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Pier diameter?

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#26 JJK

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Posted 21 August 2013 - 09:01 PM

Why not use reinforced concrete as a foundation and bolt a cylindrical metal pier to that base?

Metal piers more than adequately carry the compression, tension, and torsional loads of any telescope I could imagine ever owning. In addition, should one's needs change (raise or lower the pier height, increase its capacity), one can modify or replace the pier. Finally, it's easier to route cables through a hollow metal pier than through a concrete varietal.

I've used metal portable and permanent piers (the latter bolted to a concrete foundation). There is no movement due to torsional loads caused by the scope off to one side, and there is no seasonal movement of the system.

I think metal piers look better too.


Tubular metal piers are a lot more prone to ringing and vibrations than are concrete piers if you exceed the 1:4 ratio between diameter and height.

Steel X piers are better than tubes (O-piers?) and cool down faster as well.


Chris, your website is pretty cool & informative.

#27 Christopher Erickson

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Posted 21 August 2013 - 09:37 PM

The cool down time and ringing of cylindrical metal piers I've used are non-issues (10" to 12" diameter pier < 40" tall). The latter would generally be of concern if one was prone to constantly rapping on a pier during imaging or visual work, but I suppose it could come into play if there were strong wind gusts hitting a long OTA (a non-issue in an observatory). I've used short and long OTAs on quality metal portable and permanent piers and never had a ringing issue.

I haven't studied the effects of torsional loads on an X-bar, but I also haven't seen any such problem with decent cylindrical metal piers.

Can you please point to a FEA of X-bars versus cylindrical tubes? I've used them in my lab to support micro-manipulators that work in conjunction with microscopes. I hadn't thought about using them for astronomy pier apps.


I am not aware of anyone doing any serious FEA simulations (Finite Element Analysis) of amateur-class observatory piers but there is certainly a lot of FEA that has been done in and around larger professional observatories. The results would be interesting if someone wanted to go through all of the work to create them.

An "X pier" has more surface area, no dead air spaces and a much-higher surface-area to mass ratio than a tubular or rectangular pier. This isn't likely to matter much in a lab but in an environment where there are temperature swings and you want rapid thermal stabilization, it has advantages.

Unlike research-grade microscopes or precision mechanical laboratory stages, telescope mounts carry heavy, relatively-powerful, active motor assemblies and additional components that are vulnerable to unpredictable external influences. The real challenge for telescope mounts and piers are resonant frequencies and effective damping. In the bad-old-days, this was usually solved for telescopes by lots and lots of mass (more or less.) Nowadays FEA, advanced materials and significant fundamental research have given us much-better tools to work with.

A tubular steel pier does not have near the diagonal rigidity of a steel X pier. An FEA comparison of the two would certainly show that all other things being equal, the X pier would be much more resistant to harmonic vibrations created by wind or the mount's pulsing motors running at different speeds.

And while steel is elastic and concrete effectively is not, steel has a high thermal conductivity that has benefits in an observatory when used carefully.

In "The Design and Construction of large Optical Telescopes, © Springer 2010", Section 6 discusses telescope mount mechanical issues at length, including the challenges and limitations of FEA simulations in the design of precision telescope support structures (mount, base & foundation.) It is an excellent reference resource for anyone desiring to build a serious, effective amateur observatory. Be warned that it is filled with math.

I hope this helps.

#28 Christopher Erickson

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Posted 21 August 2013 - 10:02 PM

Chris, your website is pretty cool & informative.


Glad you like it.

It isn't much but I had to put up something because my clients expect it these days.

I need to add a lot more content to it but just haven't found the time.

#29 JJK

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Posted 21 August 2013 - 10:20 PM

The cool down time and ringing of cylindrical metal piers I've used are non-issues (10" to 12" diameter pier < 40" tall). The latter would generally be of concern if one was prone to constantly rapping on a pier during imaging or visual work, but I suppose it could come into play if there were strong wind gusts hitting a long OTA (a non-issue in an observatory). I've used short and long OTAs on quality metal portable and permanent piers and never had a ringing issue.

I haven't studied the effects of torsional loads on an X-bar, but I also haven't seen any such problem with decent cylindrical metal piers.

Can you please point to a FEA of X-bars versus cylindrical tubes? I've used them in my lab to support micro-manipulators that work in conjunction with microscopes. I hadn't thought about using them for astronomy pier apps.


I am not aware of anyone doing any serious FEA simulations (Finite Element Analysis) of amateur-class observatory piers but there is certainly a lot of FEA that has been done in and around larger professional observatories. The results would be interesting if someone wanted to go through all of the work to create them.

An "X pier" has more surface area, no dead air spaces and a much-higher surface-area to mass ratio than a tubular or rectangular pier. This isn't likely to matter much in a lab but in an environment where there are temperature swings and you want rapid thermal stabilization, it has advantages.

Unlike research-grade microscopes or precision mechanical laboratory stages, telescope mounts carry heavy, relatively-powerful, active motor assemblies and additional components that are vulnerable to unpredictable external influences. The real challenge for telescope mounts and piers are resonant frequencies and effective damping. In the bad-old-days, this was usually solved for telescopes by lots and lots of mass (more or less.) Nowadays FEA, advanced materials and significant fundamental research have given us much-better tools to work with.

A tubular steel pier does not have near the diagonal rigidity of a steel X pier. An FEA comparison of the two would certainly show that all other things being equal, the X pier would be much more resistant to harmonic vibrations created by wind or the mount's pulsing motors running at different speeds.

And while steel is elastic and concrete effectively is not, steel has a high thermal conductivity that has benefits in an observatory when used carefully.

In "The Design and Construction of large Optical Telescopes, © Springer 2010", Section 6 discusses telescope mount mechanical issues at length, including the challenges and limitations of FEA simulations in the design of precision telescope support structures (mount, base & foundation.) It is an excellent reference resource for anyone desiring to build a serious, effective amateur observatory. Be warned that it is filled with math.

I hope this helps.


Thanks Chris. I'll check that book out, as it sounds interesting. I can handle math.

#30 mich_al

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Posted 21 August 2013 - 10:45 PM

FYI cement also comes in 40lb bags.

#31 JJK

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Posted 22 August 2013 - 01:18 AM

Don't want to lug around hundreds of pounds of concrete mix, eh? Have you considered a concrete block pier? Mine is a little over 5' above the ground. I was considering filling the blocks with concrete but have found it is quite solid as is and was a cinch to put up and I had never worked with mortar and block construction before. I now have a very solid 16" square pier that will pretty much be able to handle anything I can put on it. All for my dinky 8" SCT.


Have you checked whether your scope will hit a 16" diameter pier?

#32 Christopher Erickson

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Posted 22 August 2013 - 05:14 AM

Thanks Chris. I'll check that book out, as it sounds interesting. I can handle math.


Bringing up FEA's I assumed you had no problems with serious math.

That was just a heads-up for others who might not be interested in investing in an expensive book filled with dry, college-level math.

For people who consider math and engineering to be brain-candy, you'll love it.

http://www.amazon.co...strophysics/...

#33 astrodog73

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Posted 22 August 2013 - 05:52 AM

Have you checked whether your scope will hit a 16" diameter pier?


I was about to ask the the same.... I built a 10" pier years ago, I use an 8" newt on it now, and have to keep an eye on the mount safety limits and watch slews to the meridian carefully as it gets *very* close to the pier....

#34 JJK

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Posted 22 August 2013 - 09:13 AM


Have you checked whether your scope will hit a 16" diameter pier?


I was about to ask the the same.... I built a 10" pier years ago, I use an 8" newt on it now, and have to keep an eye on the mount safety limits and watch slews to the meridian carefully as it gets *very* close to the pier....


For casual viewing in my backyard, I bolted two 4"x8" cedar beams together and used it as a pier that supported an AP900GTO mount and a few different scopes. I then widened the top of the pier by adding 3" all around (14"x14") to use an AP1200 mount and AP155 f/7. The OTA would hit the extension if I let it slew unsupervised (which I of course don't let happen).

I've seen folks add a tall, thinner "rat cages" supported by three or four 1/2" bolts atop oversized piers to make the mount level and to avoid hitting the massive pier below. Although many swear by that construction, it likely compromises the overall system's performance.

I like the X-bar concept and will look into analyzing it. I'll need a second metal pier in a few years. The X-bar is simple, can accommodate shelves and power supplies, and I might even be able to weld it myself.

#35 Raginar

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Posted 22 August 2013 - 09:42 AM

Hey Dan,

How did you bolt your pier plate to that stack of bricks? Pictures?

#36 mclewis1

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Posted 22 August 2013 - 10:00 AM

Like StarmanDan I used concrete blocks for at least part of my pier. I used rebar and poured concrete to further strengthen my concrete pier base. The primary benefits were how easy it was to construct. Each component was very easy to handle (individual blocks, smaller amounts of mixed concrete, etc.). The primary downside was the need for a larger hole to work in when constructing the base.

I used blocks arranged in a different manner for the first layer to provide a wider base and then simply alternated the blocks going up. Concrete between the blocks used as grout allowed for minor changes to the "trueness" of the pier base. My concrete base was about 8' tall (5' buried, 3' exposed above grade) and was done in 2 stages with rebar helping tie the two sections together.

This 16" square pier base turned out to be very solid. One minor compromise you must make with concrete block is some limitation with the placement of the J bolts (unless of course you drill them out later on). This limitation changed the original plan for the placement of the bolt holes in the base of the steel pier (and where the gussets went).

The 3' pier on top is steel and quite traditional. Folks who are concerned about ringing and overall stiffness of a steel pier should look into wall thickness. Schedule 40 pipe is very popular but the heavier schedule 80 isn't usually too much more difficult to get and makes a difference in the overall feel of the pier. It does however make the pier substantially heavier.

In my case a 10" sched 40 pier was originally planned and in the end a 8" diameter sched 80 pier was constructed. The smaller diameter pier made it a bit easier to affix reinforcing gussets to the top and bottom plates without those plates being too large. The larger the gussets the stiffer the pier will be overall. Bigger gussets starts to approach some of the benefits that Chris is describing with the X-bar pier design.

Yes less than robust "rat cages" do somewhat compromise the overall stiffness. For most folks this isn't too much of an issue. In my case I opted for a more robust "rat cage" - 4 x 3/4" diameter hardware spaced about 12" apart with 3/4" steel plate.

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#37 Raginar

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Posted 22 August 2013 - 02:45 PM

Interesting. Dan, can you post a picture of your pier?

#38 tomcody

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Posted 22 August 2013 - 07:27 PM

Terry,
The biggest consideration is how the mount clears the pier, you don,t want to limit the mount movement with an oversize pier.
After that, height is second.
If it were me, I would use a metal pier ( I like ATS piers) as you can easily adjust ( level) if the the foundation shifts or settles.
But a properly sized concrete pier will work fine ( Dan,s piers offer good kits for concrete piers).
Rex
PS a contractor can use a powered bucket ( looks like one of those all terain fork trucks that move pallets of sod but with a tip bucket on it) to move the concrete from the truck to the build site.

#39 StarmanDan

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Posted 22 August 2013 - 09:44 PM

Hey Dan,

How did you bolt your pier plate to that stack of bricks? Pictures?


Haven't gotten that far yet. I'm gonna try to make more progress this weekend but the plan is to stuff some newspaper in the voids of the blocks then pour concrete in just the upper block then set anchor bolts for the pier plates that the mount will bolt to. We'll see how it goes!

#40 JJK

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Posted 22 August 2013 - 10:03 PM

Hey Dan,

How did you bolt your pier plate to that stack of bricks? Pictures?


Haven't gotten that far yet. I'm gonna try to make more progress this weekend but the plan is to stuff some newspaper in the voids of the blocks then pour concrete in just the upper block then set anchor bolts for the pier plates that the mount will bolt to. We'll see how it goes!


Why not stuff the voids with broken pieces of blocks or clean stones?

#41 Christopher Erickson

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Posted 23 August 2013 - 06:52 PM

Here is another variation on the X pier that has less mass.

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  • 6042380-X-pier-3.gif


#42 TheSheriff

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Posted 23 August 2013 - 09:37 PM

Hey Dan,

How did you bolt your pier plate to that stack of bricks? Pictures?


Haven't gotten that far yet. I'm gonna try to make more progress this weekend but the plan is to stuff some newspaper in the voids of the blocks then pour concrete in just the upper block then set anchor bolts for the pier plates that the mount will bolt to. We'll see how it goes!


Why not stuff the voids with broken pieces of blocks or clean stones?


I'm an admitted "Overkill" type, but after all that work, why not just fill the entire void with concrete? Or at least, fill all but the top foot with damp sand and crete the remainder.

#43 roscoe

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Posted 24 August 2013 - 07:37 AM

Chris,
Looking at your latest sketch reminded me of the look of a z-braced radio tower - how well do you think a short section of something like Rohn 25 would work as a pier top?
Russ

#44 dawziecat

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Posted 24 August 2013 - 08:12 AM

If it were me, I would use a metal pier ( I like ATS piers) as you can easily adjust ( level) if the the foundation shifts or settles.
But a properly sized concrete pier will work fine ( Dan,s piers offer good kits for concrete piers).
Rex


Many thanks for the tip on Dan's Piers, Rex. I had not heard of them.

I have found a contractor to do the concrete work. Furthermore, he says he can get his 4X4 into the site on the very rough track the excavator has made. It just may be necessary the excavator haul the truck back out! :p

The budget will not allow one of those very fine ATS piers.

So I plan on a 12" concrete pier, topped by a plate from Dan's Piers that will directly accept the Astrophysics 119FSA adapter for the AP1100.

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#45 Midnight Dan

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Posted 24 August 2013 - 09:14 AM

Good plan! I think that should work well for you. Don't forget the rebar in the concrete. 2 pieces should be plenty, but many people use 3.

-Dan

#46 dawziecat

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Posted 24 August 2013 - 09:40 AM

Good plan! I think that should work well for you. Don't forget the rebar in the concrete. 2 pieces should be plenty, but many people use 3.

-Dan


Hmmm . . . is rebar really necessary?

#47 Midnight Dan

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Posted 24 August 2013 - 09:49 AM

Absolutely! Concrete is extremely strong in compression so rebar will not help one bit in supporting the weight of your gear. But, it is very weak in tension or shear, which means it can easily break due to relatively small sideways forces such as movement of the ground in freeze-thaw cycles.

-Dan

#48 dawziecat

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Posted 24 August 2013 - 10:10 AM

Good plan! I think that should work well for you. Don't forget the rebar in the concrete. 2 pieces should be plenty, but many people use 3.

-Dan


Hmmm . . . is rebar really necessary? I hadn't planned on it. I don't want to mess with the stuff unless I really have to!
I read all about it adding strength against shearing forces but they're not really a concern. All I see rebar doing is getting in the way of properly seating the L bolts and allowing voids to form in the concrete? :question:

#49 dawziecat

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Posted 24 August 2013 - 10:16 AM

Whoops . . . sort of a double posting done inadvertently.

Thanks Dan, I guess a couple lengths of rebar will be necessary. Not happy about it but . . .

#50 tim57064

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Posted 24 August 2013 - 11:46 AM

If you do not use rebar,it will not be long before the pier breaks apart.It is not that expensive here yet not sure where you are located.I am sure though that it would be more expensive to replace the pier.


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