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#301 John Fitzgerald

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Posted 18 May 2021 - 06:01 PM

Square tubing won't work for a couple of the pier schemes I have designed and built with a rotatable top.  One pipe is a semi snug fit inside another.



#302 macdonjh

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Posted 18 May 2021 - 06:19 PM

Square steel tube beats steel pipe of the same size and wall thickness.  For your 6x6x1/2 square steel tube, I=50.5.  It's even stiffer in the diagonal axes.  In comparison, 6" extra strong steel pipe has a wall thickness of 0.432 and I=40.5.  In this case, there is about 20% less deflection in the square tube.  For standard weight pipe, I=28.1  The flat side can be nice for mounting things on it.

 

Multiple smaller members can work very well since the moment if inertia (I) is based on the assembly rather than simply the sum of the individual moments.  So, 4 smaller pipes can easily beat a much larger single pipe when the assembly of them is spaced out.  Look at rimcarzy's assembly of smaller parts in post 269.  Also, think of a roof structure where open web steel joists are as stiff and wide flange beams but maybe less than half the weight.

For those of you wondering, the reason why things like open web steel joists and piers made of several smaller columns welded together aren't used all the time is the complexity of fabricating those assemblies as well as the cost of the labor to do so.  Sometimes it's cheaper to use something bulkier and heavier that something fabricated but light.  There's no such thing as a free lunch, as they say.

 

Square tubing won't work for a couple of the pier schemes I have designed and built with a rotatable top.  One pipe is a semi snug fit inside another.

You could make the majority of your pier from square tubing.  Then put a cap on top, a short piece of the larger diameter pipe on the cap and then nest the smaller pipe inside.  That way almost all of your pier is square tube and you only use the pipe for the part which actually rotates.

 

Another thought: install a cap plate on top of the square tube, fabricate a plate to attach your mount to, finally install a lazy Susan-type bearing between the two plates.



#303 John Fitzgerald

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Posted 18 May 2021 - 07:51 PM

For those of you wondering, the reason why things like open web steel joists and piers made of several smaller columns welded together aren't used all the time is the complexity of fabricating those assemblies as well as the cost of the labor to do so.  Sometimes it's cheaper to use something bulkier and heavier that something fabricated but light.  There's no such thing as a free lunch, as they say.

 

You could make the majority of your pier from square tubing.  Then put a cap on top, a short piece of the larger diameter pipe on the cap and then nest the smaller pipe inside.  That way almost all of your pier is square tube and you only use the pipe for the part which actually rotates.

 

Another thought: install a cap plate on top of the square tube, fabricate a plate to attach your mount to, finally install a lazy Susan-type bearing between the two plates.

Unnecessarily complicated to me.  The ones I built work fine.  I like sleek and simple.  They only rotate to facilitate initial polar alignment.  Once that's complete, they never move unless the pier itself is relocated.  That's happened once.


Edited by John Fitzgerald, 18 May 2021 - 07:54 PM.


#304 LeftCoastAP

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Posted 03 June 2021 - 12:15 PM

Greetings all.  I'm in the process of building a backyard observing station and this thread has proven tremendously helpful in designing the pier.  So at the outset, thank you to Speedster for starting the thread and for posting so many thoughtful engineering insights -- and for everyone else who has posted contributions and pictures.  It helped me settle on what I think is a good design to support a fairly light load (10Micron GM1000 with a TEC140 APO scope).  I recently completed the pier construction and thought it might help others to post some pictures.  

 

By way of quick description, I followed the advice on this thread and decided a deep, narrow footing made more sense than a spread footing.  So, ultimately, I dug a 5ft deep hole of 12" diameter with a wider 24" square top to support the planned pedestal.  The formed top required a little bit of backfilling to make room for the form but I was careful to leave undisturbed soil throughout the deeper dig so the cylindrical bottom of the footing is poured directly against undisturbed soil.  

 

I ordered a Nexdome pier with a 10Micron adapter.  Nexdome provided me a detailed schematic of the bolt pattern at the base so, while waiting for the pier to ship and clear customs, I was able to make a template using a small piece of King Starboard from Boat Outfitters.  In hindsight, I would have done the pattern in cheap wood but, at the time I ordered the starboard, I thought of mounting it permanently to the footing as a barrier between the pier and footing.  Ultimately I decided that would introduce a point of potential deflection so I just went with a paint barrier between the steel and concrete.

 

Here's a picture of the site I selected.  This location provides me a bit of shelter from a light dome to my southeast due to a local high school with bright lights and provides good lines of sight to the south, west and north.  Unfortunately the topography and hardscaping in my yard preclude any site that has good 360-degree viewing so I picked a location that put the obstructed viewing in the same direction as my brightest horizon:
20210320 172048
 
 
Some shots of the excavation, the wood form for the top base and the concrete pour:
 
20210403 101529
20210403 110301
20210403 111425
20210403 115749
20210403 101550

 

Here's the starboard template with j-bolts installed and ready to be set into the concrete:

20210403 111353
20210403 111345

 

Here's the template set in the concrete.  The second pic shows the clearance.  I used a couple offset wood slats to keep the template high enough to keep the lower bolts clear of the concrete. The ultimate goal was to have the J-bolts set in the concrete in perfect position for the pier with the threaded portions fully exposed and the unthreaded fully set in the concrete:
20210403 135727
20210403 135744

 

Once the concrete was mostly set, it was time to remove the template and finish the concrete surface:
20210403 142445

 

Letting the concrete dry:

20210404 124853

 

About 30 days after the pour, it was dry enough to paint:

20210501 161118

 

I have to admit that when the Nexdome pier arrived, there was a nervous moment of truth when the time came to set it on the bolts.  I had measured and remeasured a half dozen times to ensure that the starboard template was perfect and the bolts were exactly where they needed to be but still felt very anxious about the prospect of discovering a bolt was off by a millimeter and visions of drilling a bigger hole in the pier or cutting off the bolts and replacing them with tap-in anchors.  Fortunately, the pier fit perfectly:

20210502 145412
 
The last shot above also shows the electrical outlet I added next to the pier but that's a project for a different thread.  This shot also shows the 10Micron adapter at the top of the pier.  The mount is in transit and should be installed on my next cloudy weekend.  Unfortunately, I don't expect the TEC 140 that's ultimately going on this pier until December but I'll set it up with my small scope (Tak FSQ-85) for some summer nebula imaging while I wait, and will post pictures of the full install with the scopes.
 
Overall, this project was relatively manageable and interesting.  It seems rock solid -- although I won't really know until I take some really long exposures on the 140 -- and I'm looking forward to getting the mount installed and polar aligned/mapped so I have the option of imaging whenever the skies allow without having to re-align and create a new model every session.  I live in San Diego and we have a relatively common pattern of a night that has a few hours of clear skies and a few hours of marine layer so being nimble enough to take advantage of the clear windows when they arrive and not wasting imaging time on alignment and model building is really appealing.  
 
A few safety precautions I should also note:  The outlet is fully waterproofed, GFI protected and was installed by a qualified electrician.  I confirmed the location of electrical, gas and water lines before excavating - and had to re-route one sprinkler line to get it out of the conduit trench.  I have Telegizmo covers for both scopes and the mount as well as heavy-duty desiccants to reduce moisture under the covers and plan to move the mount inside whenever rain is in the forecast (fortunately, a rare event in Spring, Summer and Fall in San Diego).

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#305 LeftCoastAP

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Posted 03 June 2021 - 12:18 PM

Apologies for the landscape images posted in portrait mode.  I thought I figured out how to rotate them in my gallery but apparently not.  



#306 tommyboy

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Posted 17 August 2021 - 04:11 PM

Leftcoast;

 

how did you ensure that there were no voids around the j bolts beneath your template? Did you just "pound" the template into the wet concrete?



#307 macdonjh

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Posted 17 August 2021 - 10:15 PM

Leftcoast;

 

how did you ensure that there were no voids around the j bolts beneath your template? Did you just "pound" the template into the wet concrete?

The guaranteed method is to rent a concrete vibrator.  I wish I had...  However, if you push your anchor bolts/ template into your concrete and pull them out and shake and jiggle them several times, the concrete should settle in around your anchor bolts just fine.  You can also use a smallish rod to poke under your template and push the concrete around the bolts.



#308 plarche

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Posted 31 August 2021 - 03:59 PM

I’m building a pier for a Celestron CGEM II mount on solid bedrock on Georgian Bay in Ontario.
Based on all the reading here (thanks all), my plan is to use a 10” 3-foot tube.  I will drill 3 holes 10” deep and secure 3 30”x1/2” rebar with epoxy and use plumb to ensure they are vertical.  I’ll tie the rebar with tie wire.  I will then level the tube vertically over the rebar and pour cement directly into the tube onto the bedrock.
Once levelled, I will insert the 4 J bolts into the wet concrete with a wood template, ensuring all is level and set for magnetic north.
I’ve attached a picture of the bedrock and plan (not to scale).  Any advice on improvements would be appreciated.

Thanks in advance...

IMG 1345
Album: Pier
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Pier in Rock

 



#309 kathyastro

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Posted 31 August 2021 - 05:30 PM

 

I’m building a pier for a Celestron CGEM II mount on solid bedrock on Georgian Bay in Ontario.
Based on all the reading here (thanks all), my plan is to use a 10” 3-foot tube.  I will drill 3 holes 10” deep and secure 3 30”x1/2” rebar with epoxy and use plumb to ensure they are vertical.  I’ll tie the rebar with tie wire.  I will then level the tube vertically over the rebar and pour cement directly into the tube onto the bedrock.
Once levelled, I will insert the 4 J bolts into the wet concrete with a wood template, ensuring all is level and set for magnetic north.
I’ve attached a picture of the bedrock and plan (not to scale).  Any advice on improvements would be appreciated.

Thanks in advance...

 
 

 

 

Magnetic north??  Your pier cap should be oriented towards true north, not magnetic.  Admittedly, in the Georgian Bay area, the magnetic variation is probably low enough that the discrepancy will be within the adjustment range of the pier plate and/or mount.  But you may as well start with the best possible alignment.


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#310 rms40

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Posted 31 August 2021 - 07:01 PM

Kathy makes a good point. I used magnetic north once and could not get the mount polar aligned once everything was bolted down.

 

My method the next time was to hang a weighted cord over where the pier would be and then mark the shadow on the floor at the sun transit time. That worked great. You just have to be sure you use the exact time for transit. In the summer here in central Texas, it was around 1:32 pm (CDT) on the day I marked it.

 

Randall


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#311 tommyboy

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Posted 31 August 2021 - 07:57 PM

How about correcting according to the calculation found here?

 

https://www.ngdc.noa...s/magcalc.shtml

 

41° 0' 22" N
74° 31' 18" W

12° 29' W correction


Edited by tommyboy, 31 August 2021 - 08:06 PM.


#312 plarche

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Posted 01 September 2021 - 10:25 AM

Thanks for the suggestions on true north.  Much appreciated.  Any suggestions on the build would be appreciated.



#313 speedster

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Posted 01 September 2021 - 08:26 PM

Is the top half of the pier really unreinforced?  Rebar extends to 3" from top.  Layout holes in the rock so rebar doesn't interfere with anchor bolts.  Round #3 ties take care of shear loads (tie wire won't help with this). 



#314 plarche

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Posted 02 September 2021 - 09:39 AM

Thanks so much Speedster.  Just to confirm, I should run the rebar 10" deep and up to 3" below the top? I gotcha on the rebar interference with anchor bolts.  Sorry, I'm not sure what you mean on #3.  Are "ties" different from tie wire?  I really appreciate everyone's input.  New to this.



#315 speedster

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Posted 02 September 2021 - 11:17 AM

Yes for the rebar all the way up.  #3 is 3/8" rebar.  Rebar is measured in eighths so your 1/2" rebar is 4/8" or #4.  Forces in the above ground portion of concrete telescope piers are very small so people can do all sorts of crazy stuff and it still works fine.  Concrete is strong in compression but weak in everything else.  The vertical rebar takes care of tension loads but we still have shear to take care of.  Ties are reinforcing around the perimeter of the pier to take care of shear.  They might be called rings, hoops, or stirrups. They are actually "ties" if in a column and "stirrups" if in a beam.  Unless you hit the pier with a truck, you won't see much shear.  Imagine some large force pushing horizontally on the top of the pier:  One side goes into compression and the other goes into tension.  At some amount of force, the the pier will try to split vertically long the neutral axis in the middle of the pier.  The ties act to prevent that splitting.  Probably more than you want to know and you'll never see that kind of load in the part above the ground.  Ties are good practice and are also very convenient for holding the vertical bars together in a cage.  Trivia:  steel is used to reinforce concrete because is has the same coefficient of thermal expansion so the assembly doesn't tear itself apart with temperature changes.


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#316 plarche

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Posted 02 September 2021 - 12:13 PM

Thanks so much Speedster.  Totally got it.   And appreciate the education and trivia.



#317 macdonjh

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Posted 02 September 2021 - 09:05 PM

One other bit of information to add to what kathyastro and speedster said:

 

It's best practice to keep any reinforcing steel 2"-3" away from the outside surface of the concrete.  Since that won't allow for much reinforcement in a 10" pier, try to keep your rebar within an 8" circle centered in your Sonotube form.  That will provide 1" of "cover" all the way around your pier.

 

The reason for the cover is corrosion protection.  If any of your reinforcing steel gets exposed (by touching your forms while you pour your pier, or if a piece of concrete someday "spalls" off exposing the rebar) the steel will eventually rust, expand, and crack your pier.  As speedster says, this is another situation unlikely to happen with the way we load our piers, but why not use best practices when possible?

 

Oops, one more suggestion: if possible, keep you anchor bolts within your rebar cage (so within that 8" circle).  That way your top tie will go around your anchor bolts, preventing them from coming out the side of your pier.  Again, not likely to happen, but another good practice.



#318 John Fitzgerald

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Posted 02 September 2021 - 09:34 PM

I have said this before in other threads, maybe this one, but the best pier form I have found is a piece of PVC, SDR 26 sewer pipe. 

 

My last pier was poured using a scrap piece of 12 inch SDR 26 from scrap they were going to haul to the landfill from a public works site. 

 

With such a form, it can be left in place indefinitely, and makes a nice, very smooth outer finish.  Since it's in an observatory, the PVC won't be bothered by sun exposure. 

 

In a fairly large city, scrap pipe like that is easily found.  They are always replacing sewers somewhere.



#319 speedster

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Posted 03 September 2021 - 03:04 AM

+1 on PVC.  I like it.

 

IMG_2476sm.jpg



#320 macdonjh

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Posted 03 September 2021 - 08:33 AM

I have said this before in other threads, maybe this one, but the best pier form I have found is a piece of PVC, SDR 26 sewer pipe. 

 

My last pier was poured using a scrap piece of 12 inch SDR 26 from scrap they were going to haul to the landfill from a public works site. 

 

With such a form, it can be left in place indefinitely, and makes a nice, very smooth outer finish.  Since it's in an observatory, the PVC won't be bothered by sun exposure. 

 

In a fairly large city, scrap pipe like that is easily found.  They are always replacing sewers somewhere.

+2



#321 tommyboy

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Posted 03 September 2021 - 05:46 PM

I've been looking for 12 inch PVC and it's not generally available in my area. Any online recos?

 

PS I'm not in or near a large city.


Edited by tommyboy, 03 September 2021 - 05:47 PM.


#322 speedster

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Posted 03 September 2021 - 08:46 PM

https://www.pvcfitti...ft-section.html

 

Pricey.  A local well driller is liable to have some.  Corrugated HPDE culvert can be had from most any local lumber yard.


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#323 archer1960

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Posted 03 September 2021 - 08:54 PM

https://www.pvcfitti...ft-section.html

 

Pricey.  A local well driller is liable to have some.  Corrugated HPDE culvert can be had from most any local lumber yard.

The price goes down fast as the pipe size goes down. 10" is about 30% less expensive than 12".



#324 John Fitzgerald

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Posted 03 September 2021 - 08:58 PM

Mine was free from scrap.  Any fairly big sewer project will have scrap.  There's always waste, and pieces with the hubs broken.  Even our little city of 3,800 has a scrap pile (from repairs) that they haul away periodically.  There's nearly always a bit of 12 inch in it.  You don't need high pressure rated schedule 40.  Schedule 26 or 35 is just fine. Once you get to 10 to 12 inches, it's plenty thick and tough anyway.



#325 DesertCrawler

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Posted 13 September 2021 - 03:54 PM

Great thread! I very much appreciate the engineering and pragmatic approach, with math and physics to back it up.

 

If this detail was in buried within, I missed it: As to finish leveling the concrete pier top, a quick check shows some self-leveling epoxy resins show compressive strength ranging from 3900 psi to 7500 psi depending upon compound. That's JB Weld territory at the lower end. This should be sufficient to resist (significant) deflection in the mount, yes? I realize the mount needn't be level to achieve PA, and a proper GEM won't care much.




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