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Pylon Screw In Type to support telescope Pier

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#1 sberrada

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Posted 23 March 2020 - 01:29 PM

Hi,

 

i will be using pylons (screw in type) to support the 6 points of my ROR observatory.  They will be 7 feet long and 3.5” diameter. 
 

Has anyone used screw-in pylons to support their telescope pier ?  there are many types of pylons, some 10 feet long and one 5” dia. And can handle 5000 lbs of lateral load and 50K pounds compressive force   This would replace a 12inch sonotube cement pier.

i have no doubt about compressive resistance, but wonder about lateral movement ?

 

I would appreciate your thoughts

 

thanks 

Sam

 

 



#2 speedster

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Posted 23 March 2020 - 04:47 PM

A helical anchor can certainly be good for 50k pounds but that is just the anchor itself.  The soil that supports the anchor is only good for 17.36 psi, assuming "ordinary" soil.  So, a 3.5" diameter shaft with 8" diameter flight won't break until the load hits 50k pounds but your soil may shear when the load hits 700 pounds.   They just don't have the bearing area needed until you get into larger sizes.  With 7' of depth, you have the opportunity to use multiple flights to increase total soil bearing capacity.  2,500 PSF is a presumptive bearing capacity and actual capacities range from 1,500 PSF to 4,000 PSF for sedimentary rock.  The point is, that 50k pound capacity may really only be about 700 pounds in application. 

 

Lateral movement is basically a function of width and, all others things being equal, a 5" shaft sees less  than half the lateral resistance of a 12" shaft.  A helical anchor sees less lateral resistance than a driven or bored pile because the flights disturb the soil around the shaft.  No problem for your building but there are stiffer alternatives for a pier.



#3 macdonjh

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Posted 23 March 2020 - 05:51 PM

+1 to Speedster's explanation.  I've seen helical piles used often in recent industrial construction, but never a single pile for a single column.  The minimum I've seen used is two, supporting the two columns of a pipe support shaped like a football goal post.  More often, a "grid" of piles supporting a foundation, like you plan for your observatory.  That way the helical piles can be linked together to resist lateral movement as a connected "team".

 

All that said, I think your observatory foundation could be a good application.



#4 Stevegeo

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Posted 24 March 2020 - 04:03 AM

For the observatory  they are more then adequate.  As for a pier , if your scope isnt that heavy , and the pier height isnt abnormally tall ,say over 4 ft .. it will as well function. HOWEVER, to improve your pier and resistance to bending, or vibration an easy solution is a concrete mix, or sand in the tube to reduce inertia . Sand, make sure you tamp as you go along use a broomstick.. with concrete, make it slightly like molasses, pour slowly and whack the tube to release air bubbles ..

 

Stevegeo 



#5 sberrada

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Posted 24 March 2020 - 08:41 AM

Hi Everyone,
 
I appreciate your valuable comments.

In summary, it sounds like a screw-in pylon May not be ideal to support a telescope pier.
My pier will be about 4 feet high, but when you add the mount (50lbs) and scope (40lbs), this increases height and weight.  
i appreciate the idea of pouring concrete or sand in the pylon - this would increase the weight and will surely help.
 
If I put my engineering hat on, I would argue the following:

- resistance to lateral bending of pylon  = pylon side cross section which pushes against soil x centroid distance (below surface) x soil resistance force F (I have a sketch but can’t seem to attach it) 

- So a 3.5 inch x 7 feet pylon (6 feet below surface) would be = F x (3.5 x 72) x 36 = F x 9072
- A 3.5 inch x 10 feet pylon (9 feet below surface) would be = F x (3.5 x 108) x 52 = F x 19656
- A 12”dia concrete tube 4 feet below surface = F x (12 x 48) x 24 = F x 13824
- A 5 inch x 7 feet pylon (6 feet below surface) would be = F x (5 x 72) x 36 = F x 12960
 
It indicates to that a 3.5” pylon x 10 feet (9 feet below surface) would resist lateral bending more than a 12 inch cement sonotube 4 feet below surface.
 
Does this make sense ?
 
I realize things are more complex when one starts to factor in vibration and other elements ...
 
I appreciate your input 
 
Sam



#6 speedster

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Posted 25 March 2020 - 03:57 PM

You can simplify this by thinking of the pylon or pier being a vertical column hinged at its base.  The actual hinge point may be above the base but it doesn't matter in this case.  The lateral resistance of the soil is simply a matter of area.  3.5" helical shaft x 9' = 378 si.  12" concrete shaft x 4' = 576 si.  The 12" shaft, regardless of what it is made of, wins.  The flights of the helical anchor disturb the soil adjacent to the shaft as it is drilled so the lateral resistance of the soil is a good deal less than undisturbed soil.   Sonotube below grade is a no-no for telescopes.  Back fill around the sonotube will take a decade or two to consolidate back to the resistance of undisturbed soil, even if you do your best to compact it.  Pour concrete piers against undisturbed soil.  Although concrete is more flexible than steel, it makes up the difference with area.  An 8" steel pile is stiffer than an 8" solid concrete shaft.  But, if we consider the conditions in the pier engineering thread here on CN, at 48" in height, an 8" steel pipe has a deflection of 0.167" while a 12" concrete pier has much less deflection at 0.074".  A little more diameter can greatly shift the results.

 

Structurally, sand is a fluid so filling a pipe with sand changes the periodic frequency but does about nothing for deflection.  We are worried about amplitude, not frequency.  Filling a pipe with concrete can help some.  Concrete is more plastic than steel so the skinny concrete shaft inside the pipe just can't do much to help.  

 

If your underground part is in disturbed soil, like a helical anchor or sonotube scheme, your length for calculating deflection starts at the bottom of the pier.  If your underground part is in undisturbed soil, your length starts at the soil surface.  Deflection is a function of the cube of the length so this makes a big difference.



#7 sberrada

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Posted 25 March 2020 - 08:28 PM

Hi 

Thank you for the detailed explanation.

 

I am not sure that I Understand this part:

”Sonotube below grade is a no-no for telescopes.  .... Pour concrete piers against undisturbed soil.”

 

I live in Canada, so we need to dig a hole about 4 to 5 feet deep (frost), insert the cardboard tube, and fill it with concrete, so the soil would be disturbed.

 

Is there a way of pouring concrete in an underground sonotube without disturbing soil ?

thanks

Sam



#8 kathyastro

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Posted 26 March 2020 - 09:46 AM

I think the recommendation is to pour concrete straight into the hole with no form.  If you use forms, whether wood or sonotube, you have to backfill, which means the column is supported by disturbed soil, not undisturbed soil.



#9 macdonjh

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Posted 26 March 2020 - 10:49 AM

Hi 

Thank you for the detailed explanation.

 

I am not sure that I Understand this part:

”Sonotube below grade is a no-no for telescopes.  .... Pour concrete piers against undisturbed soil.”  Really, forms below grade are a no-no.  More detail below.

 

I live in Canada, so we need to dig a hole about 4 to 5 feet deep (frost), insert the cardboard tube, and fill it with concrete, so the soil would be disturbed.  Just use an auger to drill down 4ft- 5 ft, set your Sonotube on the ground above your hole and fill both the hole and your Sonotube with concrete.  The concrete will be filling the hole in the ground up against undisturbed soil, and above ground it will fill your Sonotube.  That's less work for you (smaller hole, no back filling) and better construction practice.  Win- win.

 

Is there a way of pouring concrete in an underground sonotube without disturbing soil ?  No, which is why it's recommended not to use forms below grade.

thanks

Sam

Some comments in red

 

I think the recommendation is to pour concrete straight into the hole with no form.  If you use forms, whether wood or sonotube, you have to backfill, which means the column is supported by disturbed soil, not undisturbed soil.

+1

 

I hope this helps: what we're trying to explain is whatever concrete you pour below grade (below ground level) should be poured against undisturbed soil, so do not put forms (like Sonotube) below ground level.  

 

If you put a "big" form, like a box made from plywood or something, below ground, you'll have to remove all that wood from your great big hole and then back-fill around your concrete.  As Speedster has said, even properly compacted back fill isn't as good as undisturbed soil.

 

If you use an auger to drill a smaller hole, then drop Sonotube into the hole and fill it the Sonotube with concrete, your augered hole will still be larger than your Sonotube and you'll have to back fill the gap between the Sonotube and the edge of your hole.  Even worse, over time (OK, a lot of time, but it will happen), the Sonotube will rot and there will be a gap of loose material between your concrete pier and the surrounding soil.

 

If you're going to use an auger to drill a hole for the foundation of your pier, I suggest this:

  • Pick an auger which has the same diameter as the pier you'd like to build (or maybe even a bit larger).  My local rental store has 6", 8", 10" and 12" diameter augers for post hole diggers.  I used a 12" auger.
  • Drill your hole below your frost line and clear as much soil from the bottom of your hole as you can.
  • Set your rebar cage into your hole.
  • Set your Sonotube form over the top of your rebar cage.  It's OK to push your Sonotube a couple of inches below the top of the dirt; that way you'll have nice, even looking concrete above ground.  For my pier, I put the bottom of my Sonotube 4" below grade.
  • Build some braces out of lumber to keep your Sonotube plumb- it's certainly tip a bit as you're dumping concrete into your form.  
  • Pour your concrete.  Make sure to keep your rebar as close to centered in your Sonotube as possible.  Pour slowly and either tamp the concrete with a small board (like a 2"x2"), or even better rent a concrete vibrator where you rented your auger.  
  • Set your pier plate it the wet concrete and get it oriented north (if required) and level it.  Then pack the concrete around your anchor bolts again.
  • Let the concrete start to harden, then use a trowel to smooth the top of your pier.
  • Let the concrete set.  In a day or two, remove the Sonotube (you should be able to unwrap it, like the cardboard container for Pilsbury biscuits.  If not, use a box knife to cut it.  Remove all of the Sonotube.
  • If you put your Sonotube into the ground a few inches, move some dirt around the base of your pier to fill in the "dent" and pack it down with your foot.

That's pretty quick-and-dirty, I hope I didn't miss anything.  There are variations, too.  Perhaps you only want concrete to be a few inches above (or below) grade so you can bolt a metal pier to the concrete.  Perhaps you want some conduit running through your pier so you can have your various cables inside your pier (I did this, one conduit for 12V power, another for USB cables).  You just have to decide what you want and adjust your plan accordingly.  That's one of the reasons I like using concrete for piers: you can do most anything you like yourself.


Edited by macdonjh, 26 March 2020 - 10:54 AM.


#10 macdonjh

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Posted 26 March 2020 - 11:38 AM

Here is a quick sketch of what I tried to describe above:

 

Pier.jpg

 

And, I had an idea last night: there might be a way to use the helical piles you want to use for your observatory foundation for your pier, too.  Simply sink four helical piles fairly close together in the location you want your pier.   Then, pour a small concrete slab over the tops of the helical piles to join them together.  That's called a pile cap.  It kind of makes a group of piles work together, the civil engineering version of several telescopes working together as an interferometer, making a giant telescope.

 

Pile cap.jpg

 

The pile cap could be slightly below grade, flush with grade, or slightly above grade.  Since it'll only be a few inches thick you can dig out for forms to make clean edges and then back fill around the perimeter without lessening the strength of your helical pile foundation.  You could either set a piece of Sonotube above the forms for your pile cap and make a monolithic pour, pour a concrete pier separately after the pile cap has set, or bolt a metal pier to the pile cap after the concrete has cured.

 

Just a thought.


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#11 kathyastro

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Posted 26 March 2020 - 12:03 PM

Here is a quick sketch of what I tried to describe above:

 

attachicon.gifPier.jpg

 

And, I had an idea last night: there might be a way to use the helical piles you want to use for your observatory foundation for your pier, too.  Simply sink four helical piles fairly close together in the location you want your pier.   Then, pour a small concrete slab over the tops of the helical piles to join them together.  That's called a pile cap.  It kind of makes a group of piles work together, the civil engineering version of several telescopes working together as an interferometer, making a giant telescope.

 

attachicon.gifPile cap.jpg

 

The pile cap could be slightly below grade, flush with grade, or slightly above grade.  Since it'll only be a few inches thick you can dig out for forms to make clean edges and then back fill around the perimeter without lessening the strength of your helical pile foundation.  You could either set a piece of Sonotube above the forms for your pile cap and make a monolithic pour, pour a concrete pier separately after the pile cap has set, or bolt a metal pier to the pile cap after the concrete has cured.

 

Just a thought.

I actually did something like this at my first observatory installation.  I drove four five-foot lengths of 1.5" threaded rod into the ground in a square, using a sledgehammer.  The base of my pier was a wood square, 1.5" thick and about 20" square, drilled to fit over the top ends of the rods, with a nut below and above.  The weight of the pier rested on the ground, but the rods held it rigidly in place.  It was a very successful installation, with no vibration at all.



#12 speedster

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Posted 26 March 2020 - 03:39 PM

Mac and Kathy's multiple small diameter piers with a pile cap works.  Four 3.5" shafts, if far enough apart, have 4 times the horizontal bearing surface as a single shaft.  But, the cost and complexity is sort of a Rube Goldberg solution to a single simple hole filled with concrete.

 

Kathy's driven piles can be scaled up for a very stable situation.  With some rather large equipment, we've driven 36" steel casings over 40' deep.  Just takes a big enough crane with a big enough hammer.  There are concrete piles as small as 12" diameter.  Generally not practical but, if you are in a coastal area where these smaller piles are typically used, you might have reasonable access to someone with equipment to drive you a pile, concrete or steel, and go well over 10' deep.

 

We can whack loose dirt with the end of a 2x4 and feel like we've accomplished something.  To put compaction in perspective, here is what happens to get 95% of maximum density:  choose select fill material, lab test to determine optimum moisture content for maximum density, moisture condition the fill material, place it in 6"-8" lifts (layers), compact each layer with equipment exerting about 1,000 psi for multiple passes, testing lab does a nuclear test to determine the density, if it fails - keep rolling, if it passes - spread the next lift.  Doing this in the annulus around a telescope pier simply won't work so, if you form something below grade, like using sonotube below grade, you just wait 10-20 years for gravity to do it for you.  This sounds like absurd overkill but we are talking about allowable deflection of only arc-seconds.

 

Consider a pier 5' in the ground and 4' above the ground.  If the pier is poured with forms below grade (like sonotube or a plywood box), deflection is nearly 12x that of a pier poured against undisturbed soil.  Piers are one of the few cases in astronomy where "better" is also cheaper and faster.



#13 Luke Skywalker

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Posted 27 March 2020 - 08:45 PM

I live in Northern Ontario and the idea of not using a sonotube below ground may work where their is no frost like Arizona and other similar places. Where there is frost, simply digging a hole and filling it with concrete, you will have a rough surface on your concrete post below ground giving the frost something to grab on and eventually work it up. I am talking from experience. You need to put a sonotube down at lease six feet. I have seen the frost go down to height feet where I live resulting in water main brakes. Having a slick surface will prevent the frost from grabbing and moving your concrete post/pier. When you start to backfill, only put four to six inches of gravel at a time and use the but end of your shovel handle to pack the soil down. The bracing that you put to hold the sonotube, do not remove them right away. Wait a few days and try packing again but this time add a little bit of water to the ground. You can use a plate tamper to finish packing the top.



#14 sberrada

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Posted 27 March 2020 - 11:34 PM

Hi Everyone,
 

I appreciate all your thoughts and ideas - this certainly provides more insight and leaves me with a better understanding of the alternatives.

 

The notion of having undisturbed soil is a big takeaway for me.

 

However, here are some challenges that I face:

- the temperature can be very cold in the Laurentiens with -25 to - 30C regularly during winter; this brings the frost lines down to a depth of 5+ feet.

- I have rocky soil, so I am not sure that an auger would work or reach 60+ inches deep (I can try to see);

- the freezing / thawing tends to grab a rough surface underground  and move it up (interesting that the pylon company says that they use plastic sleeves to mitigate this).

- I have a metal pier that I built which will be at the ROR floor level (about 12” above ground), and will be mounted on this underground pier (which will stick out about 12” above ground )

 

As well, I just read in a construction guide (for Quebec) the following :

“Here are a few more good practices for building stable, durable concrete pillars:

Never pour concrete directly into a drilled hole; the resulting irregular shape will increase the risk of frost heave;  Only use cardboard tubes with a diameter of 20 cm (8 in) or more;
Make sure each pillar projects at least 20 cm (8 in) above the ground;
Strengthen the pillars using reinforcing rods;”

 

Honestly, the simplest approach would have been to use a heavy duty 5-1/2” dia x 7 foot long screw-in pylon which is used for lamp posts, but I have yet to hear about any amateur astronomer that has used a pylon for the base of his pier, so it would leave me in uncharted territory.

 

The idea of small pylons supporting a concrete base leaves me concerned about the strong heaving action in my area.

 

So, unless anyone has more ideas, I believe that I am back to using a 12” sonotube with the understanding that I would use an auger if possible, or otherwise compact the soil as best I can.

 

Once again, I really appreciate your input which has been tremendous is helping me better understand the challenges and opportunities.

 

clear skies,

Sam



#15 Stevegeo

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Posted 28 March 2020 - 04:00 AM

Living north like you, most of my soil was undisturbed since the last ice age ,  and also my frost level is DEEP, 4 ft plus.

A sonotube sunk to the frost line ,has mass which makes it ideal for vibration less viewing,  also the reason. I chose to sink mine as a TUBE IN THE GROUND rather then a tube ON TOP, was this.... 

A tube in the ground sunk to below the frost line has support from the soil ( undesturbed),  while the soil around provides support laterally.  During the cold, the ground will heave ever so slightly around the tube from moisture freezing ,yet the mass of the tube will not let it move, and the tube itself will let the the soil surround SLIP as it freezes.  If the tube was on top with just a hole bored and poured I felt that the rough surface from digging ,though some would argue would provide better support ( it may in dryer soils that dont freeze), this rough surface would as the ground froze and heave move the pier as this would have more surface area to grab. 

 

I know some may argue this point,  probably an experiment of two piers side by side  4 ft poured  the same depth, with one a tube to the bottom, and one to the tube slightly in,  but on top only.  Make both piers level to each other . I say four feet apart as a long level is 4 ft.   When winter comes and ground is frozen, check the level again... if they are the same my theory is wrong, if the poured on top is slightly higher, my theory was right...and it heaved.

 

Best best is using  a bore,  undesturb as little as needed,yet get past the frost line.. the rest is up to you.. the difference may not make a difference.

 

I do like the screw in pier system, a great idea for multiple piers done quick.

 

Good luck.

 

Stevegeo 



#16 macdonjh

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Posted 28 March 2020 - 07:46 AM

 

- the temperature can be very cold in the Laurentiens with -25 to - 30C regularly during winter; this brings the frost lines down to a depth of 5+ feet.

So it sounds like your foundation needs to be six feet deep at least.

- I have rocky soil, so I am not sure that an auger would work or reach 60+ inches deep (I can try to see);

This is what I thought you were concerned about when I posted my idea of several helical piles with a concrete pile cap.  That way you don't have to dig in your rocky soil.

- the freezing / thawing tends to grab a rough surface underground  and move it up (interesting that the pylon company says that they use plastic sleeves to mitigate this).  This doesn't make any sense to me.  But then I live in almost the exact opposite conditions from you: our frost line is in the freezer in the kitchen, and we generally have either sandy or expansive clay soils.  So I don't have any experience building where the ground freezes.  The issue I still have with putting forms below grade is you'll still have to use an auger to drill a hole to drop the form into (see concern #2 above).  If that is best practice, perhaps consider some large PVC or HDPE pipe rather than Sonotube.  It will be even smoother and unlike the cardboard will never rot.

 



#17 speedster

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Posted 28 March 2020 - 05:21 PM

We think of frost heave as vertical but it is omnidirectional.  Sonotube is far from slick in terms of skin friction.  I suspect skin friction will be about the same as unformed but I'm too lazy to calculate it.  When soil freezes, it clamps the sonotube (and anything else below grade) and it is going to move it.  (Tangential heaving)  The forces far exceed the weight of any pier so going bigger isn't any better.  If a pier extended well below the frost line, so more is below the frost line than above it, then you could make a case for granular backfill above the frost line so the part above the frost line would slip. 

 

The good thing is, you can do a pier all sorts of ways and be happy with it.  We're only splitting hairs because some of us are dealing with arc-sec deflection.  In disturbed soil, you'll have much more deflection.  No way around that. 

 

If you put a pier cap on multiple helical anchors, you can pour the cap above the ground and eliminate surface soil action all together.  Not at all unusual to do this in buildings by using cardboard void forms.



#18 Luke Skywalker

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Posted 28 March 2020 - 07:37 PM

If you look at contractors that build fences using a post hole digger with out the use of a sonotube, pouring the concret directly in the hole to hold wooden or steel post, eventually the post will eave because of the frost. If you want to use an 8 inch sonotube or bigger, simply dig a hole with proper size auger. Your hole will be a bit bigger because of the ground around your hole will crumble. As long as your sonotube is below the frost line, you will be OK. If you're not sure how deep to go, find out how deep the water main is in your area. If you've never seen a water main break in your area, go as deep as the water main on your street and you will be safe.



#19 sberrada

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Posted 28 March 2020 - 08:41 PM

Hi Everyone,

 

Excellent advice and great points to keep in mind - thank you.

 

I hope to get the pier started in about 1 month when the snow melts.

 

i wonder if I worry too much about deflections or vibrations because:

- the frost and heaving movement may slightly affect polar alignment of my pier over time, but this is to be expected, so my thinking is to verify and adjust the polar alignment during late fall and spring when movement could happen (with instruments like a polefinder this only takes a few minutes)

- using a 12” cement pier, or a 5.6 inch diameter pylon (6 feet deep) as a base for my metal pier,  I doubt there will be any vibration or deflections during imaging any given night, because the pier and scope are protected from wind and detached from the floor, while the scope / mount move very slowly and smoothly ; what convinces me of this is that I sometimes set up my tripod and scope on top of my wood balcony (wood planks) where I can see the wobbling when I walk around it.  However, when I leave the balcony (after setting up my scope / mount) and view remotely the guiding, it looks great (RMS around 0.1) and I have imaged many nights in such conditions with great results (when wind speed is low).

 

Either way, I appreciate everyone’s advice because we work so hard to build a ROR that it is best to use ‘best practice’ because we want to have something that works well for a long time.

 

Cheers,

sam




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