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#51 NMCN

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Posted 07 May 2019 - 07:07 AM

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.

Indeed I do.  Thanks for the response and for putting it in layman's terms.  There will be no gravel below my pier.

 

Tim



#52 GraySkies

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Posted 28 May 2019 - 07:12 AM

So on inspection of the location I've found that where I want to place my observatory is about 2.5-3ft to bedrock, I'm assuming I just pour the concrete down to the bedrock and done? (frost line now become a moot point).

The concrete pour will be 12" diameter and 6" above grade and then I'm installing a 8" diameter metal pier on top as I sourced a used pier someone was selling.

 

From this thread sounds like I'll have an extremely stable set-up.



#53 speedster

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Posted 30 May 2019 - 03:32 AM

Howdy GraySkies!

 

Try to tie your concrete to the rock with dowels instead of just pouring it on the rock.  Setting it on the rock is fine but attaching it to the rock is super.  If you can, drill 3 holes about 8" into the rock.  A hammer drill will go through limestone easily.  Granite is another matter.  If you can do the holes, epoxy short pieces of rebar into the holes (about 24" rebar pieces).  Tie your concrete rebar onto these dowels. 

 

I happened to be at Kitt Peak last night and looked at some drawings of the pier of the 4-meter Mayall Telescope.  Mirror is 15 tons, mount is 300 tons, moving weight is something over 40 tons.  The pier is a concrete pipe, 92' high, 37' in diameter, and only about 30" thick.  There is virtually no topsoil on the peak and the pier only goes into the rock about 10'.

 

The observatory for it is 180' high and the dome weighs 500 tons.  All designed for 120 mph wind.   


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#54 GraySkies

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Posted 31 May 2019 - 02:18 PM

Howdy GraySkies!

 

Try to tie your concrete to the rock with dowels instead of just pouring it on the rock.  Setting it on the rock is fine but attaching it to the rock is super.  If you can, drill 3 holes about 8" into the rock.  A hammer drill will go through limestone easily.  Granite is another matter.  If you can do the holes, epoxy short pieces of rebar into the holes (about 24" rebar pieces).  Tie your concrete rebar onto these dowels. 

 

I happened to be at Kitt Peak last night and looked at some drawings of the pier of the 4-meter Mayall Telescope.  Mirror is 15 tons, mount is 300 tons, moving weight is something over 40 tons.  The pier is a concrete pipe, 92' high, 37' in diameter, and only about 30" thick.  There is virtually no topsoil on the peak and the pier only goes into the rock about 10'.

 

The observatory for it is 180' high and the dome weighs 500 tons.  All designed for 120 mph wind.   

I'll see if I can get a suitable drill bit (pretty sure I have a concrete drill bit somewhere around here) and anchor my rebarb bars to the rock; no short cuts I guess.

8.5" Diameter Steel Tripod
New Pier.jpg

Here is the pier with my current Losmandy GM8 mount attached above the rat cage, I took the MA adapter off the tripod to do this. At some point in 2020/2021 I'll upgrade to a larger mount so this is pretty "future proof".

Now to move it from the driveway to the Observatory, waiting for help as this thing weights... a lot... oh and apparently is also full of really fine sand.


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#55 Joshuasnoesky

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Posted 03 June 2019 - 06:01 AM

Hi Speedster,

 

I have a lot of bricks. Is it advised to throw them in the hole before pouring concrete or is this a bad thing?

This way i get rid of the bricks and save on concrete.

 

Another question: is it important what thickness the steel pipe has? In your examples there is no mention of it.


Edited by Joshuasnoesky, 03 June 2019 - 06:33 AM.


#56 Megiddo

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Posted 03 June 2019 - 08:18 AM

Hi Speedster,

 

I have a lot of bricks. Is it advised to throw them in the hole before pouring concrete or is this a bad thing?

This way i get rid of the bricks and save on concrete.

 

Another question: is it important what thickness the steel pipe has? In your examples there is no mention of it.

I was going to do the same with some of the stones I dug out, but in the end, concrete is so cheap, it's not worth the hassle of trying to get the rocks or in your case bricks carefully covered and deep enough not to cause cracks and such.    I was just too busy mixing and pouring that trying to fit rocks down there was not going to happen.   Plus your hole shouldn't be so wide you even have them much room to do that.

 

I'm sure we have some experts who know lots more about this kind of thing.



#57 Sporocyte

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Posted 07 June 2019 - 03:16 PM

Hi y'all,

 

The guidance here and elsewhere has been excellent.  Elsewhere mostly to get some idea of the deflection of different cylinder diameters anchored at one end.  My in-laws have been welders for decades so I have lots of scrap mild sheet steel, steel drill stem, and various steel water pipes to choose from.  My ultimate goal is a pier sturdy enough to use for astrophotography.

 

My pier is made from about 7" dia water pipe (I think water pipe) about 3/23" thick.  It has a massive globe valve welded on one end.  I cut the pipe to about 6 ft in length leaving the valve attached.  I used my tractor-mounted auger (farm machinery is useful for astronomy) and dug down about 5' and used a 2x4 and maul to pack the soil at the base.  

 

Farm wisdom is not to set wood posts into concrete as the concrete acts as a cup to hold water, rotting the end. Instead, the wood rests on the soil and the concrete forms a collar around the base of the post.  I assumed the same wisdom holds for steel so that's what I did.  I lowered the valve-end of the pipe into the hole and poured 7, 40# bags of mixed quickcrete into the hole by pouring one mixed sack at a time and shmooshing in with a galvanized water pipe and 2x4 to evacuate air pockets.  Then repeat 6 times.

 

My son-in-law convinced me that in twenty years I won't want to be stumbling around in a hay meadow full of fire ants so I need to build a deck.  He and I scrapped a huge pier built out into a lake and got a bunch of 2"x12"x16' planks and 2"x10" of the same length.  I had a pile of old treated power poles that I scrapped when the power company replaced the old ones with new ones.  I helped them out with disposal.  We cut them to about 4-5' lengths and I augered about 2 ft into the pasture and set the deck piers into packed earth.

 

We now have a 20'x20' deck framed up and I am installing pressure treated 2"x4"x8' as decking with 3/16" gap, screwed down on both ends. 

 

Also, it's a roll-off observatory but the building is set on to of rails (rather than the usual hanging-from rails) with the 2x4 decking mitered to the height of the rails so as not to be a trip hazard at night.  The rails are 1/4" angle iron set at about 48" parallel and mounted atop piers.  I found steel vee-shaped wheels with bearings that I welded to 1/8"x a bit over 8 ft angle iron with an inside area of 4'x8' into which I will build walls and floor.  I cut a slot in the floor so that the "observatory" (which, god help me, looks like an outhouse) will roll about 1/2 way over the telescope pier with a door to enclose it.

 

I've been taking photos of the process and will eventually get with the CN mods here to see about uploading photos and construction.  I'm about 1/2 finished with the deck and 1/4 finished with the observatory.

 

Despite all of the false starts and screw-ups along the way, it's beginning to look like something I'll be well-pleased with using.

 

Thank you for all of the guidance here on CN and the various engineering websites I visited, mostly from links here on CN.  That's one thing I didn't keep up with (useful links) and I apologize in advance.

 

Chris


Edited by Sporocyte, 07 June 2019 - 03:27 PM.


#58 speedster

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Posted 08 June 2019 - 03:33 AM

Bricks and rocks are not a good idea.  Stick with the aggregate that comes with your mix.  You want this as strong as you can reasonably make this and bricks/rocks are working against you.


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#59 speedster

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Posted 08 June 2019 - 03:45 AM

Pipe wall thicknesses in the examples are all schedule 40 steel pipe.  6" has a wall thickness of 0.280", 8" is 0.322", and 10" is 0.365".  Going with schedule 80 pipe, in lieu of schedule 40, is much heavier, much more expensive, yet does little to further reduce deflection.  Much better to bump up a pipe size than to bump up wall thickness with schedule 80.  


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#60 Scott123

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Posted 01 August 2019 - 07:26 PM

Hello all, I have been following this thread, and the previous one, with interest. I hired a local power line contractor to bore a hole for a pier. Since frost line is at least five and a half feet, I went with a seven foot hole. (Probably overdeep, but it's done.)

 

The local building supply just dropped off twenty, eighty pound bags of concrete, a piece of construction tube, and some rebar, cut to 8'. I'm picking up a mixer tomorrow. Pour will happen Saturday morning.

 

 

Some questions:

  1. Any estimate how long it will take to do the pour? I'll pour cement until it's about a foot from the top of the hole, then set and plumb the construction tube, and finish pouring. I'm trying round up some helpers.
     
  2. Do I need to work the concrete in the hole, to eliminate holes?
     
  3. The mixer is gasoline powered. How much concrete can I dump in? That is, a whole 80# bag at once?
     
  4. The building supply did not have rebar rings/rectangles, or dobies. Can I just spear the straight rebar in and call it good?
     
  5. Any good methods to make sure I point the adapter plate to true north?

Thank you,

 

Scott



#61 Steve Haverl

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Posted 01 August 2019 - 08:46 PM

1. Twenty 80s is a solid 4-6 hour job and you will need one helper to help prevent your demise. That’s a lot of concrete !
2. On a pour that deep, renting a small vibrator would be beneficial to both eliminate voids and ensure the concrete is in intimate contact with the hole sidewalls.
3. The mixer should have information on a label or plaque that defines its weight capacity. Or the rental company will. If not, start with a half bag and ensure the motor has enough snot to rotate/mix that weight plus the water.
4. Pour the shaft about halfway, then spear the vertical bars keeping a minimum of 2” off the sidewalls. That’s called ‘cover’ in concrete speak.
5. Someone else......
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#62 Steve Haverl

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Posted 01 August 2019 - 09:24 PM

Additional thought on question 1:

When you start the mix/transport/pour cycle move at a pace that ensures that the already placed concrete is not trying to setup. In other words, the new load must be placed while the previous load is still plastic, then vibrate to fully integrate the fresh load. This prevents forming cold joints; these seriously affect the integrity of the pier. Also, remember that Concrete will set faster in warm weather.
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#63 Scott123

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Posted 02 August 2019 - 05:55 AM

Additional thought on question 1:

When you start the mix/transport/pour cycle move at a pace that ensures that the already placed concrete is not trying to setup. In other words, the new load must be placed while the previous load is still plastic, then vibrate to fully integrate the fresh load. This prevents forming cold joints; these seriously affect the integrity of the pier. Also, remember that Concrete will set faster in warm weather.

The site is too far from electricity to run a concrete vibrator, I'll see if I can borrow a generator. (The mixer uses a gasoline engine for power.)

 

I've heard that adding sugar to concrete extend the set time, but I'm wary of adding too much.

 

Sadly, I can't get any help, everyone seems to be on vacation. Even the promise of free beer hasn't brought them in!



#64 Steve Haverl

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Posted 02 August 2019 - 07:31 AM

Suggest you avoid adding sugar (or any home brew admixture) due to potential to adversely affect the concrete strength.

Also suggest you pursue borrowing the generator - using the vibrator will make a much better pour.

Lastly, I don’t know about you, but I’m north of 60. Doing this job solo will be a serious challenge. Humping 80# bags of concrete is an easy invitation to an injury.
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#65 speedster

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Posted 02 August 2019 - 02:04 PM

Howdy Scot 123!

 

The good news is, you can really mess up mixing and placing but your pier will still work fine for a telescope.  Please don't add sugar.  Urine will also slow it down but there is no reason to slow it down in the first place. 

 

I'll make an assumption that the hole is 12" and that the  mixer is at least 3.5 cf since it is gas powered and likely 9 cf if you are renting it from a rental company.  The volume of your hole is about 6 cf, or about 9 #80 bags so your looking at 1 or 2 batches.  Less than an hour.  You will mix several bags at once.

 

Oops. You have 20 bags so the diameter must be larger.  20 bags = 13 cf so 3 to 4 batches depending on the mixer capacity.  Still about an hour and a bit.

 

If your concrete is mixed correctly, vibration is not needed in the hole.  Building piers are never vibrated.  Just get the mix right in the first place.  Exactly the amount of water called for and add up to 10% if needed to make it flow.  If there is any physical water on top, in the mixer, add more dry mix.  If concrete is sticking to the bottom of the mixer, add a little more water until everything flows around the paddles (like in an ice cream mixer just before it freezes). 

 

Rent a real mixer that mixes on one side and then dumps on the other side and set it up to dump right in the hole.  Commercial rentals do this.  Home Depot rentals may not.

 

In lieu of vibrating, if you just feel better working it a bit, take a 2x2 or a broom handle and rod it by stabbing the rod into it several times.  Don't stir, just stab and remove the rod slowly enough to not leave a void.  Don't stab into a later previously stabbed.  Even better, just tap the rebar cage with a hammer every few feet of pour.

 

Don't spear the rebar in.  It's asking for trouble.  They won't go in straight and may not reach the right depth.  If you concrete is mixed correctly, you won't be able to stab them very far.  Tie them in a cage and place the cage in the first 1' of concrete.  To make a cage without buying stirrups (the round hoops), find a post and bend your own.  Or, just cut short pieces and tie them into a square.  Put a stirrup at the bottom, top, and about 2' spacing.  You don't want to be wrestling loose 8' bars while you are trying to pour.

 

To get north in the daytime, use a vertical stick to cast a shadow at solar noon.  Shadows are short this time of year so this is not as accurate as marking north at night.  At night, with any telescope on any alt-az mount, set up over the hole, level the mount, center Polaris in the eyepiece (adjust for its slight offset from the pole if you want), then slew straight down to some ground object such as a fence, neighbor's house, etc. and put a mark on that object.  You now have a line from the center of the pier to the pole.  When you set the bolts, use the actual plate, or a plywood plate copy, put a short level on the plate (or any other straight edge you want to use), lined up with the axis of the plate, and sight down the level to the mark on your fence, house, or wherever you placed your mark.


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#66 Steve Haverl

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Posted 02 August 2019 - 07:45 PM

With all due respect, my observations and recommendations are based on my 30 years of experience designing and construction engineering foundations, including numerous drilled piers/ shafts. Not bragging, just the facts....
Hope your pour goes well.
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#67 Scott123

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Posted 02 August 2019 - 08:37 PM

I was able to get a friend's teenager to help, after I promised his mom NO beer for him!

 

I spent the evening dipping out the loose soil at the bottom of the hole. Even though I'd covered the hole, there was a little water in the bottom.

 

I don't know why, but the final diameter is closer to 14", so more bags. frown.gif

 

The mixer has a kill button on one side, I'm assuming that is the pour side. However, the drum doesn't turn all the way to the other side, there is a stop at the top.

 

I'm going to root around my cellar and make a chute of some kind, at least something to direct the concrete down the hole, and not bouncing off the sides, tearing off dirt. The mixer is set, and chocked, and I have an assortment of hoes, rakes, and shovels on hand.

 

Scott



#68 Scott123

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Posted 03 August 2019 - 03:41 PM

My younger son returned a day early from visiting his cousins, so he was able to help with the project. It took maybe two hours to get the hole filled to about two feet from the surface. I set in the construction tube, we had to fill that a shovelful at a time. So it went slower. Final part - inserting and leveling/aligning the adapter plate - took a half hour.

 

It's all done, I've wrapped a wet towel around the top of the structure and covered it with a tarp.

 

And I'm beat.



#69 Steve Haverl

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Posted 03 August 2019 - 08:35 PM

Congratulations, sounds like it went relatively smoothly and you’ll have a great end product !

Great that you also had some help.....
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#70 Scott123

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Posted 04 August 2019 - 04:39 PM

Steve, thank you! All went well, except two of the bags burst. We used what we could, but it came down to about four shovelfuls extra at the end. Having two helpers made it go faster. The boy I hired was game, and worked hard, but he is a little short, and thin. My son, on the other hand, is over six feet tall. It made a big difference.

 

Took about four hours, including leveling/aligning. A half hour to clean tools, then we returned the mixer to the rental company.

 

That 'stop' I thought was set on the mixer? Just the gears binding a little.

 

My wife provided water and iced tea, and cooked lunch for us.


Edited by Scott123, 04 August 2019 - 04:40 PM.


#71 Steve Haverl

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Posted 05 August 2019 - 12:58 PM

Sounds like an excellent concrete crew !

If no one has mentioned it, I would recommend waiting 3 days to strip the tube to allow the concrete to develop initial strength and minimize spalling. I apologize if it’s obvious, but Strip the tube by separating it along the spiral seam.
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#72 Scott123

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Posted 06 August 2019 - 06:13 PM

They were a great help! The younger boy (not my son) wouldn't drink enough water, even though both my son and I warned him. My son made sure he, at least, stayed hydrated, and kept offering water to both the boy and me. The boy told me he felt dizzy and headachy when we brought him home. Hope it was a learning experience for him.

 

I have some rags draped around the top and sides of the pier. I soak them with water every day, then cover it back up with a tarp. I don't plan on stripping it until this weekend, when I'll have more time.



#73 Steve Haverl

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Posted 09 August 2019 - 08:35 AM

Post a picture of the stripped pier so we can marvel at your work !

#74 Scott123

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Posted 09 August 2019 - 08:58 AM

Sounds like an excellent concrete crew !

If no one has mentioned it, I would recommend waiting 3 days to strip the tube to allow the concrete to develop initial strength and minimize spalling. I apologize if it’s obvious, but Strip the tube by separating it along the spiral seam.

No, that wasn't obvious, I would have tried to score the cardboard in straight lines. I'll strip the tube when I get home this evening.

 

Which brings up more questions:

 

  1. There may be holes and gaps, best method to repair?
     
  2. Can I use regular paint to paint it?
     
  3. I know the top is ugly, can I grind off the excess concrete? Method?
     
  4. Could I just leave the tube on the pier and paint it? It is exposed to the weather until at least next year.

 

Thank you all for your help.

 

Scott


Edited by Scott123, 09 August 2019 - 09:01 AM.


#75 darkskyrancher

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Posted 09 August 2019 - 04:14 PM

My question is for Speedster.  Having read your discourse on soils considerations, what about pier construction in sandy soil?  I recently tractor augered 24 sites to a finished depth of 4 feet for a solar array.

I had to make 2 passes of the site.  First, a rough auger with a great deal of sand collapse and the second allowed for Sonotube insertion but only after preparing the sites with a predetermined amount of water.

 

I only mention this experience as a preface to any recommendation you might have for constructing a pier in sand.  The challenges in sand must logically temper the pier construction technique choice.  What would you recommend to optimize a pier in sand?  Thank you, in advance for your contributions to our forum.

 

Darkskyrancher


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