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#1 dawziecat

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Posted 20 August 2013 - 01:21 PM

I am getting ready to commence building a Skyshed roll-off roof observatory.

I already seem to have made my first mistake. I bought a sonotube and Bigfoot thinking an 8" pier would be about right. But, when thinking through how an AP1100 would actually attach to the pier, it appears 8" is not nearly wide enough? I am now thinking a 14" pier is about right. It matters a great deal that this pier not get out of hand as concrete has to be mixed and poured by hand! Redi-mix truck access is just not possible. Going from 8" to 14" has already meant the wheelbarrow will be insufficient for mixing 23 eighty LB bags of pre-mix and I will have to rent a small mixer.

I enlist the opinion of any who have gone this pier route before for guidance on the matter. Pier footing will be put below frost line (4 feet here). I hope to have a steel plate 3/8" thick fabricated to allow mounting of the Astrophysics 119FSA ("flat surface adapter"). The plate will fit onto four threaded half inch rids or J bolts embedded in the pier, thereby providing for a level adjustment.

#2 Aquarist

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Posted 20 August 2013 - 01:40 PM

Well ATS recommends for Astro-Physics AP 900GTO Mounting (AP adapter)an 8 inch pier and for an Astro-Physics - AP 1600GTO Mounting (AP adapter), a 10 inch pier.

#3 roscoe

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Posted 20 August 2013 - 01:47 PM

Terry,

I admit to being a frugalist/minimalist, but I also have been a carpenter for many years, and mixed many a bag of premix in my day.....and it's my private belief that most ATM scope piers could nearly anchor one end of the Golden Gate Bridge......so it certainly seems to me that 8 would do the trick, 10 would be plenty rugged unless you were hanging a huge refractor on it..... Mine, built to accommodate a (still in the 'hopefully pretty soon' stage) 6" f/12 refractor, is 10 to a bit above ground level and 8 for another 4 1/2 feet. If I kick it hard, it'll jiggle an image for a couple of seconds, but I don't kick it very often in real life......... it seems to handle a 120/8 effortlessly.....

Russ

#4 Gastrol

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Posted 20 August 2013 - 02:01 PM

I agree most poured piers are a complete overkill. I personally would follow aesthetics and determine a pleasing diameter relative to length and the mount it will be supporting.

#5 Kraus

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Posted 20 August 2013 - 02:16 PM


My pier is 12 inch diamter, three feet tall. I got the form from Lowe's. A big bucket is useful for mixing cement. Six bags for a solid base and four bags to fuill the tube-a-dube-dube.

#6 Christopher Erickson

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Posted 20 August 2013 - 03:00 PM

A concrete pier can be hollow and not lose any of its rigidity.

Two concentric Sonotubes can do the trick and save you a lot of grief in concrete work, expense and weight.

The concrete wall thickness should not be thinner than 2".

You can also insert a short piece of PVC pipe through the two Sonotubes near the base and route cables inside the hollow pier.

Make sure to put metal mesh between the two Sonotubes for concrete strength.

The center of any pier contributes nothing to the rigidity of that pier.

My rule of thumb is a concrete pier diameter no smaller than the aperture of the scope that will be put on it.

If the pier is more than four times taller than it's diameter, increase the diameter of the pier to maintain at least a 4:1 ratio, or better.

If you decide to cheat a bit, do not exceed a 5:1 ratio.

I hope this helps.

#7 tim57064

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Posted 20 August 2013 - 03:12 PM

I have never heard of anyone using a hollow concrete pier for mounting a scope. If that is what you are describing,would that not present more of a problem with vibration transferring to the mount? Curious? :question:

#8 Christopher Erickson

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Posted 20 August 2013 - 03:24 PM

I have never heard of anyone using a hollow concrete pier for mounting a scope. If that is what you are describing, would that not present more of a problem with vibration transferring to the mount? Curious? :question:


Like I said, the center of a concrete pier contributes NOTHING to the rigidity of the pier.

#9 tim57064

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Posted 20 August 2013 - 03:32 PM

Would have been nice if I had tried that first instead of building a 16" diameter one 6' high.

#10 Midnight Dan

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Posted 20 August 2013 - 05:18 PM

A lot depends on how far the pier is sticking out of the ground. But for my 2 cents, 8" is not nearly enough.

I just recently put in a pier that is 4 feet in the ground (frostline) and 5.5 feet above the ground. The base is a foot thick and 2 feet in diameter. I started off thinking an 8" or 10" would be fine but I went 12" and am very glad I did. If I put a hand on one side near the top, and give it a good whack with the other hand, I can feel it vibrate. Mind you, it's a small vibration and damps very quickly, and I would never expect the pier to get hit anywhere near that hard in normal use. BUT ... I was surprised to find it vibrate at all. I thought it would be like a rock, but if you get concrete long and thin enough it can act more like a tuning fork.

That AP is a gorgeous mount. Don't short change it with the pier! If your's is 3 to 5 feet out of the ground, you could probably get by with 12", but I think your original plan of 14" is a good one. You only get one shot at this. Yes, it will take extra work at the mixer, but that work will be long forgotten when you're enjoying the observatory.

-Dan

#11 TimN

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Posted 20 August 2013 - 06:32 PM

I recently put in a pier and used a 14" sonotube with rebars. I then attached a metal pier to the concrete. The local stores had to order it as they only had up to 12". I went down 5 feet to a type of bigfoot. I used a small hand mixer and paid to have the hole dug. My climate would be similar to yours. It may be overkill but I agree with Dan. You only get one shot at it.

#12 roscoe

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Posted 20 August 2013 - 06:42 PM

Another thing to consider is that a tapered pier doesn't suffer from 'tuning fork' vibrations....so while making and securely fastening some plywood rings to your tubes to allow the pier to taper is a whole extra step, it's a good way to build a taller pier....
R

Also, Chris's 'hollow pier' method is a good one that I hadn't thought of......
Another thought, expanding on his 'pvc connector tube' suggestion, would be to install a piece of 3" or 4" PVC drainpipe, with an elbow at the bottom, allowing all manner of wiring to be hidden inside the pier.
R

#13 dawziecat

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Posted 20 August 2013 - 06:49 PM

Well, this is discouraging, really with suggestions all over the clock! :(

The pier will begin 4'6" below grade with another 4'6" extending above grade.

I appreciate concrete is dirt cheap. And now I actually have an excavator on the property, so digging a hole deep enough to get below frostline is a simple matter.

If it were reachable by a redi-mix truck, I'd say "heck with it," and use a couple of yards of concrete, using concrete as backfill instead of disturbed soil. Sadly, I am just not in a position to do that.

Handling 80 lb bags of pre-mix is not a simple matter for me. Unfortunately labour is scarce hereabouts.

I am not really concerned about "kicking" the pier while imaging. I am concerned about any flexing of the pier and any movement of the pier within the ground caused by the load on the mount shifting slowly as it tracks. This installation is pretty much for astrophotography exclusively with apertures ranging to 10 inches, although a 12" is not out of the question.

I thank all for their opinion in this matter . . . even though I didn't necessarily like the message! :D

I do appreciate that this is definitely something you don't want to scrimp on! I think that likely goes a long way to explaining why some folks end up with a support that would "hold up the Golden Gate Bridge!" :)

#14 roscoe

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Posted 20 August 2013 - 10:10 PM

Terry,

With 9 feet of total pier length, I agree 8" isn't sturdy enough, I think 10" is a better choice here, and won't use a whole lot more concrete.
Two suggestions for keeping it from moving in the ground: If you can, have the excavator make the bottom of the hole as flat as is easily attainable, so the bottom of your pier footing doesn't want to roll around on the base, and as you backfill, tamp or stomp the dirt as firmly as you can - like perhaps in 2" layers, so as to anchor the pier in place as much as possible. Loose backfill, that which has been shoveled back in or dropped with an excavator bucket, does very little to stop pier motion higher up. If your excavator has, or can borrow, a 'thumper', a gas-powered compactor (either the pogo-stick version or the vibrating base-plate version) it'll make your backfill a lot more stable.
You might also find it easier to mix your concrete in a mortar trough, a wood box with vertical sides and slanted ends around 18 x 36 x 8 tall, that you leave on the ground, and mix with a hoe. Much easier than lifting of bags of cement mix up into a wheelbarrow.....

Russ

#15 wolfman_4_ever

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Posted 21 August 2013 - 01:14 AM

I have never heard of anyone using a hollow concrete pier for mounting a scope. If that is what you are describing,would that not present more of a problem with vibration transferring to the mount? Curious? :question:


It's one of the reasons why your bones are hollow not to mention bend ability before breakage.

#16 Midnight Dan

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Posted 21 August 2013 - 06:31 AM

Handling 80 lb bags of pre-mix is not a simple matter for me. Unfortunately labour is scarce hereabouts.


When I did my pier, I was able to find someone with a Bobcat that had a fork mounted mixer. This was awesome because the crew could dump several bags into the mixer near the water supply, then take the mix over to the pier and pour it in the top which was over 5 feet off the ground. No wheelbarrows and no lifting!

Bobcats are pretty small and much easier to transport and get to remote locations than a big cement truck. You may want to look into it and see if there's someone nearby that can do this. If they're within 50 miles, it shouldn't be a big deal to get the equipment to you. If you can arrange it, it's well worth the money! Quick and painless:

-Dan

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#17 dawziecat

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Posted 21 August 2013 - 10:01 AM

Wow, Dan! That's quite the rig. Never heard of that before!
As to one being availabe in my area? Well, odds are not so good. "Slim to none." Likely :bawling:

I went to a local metal fabricator today. Seems no problem getting a pier-to-119FSA (AP1100 flat surface adapter) interlink fabricated.

It's going to cost me, but I've decided I want no part of humping 80-lb bags about. I will try the local "No-Job-Too-Odd" crew and pay whatever they will charge me. ( I hope they don't monitor this board!!) My wallet is stronger than my back at this stage of life!

(Did I tell you of the morning I threw my back out just before visiting Kitt Peak? Oh, never mind! :) )

#18 stmguy

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Posted 21 August 2013 - 10:45 AM

That was one of the main reasons I went with 4ft 12" concrete pier with bigfoot and 40 "'s of well casing cause I didn't want to lift the concrete up into a pier. I parked my mixer right over the pier and poured it right in. It works just fine and reasonably easy to modify if I need to change the height later. I almost wished I went with 14 inch concrete pier as the weldable pipe flange put the anchor bolts pretty close to the outside of the pier but I don't see any issues so far
Norm

#19 Midnight Dan

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Posted 21 August 2013 - 12:33 PM

Another way to make things easier, again if the pier isn't too tall, is to put the concrete mixer on the back of a pickup truck.

This is especially handy because you can have the building materials store people load the concrete bags towards the front of the pickup bed when you buy it. Back the pickup up to the pier till the lowered tailgate nearly touches it. Put the mixer on the bed/tailgate and flip the mixing barrel towards the concrete in the front of the bed. Throw a couple bags in, mix with water, and flip the barrel to the other side to pour into the top of the pier.

You, or your crew, will still have to lift the bags to put them in the mixer. But it eliminates handling heavy mixed concrete. If putting the mixer on the pickup bed is just a few inches short of high enough, you can stack some lengths of 2x12 on each other to make a ramp of sorts and gain a few inches. You also have to be sure the pickup tailgate is in decent condition so it will support the weight. Use a drop cloth to keep concrete from spilling onto the tailgate and pickup bed.

Even if you pay a crew, this can really speed things up so you'll have to pay less - assuming you're paying by the hour.

-Dan

#20 dmdouglass

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Posted 21 August 2013 - 01:19 PM

Check out Home Depot (or Lowes) or any equipment rental location. They rent small portable cement mixers that can be wheeled into location. They usually will do 1 or 2 bags at a time. Easy on the back. Set it up right next to the pour...

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#21 roscoe

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Posted 21 August 2013 - 01:36 PM

Another way to make things easier, again if the pier isn't too tall, is to put the concrete mixer on the back of a pickup truck.-Dan


Dan speaks well here......this is a common method for construction crews installing sewer grates and manholes and the like, where they need small quantities in many locations.....

and as was also mentioned, many big-boxes and most all rental centers have mixers to loan out, and many carpenters and farmers have one in the yard somewhere that you might borrow.
R

#22 JJK

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Posted 21 August 2013 - 02:13 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.

#23 Christopher Erickson

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Posted 21 August 2013 - 03:23 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.

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#24 JJK

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Posted 21 August 2013 - 07:37 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.


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.

#25 StarmanDan

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Posted 21 August 2013 - 08:38 PM

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.

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