817 replies to this topic

[akulapanam]

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).

 

I was actually thinking about doing several steel piers to the slab so I assume it would be better to do mini slabs for each one with motel o scopes on top?  Separately what about in a large roll off roof?  One main slab and then isolated ones for each pier?

Edited by akulapanam, 16 April 2019 - 10:19 PM.

[speedster]

Just thicken your slab at the steel piers so you have enough depth for embedding the bolts.  If you thicken to 12" at the piers, you could use bolts embedded 8" and still clear the bottom of the concrete by 4".  You can also thicken an area, pour concrete, and come back later and drill in some epoxy anchors.  Even the garden variety 1/2" diameter anchors have a withdrawal force of 7,000 pounds or more.  Plenty.

[Megiddo]

Building a pier soon... here are my plans (opinions are very welcome)

Has anyone used the expanding foam vs. concrete?    I plan on going down 3ft with 4ft exposed.

 
I have a 2012 Chevy Malibu Rear Rotor and everything looks to fit nicely.
 
The pier is going to be 4 - 4x4's  and at this time are going to be arranged in this cross pattern:
Placing them together doesn't give me room to bolt through the rotor flange.   The rotor will only be higher than the lumber enough for me to get a wrench in to tighten the center bolt.

(In the drawing the rotor is sunk in the wood so I can see the layout.)

Edited by Megiddo, 24 April 2019 - 03:14 PM.

[macdonjh]

Megiddo, expanding foam for what?

+1 to everything speedster has said. Good advice, all of it.

One more "contractorism": rebar is measured in 1/8s of an inch. So #3 rebar is 3/8" in diameter, #5 is 5/8" diameter.

[Megiddo]

Megiddo, expanding foam for what?

+1 to everything speedster has said. Good advice, all of it.

One more "contractorism": rebar is measured in 1/8s of an inch. So #3 rebar is 3/8" in diameter, #5 is 5/8" diameter.

 

Expanding foam vs. concrete.    The part that sticks in the ground.      Apparently it is being successfully used for fence posts and even telephone poles.

[speedster]

Short answer:  never use foam in the ground.  That foam is a great product with many great uses and an overzealous marketing department.  Big box store concrete is $4.50 per cubic foot compared to $17.90 for foam.  Foam density is about 3 lb per cf and you can easily cut it with a knife.  Concrete is about 145 lb per cf.  Foams are flexible and not very elastic.  Lateral earth forces can easily compress the foam and it doesn't fully recover.  Concrete is far less elastic but that is not an issue with concrete since is does not deform at all so it has no need to be elastic enough to recover.  The claim that foam prevents rot is very questionable.  It can certainly slow it down if it is installed to do that.  Nature still wins in the end. 

[Megiddo]

Rot and compression are not a big concern, but that weight certainly is.   For our use, I'm concerned that the top end is balanced out by the bottom.   Having that big bulb of weight at the bottom has got to help offset that mount and scope (I'm thinking I have about 100lbs up there).

[spacemunkee]

Thanks for the lengthy food for thought speedster, as I'm preparing to put in a pier.

Note that at present I'm not putting in a lifetime installation. Eventually in next couple years or so as lot changes(new garage in one spot, large shed torn down elsewhere) I will be relocating to a permanent location where present shed sits.

Anyway, question is regarding this whole forming below grade issue. In many years of construction(but just doing electric), I know I've seen many instances of things such as deck peirs or such being formed(usually sonotube)somewhat below grade atop a non-formed footer.

I just don't see the difference in formed above or below, as once it's well cured, and understand it cures more with time, once it's set it's set..

Where my hole is its solid clay that you practically have to chip away at after about a foot down, and even with the tons of rain lately this spot has been dug for a couple days and no water seeping into it at all. Down a foot past where the average frost line is for my area.

My plan was a good diameter non-formed footer with rebar stub-ups and set a 12x12 form atop that to get just above grade(don't want to fill that whole hole with concrete to grade) with rebar up from there for an 8" sonotube pier. Planned to brace things up to ensure it all stays vertical and pour the 12x12 and pier in one pour.

Seems like it would last a while, and not like I'm setting the Brooklyn Bridge on top of it.

Thoughts, advice? Thanks.

[macdonjh]

Rot and compression are not a big concern, but that weight certainly is.   For our use, I'm concerned that the top end is balanced out by the bottom.   Having that big bulb of weight at the bottom has got to help offset that mount and scope (I'm thinking I have about 100lbs up there).

Compression is a concern: if the foam compresses and does not recover, there will be a gap between the soil and your pier, and your pier will no longer be stable.  The fencing contractors you have seen must be using different foam than I am familiar with.  The polyurethane foam I know gets brittle after a couple of years and does not bounce back, it just crushes and turns to dust.  I will stay with concrete for my below grade applications.

 

That bulb of weight isn't that big a deal if you have good contact between the soil and your pier foundation.  It doesn't hurt anything, but the weight by itself doesn't help much, either.  Several of my observing friends have big foundations under their mounts: 1 cubic yard, 1.5 cubic yard.  They dug really big holes and poured a lot of concrete.  I used about 6- 7 cubic feet of concrete in a 12" "pile" just like speedster has championed and my mount has been just as stable as my friends'.  

 

However, it will be your project so you have to do what makes you sleep well at night.  

[Megiddo]

I'm of the mind to use concrete.    And yeah, since my pier will be using a cross shape, I don't want to be digging a wider hole than I have to.

 

I have to admit the use of the foam was going to make my back happier

 

I noticed fence posts that I put in 10 years ago barely wiggle at all.   And I dug these hole not much bigger than the 4x4.   I even used that concrete that you pour in dry and let the moisture in the soil water it.  (not that I'm planning on doing that)

 

Glad to hear a large bulb of concrete is not needed.

[macdonjh]

I'm of the mind to use concrete.    And yeah, since my pier will be using a cross shape, I don't want to be digging a wider hole than I have to.

 

I have to admit the use of the foam was going to make my back happier

 

I noticed fence posts that I put in 10 years ago barely wiggle at all.   And I dug these hole not much bigger than the 4x4.   I even used that concrete that you pour in dry and let the moisture in the soil water it.  (not that I'm planning on doing that)

 

Glad to hear a large bulb of concrete is not needed.

My back was sore for a couple of days after pouring my pier, but that passed.  My pier is still solid.  What do the jocks say?  "Pain is temporary, glory is forever"?

 

The only reason for putting concrete into post holes for fences is to ensure tight contact between the post and the soil.  That prevents the wiggle.  The same effect can be achieved with gravel or the soil that is dug out.  The secret is the gravel or soil has to be well compacted around the post.  That is usually more work than pouring a bit of concrete into the hole.

 

The trick with a "straight" pier is to go deep.  If you live where it gets cold you must use your auger to dig below the frost line, otherwise your pier will heave when the ground freezes.  I live in sunny Texas, so that wasn't a problem.  I used a rented auger to dig about 78" below grade and my pier is about 42" above grade.  At my site the soil is 36" of sand on top, then 36" of clay, and then another sand layer (which is why I stopped at 78" deep).  My Sonotube form extended 4"- 6" below grade to ensure a finished appearance when my observatory was done, but there are more than 72" of concrete poured directly against undisturbed soil below that (and I removed all the Sonotube after the concrete had set).

 

Good luck with your project.  There is lots of good information and ideas in the Observatories forum here...

[Mel M]

Yes, the pier is stronger if there are no forms below grade unless you have a deep frost line. If you want a 12" pier, dig a 12" diameter hole. The strength comes from pouring the concrete against the undisturbed soil in the hole. The sonotube should be placed only a few inches into the hole.

Digging a hole larger than the form and then shoveling soil against the form makes a weaker pier.

Edited by Mel M, 29 April 2019 - 12:56 PM.

[Crashcourse]

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

Hi Scott, I'm in Manitoba Canada and we get frost to 4ft here. My pier consists of a 24" rebarred concrete column 7ft underground and that is tied into a 12" column 5ft above ground. I hired an auger to come in to cut the 24" hole, built the rebar cage, dropped it in and poured against the bare hole. I poured to about 6" below grade and let it set overnight, tied in the rest of the rebar cage with 3ft x 3/4" galvanized threaded rods at 5' 4" (for the pier adapter) and placed a 5ft 12" sonotube over the cage and poured the tube. Pulled off the tube the next day.
I run a C11 Edge loaded with imaging gear (about 43lbs) with no complaints from the pier.

[GraySkies]

I'm looking at making a pier in Ontario (frost line is... whatever I plan to go 2 ft lower).

I'm looking at two options:

A) 12" Concrete pier to 6" above grade/floor - 4 rebar core, then an Aluminum 8" Pier to the proper height (~32")

 

B) 8" Concrete pier the whole distance

A Losmandy MA will top each (6" Diameter)

 

What I like about A, is that if/when I move I can take the metal pier with me and leave the 12" without issue (its in the country, maybe put a birdfeeder/house in place of the telescope). where as option B that concrete is going to be harder to get rid of at the end of the day.

Thoughts?

Equipment is listed below, but I'll likely upgrade to a A-P mount (Mach2GTO?) in a few years for better operation but I can't see myself buying larger telescopes then 8".

Edited by GraySkies, 02 May 2019 - 12:44 PM.

[speedster]

Howdy GraySkies!

 

Part 9 of the Ontario Building Code states frost depth is 1.2m which is 4' feet.  There is no "wrong" way when the final decision boils down to personal preference.  This thread was started so we all have some accurate info to make informed decisions and get the most bang for the buck.  Ultimately, we are after stiffness which is measured by modulus of elasticity.

 

Steel = 29,000,000 psi

Aluminum = 10,000,000 psi

Concrete = 4,350,000 psi

 

Specific metal alloys make a difference, as does concrete mix design but, a general comparison of structural metals and 3,000 psi concrete shows that Aluminum is 3x stiffer than concrete and steel is 3x stiffer than aluminum.  The shape and length of the piece also affect stiffness so a 16" concrete column is stiffer than an 8" steel pipe even though concrete is 7x more flexible than steel.

 

The 8" concrete pier scheme has a practical problem with embedding anchor bolts in the top of it.  Embedded bolts need to be at least 2-1/2" from the edge of the concrete which really limits your bolt pattern on an 8" concrete pier.  You could do a funnel shaped bolt cage and embed the funnel so there are ways around this if you want an 8" concrete pier.

 

The aluminum option is personal preference but bare in mind that an 8" aluminum pipe is way more expensive than 8" steel and also way more flexible.  Weight is our friend and steel is heavier.  At our thicknesses, corrosion is not an issue for the next 100 years unless aluminum is in contact with concrete. 

 

The one certain thing is that our needs and wants will change over time and the metal pier on top of concrete pier gives you a great deal of future flexibility since it is relatively easy to cut some off or weld some on later.  Also easy to attach stuff to it and run things through it.

 

Looks to me like steel (or Al) on concrete is the way to go.  A local shop can typically weld up a pier to your specifications cheaper than the store-bought ones and you can get exactly what you want.

 

Just to confuse things, since you are going 6' deep (excellent), you could just suspend an 8" steel pipe in the hole and concrete it in like a big fence post.  No bolts, no pier fabrication cost and, if you move, just cut it off and take the top with you.  Then, weld a base plate on it and you still have your pier. 

[GraySkies]

Howdy GraySkies!

 

Part 9 of the Ontario Building Code states frost depth is 1.2m which is 4' feet.  There is no "wrong" way when the final decision boils down to personal preference.  This thread was started so we all have some accurate info to make informed decisions and get the most bang for the buck.  Ultimately, we are after stiffness which is measured by modulus of elasticity.

 

Steel = 29,000,000 psi

Aluminum = 10,000,000 psi

Concrete = 4,350,000 psi

 

Specific metal alloys make a difference, as does concrete mix design but, a general comparison of structural metals and 3,000 psi concrete shows that Aluminum is 3x stiffer than concrete and steel is 3x stiffer than aluminum.  The shape and length of the piece also affect stiffness so a 16" concrete column is stiffer than an 8" steel pipe even though concrete is 7x more flexible than steel.

 

The 8" concrete pier scheme has a practical problem with embedding anchor bolts in the top of it.  Embedded bolts need to be at least 2-1/2" from the edge of the concrete which really limits your bolt pattern on an 8" concrete pier.  You could do a funnel shaped bolt cage and embed the funnel so there are ways around this if you want an 8" concrete pier.

 

The aluminum option is personal preference but bare in mind that an 8" aluminum pipe is way more expensive than 8" steel and also way more flexible.  Weight is our friend and steel is heavier.  At our thicknesses, corrosion is not an issue for the next 100 years unless aluminum is in contact with concrete. 

 

The one certain thing is that our needs and wants will change over time and the metal pier on top of concrete pier gives you a great deal of future flexibility since it is relatively easy to cut some off or weld some on later.  Also easy to attach stuff to it and run things through it.

 

Looks to me like steel (or Al) on concrete is the way to go.  A local shop can typically weld up a pier to your specifications cheaper than the store-bought ones and you can get exactly what you want.

 

Just to confuse things, since you are going 6' deep (excellent), you could just suspend an 8" steel pipe in the hole and concrete it in like a big fence post.  No bolts, no pier fabrication cost and, if you move, just cut it off and take the top with you.  Then, weld a base plate on it and you still have your pier. 

Thanks for your insights!

6ft will be "fun" to dig next month :/ that's what I get for building an observatory on a hillside.

I've contacted a local metal supplier, if I can get steel at a better price than aluminum I'll go with steel.

I'll be filling the pier with material to help dampen the vibrations (I heard Tires rubber makes a good fill), so it won't be a hollow tube (although whatever I fill it with will be removable).

I'm also going to paint/coat the metal tube to help reduce rusting.
 

[Joshuasnoesky]

Why not use a stove/flue pipe 8 inch 2mm thickness. Steel and already painted black. Fill it up with sand to damp vibrations.

In the netherlands it sells for about $50 for 1 meter.

 

I'am about to move/renew my observatorium to a new part of the garden. My current pier is of solid concrete, 14 x 14 inch.

But it is to massive.

For the new pier i want to use concrete in the ground and upon that the steel pipe.

Edited by Joshuasnoesky, 03 May 2019 - 12:12 AM.

[Messierthanwhat]

Interesting thread. One thing I don't recall ever seeing discussed (and don't know why) is pouring a round concrete pier in PVC pipe, and leaving the pipe in place as the outer sheath of the pier. My own pier is already done that way, and has been in use trouble-free for about four years. Leaving the pipe in place did make for a much nicer-looking final product than most bare concrete. I haven't noticed any resulting issues, but I wonder if there were reasons not to go this route.

[Joshuasnoesky]

My first idea was also to use a pvc pipe, pour concrete in it and leave the pvc pipe as it is. A little grinding and paint and Bob's your uncle.

But when a saw a beautiful stove pipe i changed my mind.

Edited by Joshuasnoesky, 03 May 2019 - 09:29 AM.

[GraySkies]

Interesting thread. One thing I don't recall ever seeing discussed (and don't know why) is pouring a round concrete pier in PVC pipe, and leaving the pipe in place as the outer sheath of the pier. My own pier is already done that way, and has been in use trouble-free for about four years. Leaving the pipe in place did make for a much nicer-looking final product than most bare concrete. I haven't noticed any resulting issues, but I wonder if there were reasons not to go this route.

I thought about that as well, mainly it’s the removability I’m concerned about as one day it will have to come out...

[speedster]

PVC is an excellent solution for above ground forming.  It does nothing for stiffness so you can only count on the stiffness of the concrete itself but a 12" concrete pier is much stiffer than an 8" pipe.  Now that online retailers are selling cut lengths, it's a very economical way to get a very stiff pier with a much cleaner finish than Sonotube.  PVC doesn't take paint well but the printing can be removed with solvent for a pretty clean white finish and you can also put a vinyl wrap on it to have whatever finish you like.

 

For comparison, using our example of a 5 pound horizontal force applied at the top of a 48" high pier:

 

8" schedule 40 steel pipe deflection = 0.167 arc-sec

8" concrete deflection = 0.91 arc-sec

10" concrete deflection = 0.3708 arc-sec

12" concrete deflection = 0.074 arc-sec

 

5' of 12" schedule 40 PVC is about $110.  Dig a 12" hole 5' deep, pour concrete to ground level with four 3/8" vertical rebar, then slip the PVC over the rebar cage, brace it thoroughly, pour the rest of the concrete, embed your bolts, and for the cost of the pipe, rebar, bolts, and about 16 bags of concrete (about $200 total), you have a pier 5 times stiffer than a $2,000 store bought pier bolted to a 4,000 pound block of concrete.  Calculate your actual savings, divide by 2, send me a High Point gift card for that amount and then Bob is your uncle. 

 

All sorts of working solutions and none are wrong.  But, when we look at properties of materials and soils and get a handle on how much deflection is acceptable, the stiffest, least moving solutions turn out to also be the least expensive.

 

Someone mentioned curing concrete inside pvc earlier in this thread.  Not an issue.  The concrete will cure even under water.  It will cure slower which is a good thing.  we want to retard the curing of the surface as much as we can.  Throw a wet towel over the top of your PVC pier and keep it wet for a week.  Follow directions on the concrete bag and don't mess up the water content.  When the concrete is cured, you will notice a hairline gap between the PVC and the concrete.  Not a defect.  The concrete simply shrinks a bit and the PVC isn't doing anything for us in terms of stiffness anyway.

[SteveInNZ]

8" schedule 40 steel pipe deflection = 0.167 arc-sec

8" concrete deflection = 0.91 arc-sec

I find these two numbers interesting and convenient. For the same diameter, I can have a pier that I can unbolt and remove, without compromising on the performance. Am I missing something ? The universe doesn't usually work that way.

 

Also, one of the net.wisdom reasons for long gussets (?) is to change the resonance characteristics. As I'm sure you know, astronomers love to go outside and whack their pier with a hammer. Do you have any comment on that - fact or fallacy ?

 

Steve.

[speedster]

No, you are not missing anything.  That's really the way it works. 

 

Gussets are unnecessary but they look cool if you like rocket fins. Think '57 Cadillac with the fins pointing up with working tail lights.  If you have an 8" pipe and your deflection is only 0.167 arc-sec and fins take it down to 0.15, is it worth the expense of fins?  Again, certainly not wrong but they just don't do anything unless your pipe is undersized to begin with.  Even with a 4" pipe, with the load we are talking about, deflection is only 1.63 arc-sec.  Ten times an 8" pipe but still less than 2 arc-sec and that is only if someone bumps it.  Gussets are a marketing ploy in telescope piers.  Oh, but they stiffen the base plate.  Yes, they do but, again, it's such overkill it is completely unnecessary.   You can completely ignore base plate deflection unless you get thin, like 1/8".  It's not even worth running the numbers on.

 

For resonance, put on your tin foil hat.  If you can whack your mount and it only moves 0.167 acr-sec, does anyone care if it vibrates at 440 Hz or 280 Hz?  Frequency has absolutely nothing to do with deflection.  Every material or assembly has a natural periodic frequency.  Like tuning forks:  change the shape or the mass and you change the frequency.  You can change the frequency of your steel pier by changing the mass or dampening.  Adding sand dampens the vibration and changes the frequency but has no effect on deflection.  The only variables for deflection are length, force, elastic modulus, and moment of inertia.  Anything else is basically snake oil.  Again, it's certainly not wrong but it may not be doing what you think it is doing.  If we hyper-tune our mounts, we could also hyper-tune our piers to our favorite note.  "Oh, I see your pier is a crisp, clear A while mine is merely a muddy B#.  You are clearly the better man!"

[NMCN]

Speedster, you have mentioned several times that removing loose material at the bottom of the hole should always be done to prevent settling.  I've seen several people talk about using pea gravel at the bottom to promote drainage.  Is that still a good idea to follow?

[speedster]

Howdy Mariner2!

 

Good question.  We see someone putting gravel in a hole and assume they know there is some good reason to do so.  After all, we have gravel roads and gravel stuff is put under buildings so it must be good, right?  That gravel is typically crushed limestone, graded, and has enough fine particles to bind the whole pad/road into a big block.  No reason for anything like that at telescope piers.  As for drainage, gravel around the pier provides a path for water to run to the bottom of the pier.  If the gravel allows water to drain around the pier, where does the water eventually go?  There is no drain so it ponds in the gravel at the bottom and eventually soaks into the ground which causes the soil to expand which tends to move the pier.  Then it dries out and moves again, gets wet and moves again, etc.  Will it make a difference in your images or will you have to re-align every season?  In some soils, definitely.  No way to know for sure everywhere, but why use gravel, or sand, when it is working  against what we want to accomplish?

 

If you pull a foundation that has been set on gravel for a while, the air gaps in the gravel have been filled in with dirt.  That dirt came from somewhere which means things have been moving a little for the soil to displace the air voids.  Will it be a measurable movement?  Certainly not if we are talking about fence posts but, we are talking about arc-secs.  Again, why would we do things that work against us?

 

For loose fill around a pier in the pier hole, think about a simple lever.  Force times distance.  Consider a pier, 6' in the ground and 3' above the ground.  If the in-ground part is poured to fill the hole, it is restrained by the undisturbed earth and we have a 3' lever arm from the ground to the top of the pier. If we over excavate the hole and form the underground part of the pier and then fill in the annulus with sand/gravel/dirt, The sand or gravel is a fluid as far as structure is concerned so the pier is not restrained by the earth and the lever arm is now 9' instead of 3' and the base of the pier is a hinge.  It's not quite that simple but you get the idea.