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# Pier engineering

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### #1 speedster

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Posted 26 February 2019 - 04:05 AM

I'm beginning a 13' pier and was surfing the net and found many people who sincerely want to do a good pier, some who's piers work for them but for the wrong reasons, some real mistakes, some folks completely off base, and few youtube vids that beg for the presenter to don his tin foil hat.  I had to run some numbers for my tall pier and, being architects and engineers, we thought others might be interested in what is really going on in piers.

The first misconception is the load involved.  They are tiny but, so is the acceptable deflection.  A 4,500 pound car hitting a 4" curb at 20 mph experiences a force of nearly 500 pounds.  A C8 on a tripod in a 10 mph wind sees 0.84 pounds and 3.36 pounds at 20 mph.  The first question is what load do we design for?  Someone else used a 5 pound horizontal load at the top of the pier.  His numbers seemed off but his logic is sound.  5 pounds is a good wind gust or a bump.  Using 5 pounds, let's limit the angular deflection to 0.5 acrsec which is acceptable for imaging and beyond the resolution of many scopes.  With a deflection that small, vibration and dampening almost become mute points.  Do we care if it's vibrating as 57 hertz if the amplitude is too small to detect?  No.

Here is the angular deflection at the top of the pier for various pipes and concrete piers of different lengths with a 5 pound horizontal load at the top:

Shaded cells are most economical pipe sizes for deflection less than 0.5 arcsec at each length.  Concrete section modulus assumed to be 2,465 ksi, varies with aggregate mix properties and varies over time.  Can range from 2,030 ksi to 5,946 ksi.  If you don't like a force of 5 pounds, divide the table values by 5 and then multiply by whatever force you want (in pounds).   Filling steel pipe with sand or concrete will change the harmonic frequency but not the deflection under load.  The sand is a mass fluid and the concrete is more flexible than the pipe that contains it.  Concrete may stiffen thin wall tubing but it won't help much on these pipes which are garden variety A36 schedule 40 structural shapes.  Masonry piers of concrete block or brick, with the same cross section size as the concrete piers, are about 10x more flexible.

But, pier flexure is only a part of the whole pier.  There are also connections to deal with.  So, let's look at a whole pier with foundation:

Starting at the top, this drawing does not show a bolt cage.  This can add very little flex or can add a lot.  Having that cage is very handing when mounts have to be attached from below and when there is a possibility of future equipment changes.  If they are 3/4" bolts and rigidly attached, even 5" of bolt height might add only 0.25 arcsec of deflection.  The bigger issue is things not being rigid and the whole cage wiggling.  Attach the mount to the top plate and then adjust the plate down as far as it will go and then tighten everything up to minimize deflection at this point.

The next thing down is the 6" pipe which has a deflection of 0.108 arcsec.  Then 2 plates bolted together which adds 0.023 acrsec of deflection.  This could be eliminated but this connection allows a smaller diameter pier where the scope may hit it and also allows changing the height by replacing this short piece while leaving the bigger stuff below alone.

Next is the 10" pipe with a deflection of 0.404 arcsec and the bolted connection to the foundation which adds 0.036 arcsec of deflection.  So, our total, with no cage on top, is 0.571 arcsec of deflection.  Pretty close to our max target of 0.5 arcsec of deflection.

If you don't agree with the numbers, that's fine.  It's just plain old physics and pretty simple stuff.  If you just want to shoot from the hip based on your intuition, that's fine too.  This is just for those who want to understand the mechanics and what can be done to improve their designs.  Structurally speaking, there is some crazy stuff out there and pushing the envelope is great but some want to push it in the right direction.  If you glue concrete blocks together with foam in a can and strap a \$1,200 mount to it and your happy, I don't understand you but I'm happy too.

Now, some details of the pier before looking at the foundation.  Personally, I like the top of my pier level.  It doesn't need to be.  All we are after is the polar axis being truly polar.  The base of the mount can be stuck to a wall and the thing will still work.  I just bugs me when things aren't level and plumb.

Rocket fins on the tube.  We put gussets on lot's of things but not telescope piers.  The little short ones do virtually nothing unless you just like the look.  If you pick the right size pipe or concrete diameter, fins are just decoration.  If you skimp on the diameter, long wide fins will help but why go to the trouble and cost?  Most fins I see are a gimmick.

Bolts.  Don't waste your money on Grade 8 or stainless.  All steel bolts have virtually identical stress/strain curves independent of alloy or heat treatment.  The more expensive bolts just make it farther up the curve and stretch more before the fail.  Avoid Chinese bolts.  They are notorious for failure.  Always use galvanized bolts in concrete.  Drill your bolt holes 1/16" over.

Aluminum pieces.  If you just like aluminum, use it.  You are saving weight where we don't want to save weight and introducing a dissimilar metal but it will certainly work.  Never put aluminum in contact with concrete.  If a part is steel, weld it.  If aluminum, braze it.  Brazing aluminum is easier than welding steel and can be done easily with a cheap MAP gas bottle torch from a  big box store.

The foundation is the weakest link in what I see being done on the net.  Throw enough concrete in a shallow hole and it will probably work but here are some things that can be improved.

Never embed your steel pier shaft in your foundation.  It will corrode later, if not sooner and the expansion will break your foundation.

Freezing also causes soil movement.  Put the bottom of your foundation 12" below your frost line.  In central Nebraska, that's 5' deep.  But, that's a good thing because soils get more stable as you go deeper.  The moisture content is more constant as  you get below the active surface zone.

Instead of a shallow spread footing in the most active zone, go vertical with no bell at the bottom.  In the diagram above, the foundation is 60" deep.  That is the deepest a Bobcat auger will go without an extension.  Oh, but renting equipment costs money!  Yes, but you make it up in less concrete, less labor, and you get a better, more stable foundation.  The pier example above is only putting 0.046 psf of horizontal load against soil that will take 2,000 psf before deforming.  Very strong and out of the active zone.

If you still want to do spread footings, why dig 2' deep and fill the hole with concrete?  You could just as well place a 3' x 3' footing 8" thick right on the surface and get the same result.

With drilled piers, always clean out the bottom of the hole to get rid of loose material.  With spread footings, always compact anything you disturb.  Rent a vibratory plate and greatly reduce the effect of settling.  Whatever you leave fluffed up will take years to consolidate on its own.

Don't form concrete below grade.  We want the concrete bound to the soil.  If you form below grade with wood and leave it, it rots, forms a void, and the foundation easily moves.  If you Sonotube below grade and don't remove it, it rots, forms a void and the foundation easily moves and you also create a path for water to expand soil adjacent to the pier and things move.  On larger drilled piers, the skin friction can take more load than the bearing at the bottom of the pier.  There is good reason utility poles, signs, and parking lot lights are bolted to drilled piers:  it's much more stable and moves less.  The little drilled pier in the example above is not much smaller than what we put 40' light poles on and expect them to survive 100 mph wind.  Would the typical shallow telescope pit full of concrete do that?  And, the drilled pier is cheaper and faster.  Think vertical.

Some parts of structural engineering don't always make intuitive sense and there is,unfortunately, plenty of misinformation on the web from really good people with really good intentions.  Keep on experimenting!  That's one of the beauties of this activity.  You know, I didn't notice anyone driving piles.  You can put a 500 pound hammer on a tractor PTO and bang a pipe down for an excellent foundation with no digging, no concrete, and only a little welding.  Hmmm.

### #2 speedster

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Posted 26 February 2019 - 04:13 AM

And another thing I forgot:  anchor bolts.  Anchor bolts are there to resist uplift.  How much uplift do we have on a telescope pier?  Yep, pretty much none at all.  Long anchor bolts aren't helping you except to lighten your wallet a bit.  Threaded rod is not a good anchor bolt as the threads don't provide much withdrawal strength in concrete (great in epoxy anchors though) and they don't resist twisting too well either.  You can take a simple 12" galvanized bolt, tack weld a fender washer to the head, and have wonderful anchors for a telescope pier.

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### #3 OldManSky

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Posted 26 February 2019 - 12:01 PM

I appreciate the research and common sense that went into your post.  Thanks for doing that.  Tons of good info.

But I probably fit this part: "If you glue concrete blocks together with foam in a can and strap a \$1,200 mount to it and your happy, I don't understand you but I'm happy too."

Pretty close.  I glued concrete blocks together, but with good construction adhesive.  And only put a \$950 mount on it.  And did a reasonable footer.

https://www.cloudyni...d/#entry9174545

But then, my load is only going to put about 1.5 pounds of deflection load at the top in a 20MPH wind.  And if the wind hits 20MPH, I'm not going to image.  So we're both happy

Based on your info, I did some things well and some things...not so well.  But it'll be great for its purpose.

Thanks again.

Edited by OldManSky, 26 February 2019 - 12:14 PM.

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### #4 Stelios

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Posted 26 February 2019 - 01:21 PM

Moving to Observatories for a better fit.

### #5 speedster

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Posted 26 February 2019 - 05:29 PM

Hey OldManSky - I love the foam and blocks.  Creative thinking way outside the box and I think you are ahead of your time.  There are some pretty incredible foams being developed that are looking for applications.  We tried a new foam to install steel door frames in masonry walls and, once you figure it out, it's awesome.  Woe to the man who has to remove them. Keep it up!  We all may be doing foam adhesives in the near future!

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### #6 macdonjh

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Posted 26 February 2019 - 07:36 PM

I'm beginning a 13' pier and was surfing the net and found many people who sincerely want to do a good pier, some who's piers work for them but for the wrong reasons, some real mistakes, some folks completely off base, and few youtube vids that beg for the presenter to don his tin foil hat.  I had to run some numbers for my tall pier and, being architects and engineers, we thought others might be interested in what is really going on in piers.

The first misconception is the load involved.  They are tiny but, so is the acceptable deflection.  A 4,500 pound car hitting a 4" curb at 20 mph experiences a force of nearly 500 pounds.  A C8 on a tripod in a 10 mph wind sees 0.84 pounds and 3.36 pounds at 20 mph.  The first question is what load do we design for?  Someone else used a 5 pound horizontal load at the top of the pier.  His numbers seemed off but his logic is sound.  5 pounds is a good wind gust or a bump.  Using 5 pounds, let's limit the angular deflection to 0.5 acrsec which is acceptable for imaging and beyond the resolution of many scopes.  With a deflection that small, vibration and dampening almost become mute points.  Do we care if it's vibrating as 57 hertz if the amplitude is too small to detect?  No.

I think most people would design for the 5 MPH wind because other connections in their system (saddle, OTA rings, etc.) would deflect in such wind.  I don't know anyone who observes or images in winds of 20 MPH.

Here is the angular deflection at the top of the pier for various pipes and concrete piers of different lengths with a 5 pound horizontal load at the top:

Shaded cells are most economical pipe sizes for deflection less than 0.5 arcsec at each length.  Concrete section modulus assumed to be 2,465 ksi, varies with aggregate mix properties and varies over time.  Can range from 2,030 ksi to 5,946 ksi.  If you don't like a force of 5 pounds, divide the table values by 5 and then multiply by whatever force you want (in pounds).   Filling steel pipe with sand or concrete will change the harmonic frequency but not the deflection under load.  The sand is a mass fluid and the concrete is more flexible than the pipe that contains it.  Concrete may stiffen thin wall tubing but it won't help much on these pipes which are garden variety A36 schedule 40 structural shapes.  Masonry piers of concrete block or brick, with the same cross section size as the concrete piers, are about 10x more flexible.

But, pier flexure is only a part of the whole pier.  There are also connections to deal with.  So, let's look at a whole pier with foundation:

Starting at the top, this drawing does not show a bolt cage.  This can add very little flex or can add a lot.  Having that cage is very handing when mounts have to be attached from below and when there is a possibility of future equipment changes.  If they are 3/4" bolts and rigidly attached, even 5" of bolt height might add only 0.25 arcsec of deflection.  The bigger issue is things not being rigid and the whole cage wiggling.  Attach the mount to the top plate and then adjust the plate down as far as it will go and then tighten everything up to minimize deflection at this point.

The next thing down is the 6" pipe which has a deflection of 0.108 arcsec.  Then 2 plates bolted together which adds 0.023 acrsec of deflection.  This could be eliminated but this connection allows a smaller diameter pier where the scope may hit it and also allows changing the height by replacing this short piece while leaving the bigger stuff below alone.

Next is the 10" pipe with a deflection of 0.404 arcsec and the bolted connection to the foundation which adds 0.036 arcsec of deflection.  So, our total, with no cage on top, is 0.571 arcsec of deflection.  Pretty close to our max target of 0.5 arcsec of deflection.

If you don't agree with the numbers, that's fine.  It's just plain old physics and pretty simple stuff.  If you just want to shoot from the hip based on your intuition, that's fine too.  This is just for those who want to understand the mechanics and what can be done to improve their designs.  Structurally speaking, there is some crazy stuff out there and pushing the envelope is great but some want to push it in the right direction.  If you glue concrete blocks together with foam in a can and strap a \$1,200 mount to it and your happy, I don't understand you but I'm happy too.

Now, some details of the pier before looking at the foundation.  Personally, I like the top of my pier level.  It doesn't need to be.  All we are after is the polar axis being truly polar.  The base of the mount can be stuck to a wall and the thing will still work.  I just bugs me when things aren't level and plumb.

+1

Rocket fins on the tube.  We put gussets on lot's of things but not telescope piers.  The little short ones do virtually nothing unless you just like the look.  If you pick the right size pipe or concrete diameter, fins are just decoration.  If you skimp on the diameter, long wide fins will help but why go to the trouble and cost?  Most fins I see are a gimmick.

But gussets are a good way to either correct a mistakenly sized pier, or make do with a pier that's smaller than you want but easily available.  Also, in a DIY hobby, sometimes material is harder or more expensive to come by than labor.

Bolts.  Don't waste your money on Grade 8 or stainless.  All steel bolts have virtually identical stress/strain curves independent of alloy or heat treatment.  The more expensive bolts just make it farther up the curve and stretch more before the fail.  Avoid Chinese bolts.  They are notorious for failure.  Always use galvanized bolts in concrete.  Drill your bolt holes 1/16" over.

Aluminum pieces.  If you just like aluminum, use it.  You are saving weight where we don't want to save weight and introducing a dissimilar metal but it will certainly work.  Never put aluminum in contact with concrete.  If a part is steel, weld it.  If aluminum, braze it.  Brazing aluminum is easier than welding steel and can be done easily with a cheap MAP gas bottle torch from a  big box store.

The foundation is the weakest link in what I see being done on the net.  Throw enough concrete in a shallow hole and it will probably work but here are some things that can be improved.

Never embed your steel pier shaft in your foundation.  It will corrode later, if not sooner and the expansion will break your foundation.

Freezing also causes soil movement.  Put the bottom of your foundation 12" below your frost line.  In central Nebraska, that's 5' deep.  But, that's a good thing because soils get more stable as you go deeper.  The moisture content is more constant as  you get below the active surface zone.

+1

Instead of a shallow spread footing in the most active zone, go vertical with no bell at the bottom.  In the diagram above, the foundation is 60" deep.  That is the deepest a Bobcat auger will go without an extension.  Oh, but renting equipment costs money!  Yes, but you make it up in less concrete, less labor, and you get a better, more stable foundation.  The pier example above is only putting 0.046 psf of horizontal load against soil that will take 2,000 psf before deforming.  Very strong and out of the active zone.

+1, this is the design I used (but I went almost seven feet below grade)

If you still want to do spread footings, why dig 2' deep and fill the hole with concrete?  You could just as well place a 3' x 3' footing 8" thick right on the surface and get the same result.

Yes, but you'd be walking on your foundation, causing vibrations in your imaging train.  If a spread footer is desired, even with the recommendations against it above, put the footer below grade to isolate it from footfalls.

With drilled piers, always clean out the bottom of the hole to get rid of loose material.  With spread footings, always compact anything you disturb.  Rent a vibratory plate and greatly reduce the effect of settling.  Whatever you leave fluffed up will take years to consolidate on its own.

Don't form concrete below grade.  We want the concrete bound to the soil.  If you form below grade with wood and leave it, it rots, forms a void, and the foundation easily moves.  If you Sonotube below grade and don't remove it, it rots, forms a void and the foundation easily moves and you also create a path for water to expand soil adjacent to the pier and things move.  +1 On larger drilled piers, the skin friction can take more load than the bearing at the bottom of the pier.  There is good reason utility poles, signs, and parking lot lights are bolted to drilled piers:  it's much more stable and moves less.  The little drilled pier in the example above is not much smaller than what we put 40' light poles on and expect them to survive 100 mph wind.  Would the typical shallow telescope pit full of concrete do that?  And, the drilled pier is cheaper and faster.  Think vertical.

Some parts of structural engineering don't always make intuitive sense and there is,unfortunately, plenty of misinformation on the web from really good people with really good intentions.  Keep on experimenting!  That's one of the beauties of this activity.  You know, I didn't notice anyone driving piles.  You can put a 500 pound hammer on a tractor PTO and bang a pipe down for an excellent foundation with no digging, no concrete, and only a little welding.  Hmmm.

Ah, but then you'll have steel in direct contact with soil.  It'll rust up to three feet below grade eventually.  Of course, that may take twenty years or more...

Some comments in red above.  I didn't do nearly the amount of research or calculating speedster did, but came to the same conclusions he did.  My pier is concrete.  I used a an auger to go about seven feet below grade (42" above grade).  No troubles to date, but I do have to touch up my polar alignment (after nearly four years).

 I should also add: I agree with everything speedster says about making sure your pier/ foundation is below the frost line for your area, not leaving form material below grade, doing your best to make sure what ever concrete you pour is poured against undisturbed soil, don't allow reinforcing steel to be exposed to the air or soil, et cetera.

Edited by macdonjh, 27 February 2019 - 08:54 AM.

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### #7 NMCN

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Posted 01 March 2019 - 11:32 AM

Noob question -- why do you want to avoid contact between aluminum and concrete?

### #8 macdonjh

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Posted 01 March 2019 - 02:04 PM

Aluminum corrodes when in contact with concrete.  There's a chemical reaction.

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### #9 speedster

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Posted 01 March 2019 - 03:39 PM

Steel pilings do corrode but a steel pile is good for 80 years, 120 years if capped.  A cap on the bottom makes them much harder to drive but keeps moisture out of the interior.    Small piles can be hot dipped galvanized but it's not common.  We see more "H" piles than pipe piles.  There are also prestressed concrete piles in small diameters, like down to 12".

### #10 NMCN

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Posted 02 March 2019 - 12:15 AM

Aluminum corrodes when in contact with concrete.  There's a chemical reaction.

Thanks.  I had no idea.  I'm in the process of designing a concrete pier and was contemplating placing my aluminum adapter right at the top of the pier. I certainly won't be doing that now!

### #11 macdonjh

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Posted 02 March 2019 - 09:34 AM

Thanks.  I had no idea.  I'm in the process of designing a concrete pier and was contemplating placing my aluminum adapter right at the top of the pier. I certainly won't be doing that now!

You could if you separate the aluminum from the concrete.  Say with a thin rubber sheet, or a steel plate (you might set up a galvanic cell mixing steel and aluminum...).

Here's a question for those that know more about this than me: if aluminum is in contact with concrete, but the connection is kept dry, with there still be corrosion/ chemical attack?  Also, would anodizing protect the aluminum from the alkalis in the concrete (the aluminum oxide might be resistant to the alkalis)?

### #12 speedster

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Posted 02 March 2019 - 04:45 PM

We are kinda splitting hairs here as we are talking about some long term processes but, I if we are going to make the investment in time and money to do it, we may as well do it right.  You want to keep all your metals dry but you can't eliminate moisture in the air which is why we paint, anodize, etc.  A piece of bare steel sitting in your "dry" garage will still rust in time.  Aluminum oxidation actually starts in minutes.

Nix the rubber sheet.  That introduces opportunity for deflection.  Paint is just fine to separate an Al plate from the top of a concrete pier.  No special paint needed other than something designed to stick to bare metal.  A rattle can of high-build sell-etching primer from your local auto store is wonderful. For the concrete pier, there is paint called "block-filler" (or whatever trade name) that is high solids, high dry film thickness, that is made for partially filling the voids in concrete block before a finish coat.  That is wonderful for the concrete pier.

Concrete corrodes most anything it touches, steel included.  Critical embedded steel parts are galvanized or epoxy coated.  So, why is only steel used for reinforcing?  Steel and concrete have the same coefficients of thermal expansion.

### #13 Tonk

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Posted 03 March 2019 - 10:32 AM

Aluminium oxide reacts with both acids and alkalis ( I'm a chemist ). The latter reaction forms aluminate salts.

Pity that pounds and inches, miles etc is largely a USA thing. I'm getting out my conversion tables to put all this into SI units - i dont want to miss Mars - lol

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### #14 Edd Weninger

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Posted 04 March 2019 - 02:50 PM

I've built 2 piers of concrete topped with an aluminum plate as shown here.

The first pier in SoCal was removed in 2017 upon sale of the house.  The 3/4" unanodized plate showed no signs of corrosion after 15 years in place.  My new pier in northern Arizona (now 13 years) is built similarly, and I'm not worried about it.

Cheers,

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### #15 earthltd

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Posted 06 April 2019 - 11:50 PM

Thanks, speedster, for all the tips.  But do you have numbers comparing flexture of a steel design to a concrete pier?

### #16 Stozhary

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Posted 08 April 2019 - 08:56 PM

Hi there, as speedster said, there are some rules of machinery based on chemistry, Aluminum and steel are not compatible metals due to corrosion. However from practical point to reduce that risk you can use stainless bolt, rods and nuts. Corrosion on Al-Fe pair usually starts on Fe and over time is hard to unscrew nut from steel bolt. Ask auto mechanic about.

About flexture. Nobody builds 3"-5"diameter concrete piers. Minimum 8"x8" blocks or 8" tube. But its rare amateur steel pier 8" diameter or square. It's wrong to compare apples and oranges. Usual concrete piers are MUCH more heavier and stable then steel ones.

Thanks

Alex.

### #17 speedster

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Posted 08 April 2019 - 11:37 PM

earthltd, yes there is a comparison on the first post of this thread.  Here is a jpg of that doc:

The deflections are for a 5 pound horizontal load applied at the top of the pier.  I think that elsewhere in this thread there is discussion of adding deflection for bolts, etc., with the goal of keeping the total deflection under 0.5 arc-sec.  The 5 pound horizontal load is an arbitrary number that is big enough to take care of wind gusts, minor bumps, etc.

Here is an example for a steel pier top on a concrete pier.

7' of concrete above ground, at 16" diameter, gives 0.125" of deflection.  The 4' of 8" pipe gives 0.167".  And the bolts to the concrete and to the cap give about 0.05" for a total of about 0.342".

Before you rush out and spend a lot of money on a pier and an adapter on top, this pier is entirely custom and it cost \$424 primed, and delivered over 100 miles.  Drill the cap as needed for you mount.  The cap, and your mount can turn 360 degrees in the pier so you have no limitations on finding north.  When you change mounts, just drill more holes for the new one.  If you need a recessed cup on top for the bottom of you mount, have them cut you a ring to go on top of the cap.  Nothing against expensive pier adapters but you can have more flexibility and less deflection other ways.

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### #18 Scott123

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Posted 15 April 2019 - 02:26 PM

If you Sonotube below grade and don't remove it, it rots, forms a void and the foundation easily moves and you also create a path for water to expand soil adjacent to the pier and things move.  On larger drilled piers, the skin friction can take more load than the bearing at the bottom of the pier.  There is good reason utility poles, signs, and parking lot lights are bolted to drilled piers:  it's much more stable and moves less.  The little drilled pier in the example above is not much smaller than what we put 40' light poles on and expect them to survive 100 mph wind.  Would the typical shallow telescope pit full of concrete do that?  And, the drilled pier is cheaper and faster.  Think vertical.

Hi Speedster, thank you for the information. I hope to set a pier this year, I live in Maine, so the frost line is at least five feet down. If I drill a six foot hole and drop in a sonotube, how do I remove the sonotube after the concrete sets?

Or, could I use Thinwall PVC Duct Pipe (or something similar), and backfill as I pour? No rot problem, and I would guess uplift would be decreased.

If I did that, would I need the spread footing?

I went to the soils website, the soil plasticity in my area is either 2% or 5%, depending on which side of the line I m on.

Thank you,

Scott

### #19 speedster

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Posted 15 April 2019 - 08:42 PM

Howdy Scott 123!

Never form anything below grade.  You want skin friction, not smooth sides.  6' deep hole is great.  In your case, you have to do it to get below frost heave that will definitely mover your pier if you are above it.  The big benefit is getting as far as you practically can below more active soils nearer the surface.  Soil stability increases with depth.  Most soil movement is caused by variation in moisture content and that variation decreases rather dramatically with depth.

No spread footing.  The only reason for a spread footing is to increase bearing capacity and a telescope pier has a tiny load relative to soil bearing capacity.  A square foot of "average" soil will hold 2,500 pounds so there is no need for spread footings.  When you over-excavate to have a hole big enough for a spread footing, you lose most or all of the undisturbed soil's lateral load carrying ability.  In other words, it's like you put the spread footing right on the surface except that  you are sitting on more stable soils due to depth below the surface.  That's why piers are under-reamed to bell the bottom portion rather than over excavating the hole and forming the shaft.

I doesn't much matter how much weight you put on a shallow footing - it will move as a result of soil movement.   More depth = less movement , both lateral and vertical.  Your 12" pier, 6' deep, will weigh only about 650 pounds but will move much less than a 3' x 3' spread footing 3' deep even though the latter weighs about 2,500 pounds.

If you have a spread footing, should you rip it out and put in a pier?  Certainly not.  This discussion is for a new foundation, comparing deeper vs wider and the importance of not forming anything below grade whichever way you go.  I suspect the people with really stiff foundations (piers) rarely "need" that much stiffness but, if you are going to do a foundation and would like it to be as staff as you can reasonably make it, it's nice to have some info on what the choices in foundations are really doing.

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### #20 Scott123

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Posted 15 April 2019 - 09:32 PM

Hello Speedster!

I will rent a hole excavator, this would be easier than borrowing a backhoe and excavating a pit! I can bore a hole for the pier, and then smaller holes for the shed. And maybe set some posts for a goat pen.

I am still thinking about the sonotube rotting. Should I just bore a 12" hole, scoop out the loose soil, and fill it to ground level, using the ground as the form? Or bore a larger hole, and reach down the hole and peel the sonotube away once the concrete has cured? Or am I worrying about this too much?

Thank you,

Scott

### #21 speedster

speedster

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Posted 15 April 2019 - 11:37 PM

Howdy Scott123!

You are right to worry but don't lose any sleep over it.  No forms below grade so the earth is the form for the below grade portion.  If you over excavate to have room to remove the cardboard, you lose the lateral resistance of the soil.  You want concrete against undisturbed soil.  Drill your hole, put a can on the end of a stick and scoop out loose material, and pour.  if you are only going to ground level, it's nice to put a ring of sonotube just a few inches tall on top just for looks.  It gives you a nice round finished side right at the top while the rest of the pier is formed to earth.  If you are going higher than ground level, you have a choice:  pour the in-ground part first and then come back and pour the top, or pour it all at once.

### #22 SteveInNZ

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Posted 16 April 2019 - 03:04 AM

For those of us who don't speak fluent contractor, what does "Never form anything below grade" mean ?

Steve.

Edited by SteveInNZ, 16 April 2019 - 03:05 AM.

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### #23 speedster

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Posted 16 April 2019 - 12:25 PM

Howdy Steve!

Whatever shape hole you dig, pour your concrete against the dirt (no forms).  Pouring directly against the dirt makes a much stiffer foundation.  It's one of those rare cases when "cheaper and easier" is actually better.

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### #24 akulapanam

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Posted 16 April 2019 - 09:31 PM

What is your recommendation on pouring a slab for multiple telescopes in or out of a roll off roof

### #25 speedster

speedster

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Posted 16 April 2019 - 10:13 PM

If you are on tripods, a 4" slab with #3 rebar at 16" both directions is fine.  Remove the top 6" of existing soil to get rid of loose and organic matter.  Backfill with sand, crushed limestone, or whatever is typically used locally in your area.  If you go more than 120 square feet, I'd put a grade beam around it.  Grade beam would be something like 10" wide and 18" deep or more with 2 #5 top and bottom.  I should do some standard details for anyone to use.  Won't be able to get to it this week but I'll put it on my list.

If you are doing concrete piers, isolate the piers from the slab (don't let them touch).

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