to a two story observatory. The floor of the observatory is 10-feet off the ground and I still need to get the pier above that by about 4 feet. 



#8 mmalik

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Posted 06 September 2021 - 07:59 AM

I'm building what amounts to a two story observatory. The floor of the observatory is 10-feet off the ground and I still need to get the pier above that by about 4 feet. 

I would advise sharing your actual (observatory) plan before gong too far. Why such an high observatory? What's driving that? There could be other options. Regards



#9 *skyguy*

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Posted 06 September 2021 - 08:23 AM

 I'm building what amounts to a two story observatory. The floor of the observatory is 10-feet off the ground and I still need to get the pier above that by about 4 feet. 

Using concrete blocks to build a tall pier might be a viable option for your observatory.

 

Twenty years ago I built a roof top ROR observatory on my attached garage.  I used 16"x16"x8" chimney blocks to build the 14' pier. Only about 3 feet of the pier was filled with concrete and the rest was left hollow. It cost about $75 for the materials and was built by myself with the help of a friend. There's a Meade 12" LX200 w/wedge mounted on top, used mainly for astrophotography and I've never had any problems with vibrations. The pier has proven to be rock solid.

 

https://www.flickr.c...57644177074161/

 

observatory pier.jpg
 


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#10 kathyastro

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Posted 06 September 2021 - 09:20 AM

While I am a math nerd, I am not an engineer and I do not know how to calculate deflections and vibration modes.  I tend to just over-engineer and call it good.

 

So, looking at your design sketch, I cannot see a good reason for having three separate columns.  I can understand using them in lieu of an unavailable larger size.  But if I was doing that, I would place the three columns in contact with each other and have them welded together.  That strikes me as a more rigid option than the independent columns.


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#11 macdonjh

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Posted 06 September 2021 - 02:03 PM

Do you know if you can fill a sonotube in multiple pours? I would add rebar to each section embedding it half the way in the wet pour and leaving the other half of the rebar exposed for the new pour.  I'm building what amounts to a two story observatory. The floor of the observatory is 10-feet off the ground and I still need to get the pier above that by about 4 feet. 

You can, but to do it right is a pain, so it's if it was me, I'd try to pour it all at once.  To do it right, you should let the first pour cure for a few days.  Then you need to get access to the top of the concrete to roughen and then clean the surface.  Roughen it to make a good bonding surface for the second pour, clean it to make sure there is nothing interfering with the bond between first and second pours.  The you need to moisten the top to make sure water isn't wicked out of the wet second pour into the surface of the dry first pour.  All that access likely means you'll need to take your forms down and then install a new form for the second pour.  It's not the end of the world, but hiring a concrete truck and pouring the whole thing at once might be easier than getting that second set of forms nice and straight and lined up with the first pour and keeping everything plumb.

 

 

While I am a math nerd, I am not an engineer and I do not know how to calculate deflections and vibration modes.  I tend to just over-engineer and call it good.

 

So, looking at your design sketch, I cannot see a good reason for having three separate columns.  I can understand using them in lieu of an unavailable larger size.  But if I was doing that, I would place the three columns in contact with each other and have them welded together.  That strikes me as a more rigid option than the independent columns.

Yeah, that is where engineering comes in.

 

Those three columns welded together will be more rigid that each column acting singly.  However, there are benefits in rigidity to having the individual columns separated.  Since you've admitted to being a math nerd, the area moment of inertia (one factor in figuring out how stiff something is) of a composite section is dependent on the square of the distance between the composite pieces and the centroid of the composite area.  That's at the heart of why I-beams and trusses work: they take the load-bearing part of the material and move it away from the centroid, almost like creating a lever arm to multiply your forces (or resistance to forces).

 

But where you might run into trouble is if the individual columns deflect, or worse buckle, under the loads applied to the structure.  It's not likely to happen with telescope loads (we do balance things, after all), but it could happen.  That's why I-beams are braced perpendicular to their axes, and trusses have all that cross bracing between the main members: to stop buckling. 

 

All that to say, the calculations aren't that difficult and the OP could figure out if there's an advantage to using his trefoil design to "simulate" a 12" pier, or simply look for some 6" or 8" pipe which he may find is plenty stiff enough for his L350 (and its properly balanced load).  The effects of adding lateral bracing to the trefoil design can also be calculated (adding braces in at the midpoint should double the resonant frequency and will increase the load which can be applied before the columns buckle).


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#12 WalterG

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Posted 06 September 2021 - 03:21 PM

I would advise sharing your actual (observatory) plan before gong too far. Why such an high observatory? What's driving that? There could be other options. Regards

That ship has sailed as the expression goes.  We have a dense wooded lot and building that tall gets me a good view.



#13 WalterG

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Posted 06 September 2021 - 03:23 PM

Using concrete blocks to build a tall pier might be a viable option for your observatory.

 

Twenty years ago I built a roof top ROR observatory on my attached garage.  I used 16"x16"x8" chimney blocks to build the 14' pier. Only about 3 feet of the pier was filled with concrete and the rest was left hollow. It cost about $75 for the materials and was built by myself with the help of a friend. There's a Meade 12" LX200 w/wedge mounted on top, used mainly for astrophotography and I've never had any problems with vibrations. The pier has proven to be rock solid.

 

https://www.flickr.c...57644177074161/

 

attachicon.gifobservatory pier.jpg
 

Nice!  I have seen this before, great job.  I was thinking of chimney blocks also.  What was the reason for leaving most of the blocks hollow... just wondering. 



#14 speedster

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Posted 06 September 2021 - 10:01 PM

Yes, you can do three pipes and make it as stiff as you want.  But, far from the best scheme.  With 3 pipes the deflection is not the sum of the 3 pipes but rather depends on the moment of the assembly.  Pipes separated a bit are a stiffer assembly than pipes touching (provided the plate deflection doesn't become a factor.)

 

Plenty of pipe in Michigan.  Your just not looking in the right place.  I try to understand people going overboard on their piers but you might rethink the 12"-14" diameters.  At 50", 8" schedule 40 pipe provides 0.167 arc-sec of deflection per the conditions in the pier engineering thread.  6" pipe has 0.431 arc sec.  That's just the pipe but the deflection in the bolts and plates are so tiny they can be ignored unless you do something crazy.  We shoot for less than 0.5 arc-sec total.

 

1/4" plate is fine.  That's not where the deflection is happening unless you put your bolts far from the pipe.  Leave just enough distance between pipe and bolts to get a wrench on.

 

With proper reinforcing, you can make multiple pours in a sonotube.  Not good practice and it's not very hard to just do the whole thing at once.  Multiple pours are not a good idea but it's not going to break because of it.

 

Love the creative thinking of frames and even trusses but for a telescope pier, everything about those are working against you.



#15 mmalik

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Posted 07 September 2021 - 02:12 AM

That ship has sailed as the expression goes.  We have a dense wooded lot and building that tall gets me a good view.

Bring that ship back. Cut a few trees or go the edge of the lot (with a view) if there is such a thing. If you ask me, this high and mighty plan doesn't make sense. Rethink; three is always a simpler solution to the problem. Regards


Edited by mmalik, 07 September 2021 - 02:13 AM.


#16 WalterG

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Posted 07 September 2021 - 06:06 AM

Yes, you can do three pipes and make it as stiff as you want.  But, far from the best scheme.  With 3 pipes the deflection is not the sum of the 3 pipes but rather depends on the moment of the assembly.  Pipes separated a bit are a stiffer assembly than pipes touching (provided the plate deflection doesn't become a factor.)

 

Plenty of pipe in Michigan.  Your just not looking in the right place.  I try to understand people going overboard on their piers but you might rethink the 12"-14" diameters.  At 50", 8" schedule 40 pipe provides 0.167 arc-sec of deflection per the conditions in the pier engineering thread.  6" pipe has 0.431 arc sec.  That's just the pipe but the deflection in the bolts and plates are so tiny they can be ignored unless you do something crazy.  We shoot for less than 0.5 arc-sec total.

 

1/4" plate is fine.  That's not where the deflection is happening unless you put your bolts far from the pipe.  Leave just enough distance between pipe and bolts to get a wrench on.

 

With proper reinforcing, you can make multiple pours in a sonotube.  Not good practice and it's not very hard to just do the whole thing at once.  Multiple pours are not a good idea but it's not going to break because of it.

 

Love the creative thinking of frames and even trusses but for a telescope pier, everything about those are working against you.

 

I'll have to go another search for pipe in Michigan.  It seems like 8" is a good solution if I don't go with the Trefoil design.  Thank you



#17 WalterG

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Posted 07 September 2021 - 10:12 AM

Just to let you all know, I really appreciate all of your input. I have been convinced to go with a single pipe, an 8", and a top and bottom plate. I found a place very near my home, not listed on any google search that has the pipe and will do the required welding...win-win. 

 

Thanks again for all of your help!

 

Walter 


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#18 macdonjh

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Posted 07 September 2021 - 12:25 PM

Excellent.  Perseverance pays off.

 

Oh, the only time I've heard of a truss being used as a telescope pier was in a discussion about using radio antenna towers as the basic building block.  You know, those three-pole towers with the zig-zag bracing holding the three poles together.  The advantage there is they are prefabricated and easy to cut to length.  I don't know in anyone actually tried it.  Trusses and other fabricated structures can be very weight-efficient.  However, in working with civil-structural guys for a while I've come to realize that simple shapes and simple construction is almost always less expensive and just as functional.  Only go with complex fabricated things if you want to look fancy or you have an abnormal restriction (e.g. for some reason your permanent pier must not weigh more than 100 lb :-) ).


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#19 555aaa

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Posted 08 September 2021 - 10:35 PM

If you are using the L350 in alt az mode you would be asking for trouble with the original approach because there is a large torsional moment between the az axis and the pier that the pier has to react and the whole thing can go unstable if the pier isn’t torsionally stiff relative to the inertial load of the mount and payload. Even in a wedge you have to consider that in a direct drive mount the force that gets reacted by the pier is large and can cause instability at worst or bad tuning at best, where the mount doesn’t want to settle down after a slew. The linear deflection of the pier is irrelevant. If PW tells you the torsional stiffness required then you can determine if the design is stiff enough. Maybe it would have been fine and they’ve sold a ton of these. It would be stiffer as a box structure with sheet sides especially if it were tapered. People really need to stop talking about the deflection at the top of the pier, especially with this kind of mount where the only thing that keeps it stable is the control loop.

Edited by 555aaa, 08 September 2021 - 10:41 PM.


#20 555aaa

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Posted 08 September 2021 - 10:36 PM

BTW trusses are used all the time in piers.
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#21 WalterG

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Posted 09 September 2021 - 07:24 AM

If you are using the L350 in alt az mode you would be asking for trouble with the original approach because there is a large torsional moment between the az axis and the pier that the pier has to react and the whole thing can go unstable if the pier isn’t torsionally stiff relative to the inertial load of the mount and payload. Even in a wedge you have to consider that in a direct drive mount the force that gets reacted by the pier is large and can cause instability at worst or bad tuning at best, where the mount doesn’t want to settle down after a slew. The linear deflection of the pier is irrelevant. If PW tells you the torsional stiffness required then you can determine if the design is stiff enough. Maybe it would have been fine and they’ve sold a ton of these. It would be stiffer as a box structure with sheet sides especially if it were tapered. People really need to stop talking about the deflection at the top of the pier, especially with this kind of mount where the only thing that keeps it stable is the control loop.

I'll be using it in equatorial mode, thank you for the insight



#22 speedster

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Posted 09 September 2021 - 10:09 PM

Hey Bruce, you are turning out some boss equip.  It would be interesting to measure the torsion load of a big payload.  For an 8" schedule 40 pipe, 36" long, you're probably going to be well above 100,000 lb-ft of moment before even thinking about hitting the yield strength and at 100 lb-ft the angular deflection is only 4 arc-sec and that only happens at start and stop of slews. 


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#23 Star Shooter

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Posted 10 September 2021 - 08:47 PM

I buy metal from Alro Steel. They have several outlets in Michigan. www.alro.com


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