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Isolating concrete pier

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#1 OrionStrider

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Posted 22 May 2020 - 01:55 PM

Hello,

 

I am planning to build a 10'(L) x 10'(W) x 8'(H) poured concrete foundation for an observatory. I have two questions that I hope to get comments on:

 

1. I am planning to pour a concrete slab roof on this foundation, which will serve as the base for my observatory (an old Sky Tent for now). Must I isolate the pier from this slab, or could I anchor the slab to the pier without introducing vibration issues?

 

2. Given that the pier will be approximately 10' tall, is a 12" diameter Sonotube (with rebar) adequate, or should I consider something larger? I guess this answer may depend on the answer to my first question.

 

Thank you for sharing your experience.

 



#2 William Russell

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Posted 22 May 2020 - 02:17 PM

I've done the same thing. 

 

Yes! Isolate it. A 12x12 1.5' deep pier base should do it. I isolate mine with some rubber between a 8x8 4" concrete slab. Esentuly pour the slab with a hole in it, then pour the footer in the middle. I didnt use rebar on mine. for a 12" lx200 with super wedge. 

 

If its bolted directly to the slab, It caries a-lot of vibration to the mount from walking on it. . You dont want that. 

 

 

 

Bill

 


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#3 appicloudy

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Posted 22 May 2020 - 03:23 PM

Hello

 

That is some foundation! - by by metric to imperial calc, about 14800 pounds of concrete in the foundation.

 

What is often done is the pier base is poured first, the size is a factor of how big your wallet is, big equals getting a structural engineer to design it, less big is bypass the engineer and over build it, joe average is make it as big as you can afford and hope for the best, I'm in the latter group. My pier base is about 4 by 4 feet by 4 feet deep with a riser coming out of that. 

 

The top of the pier base finishes BELOW grade, when you pour your slab for the obs, you would isolate the pier form the slab, I used a light insulation foam, that is  used on any vertical surface or horizontal surface. 

 

Footprint
 
Hope this helps
 
Ed
 
 
 
 

 


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#4 OrionStrider

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Posted 22 May 2020 - 03:49 PM

I've done the same thing. 

 

Yes! Isolate it. A 12x12 1.5' deep pier base should do it. I isolate mine with some rubber between a 8x8 4" concrete slab. Esentuly pour the slab with a hole in it, then pour the footer in the middle. I didnt use rebar on mine. for a 12" lx200 with super wedge. 

 

If its bolted directly to the slab, It caries a-lot of vibration to the mount from walking on it. . You dont want that. 

 

 

 

Bill

Thanks for the reply Bill. It appears that I wasn't as clear as I'd hoped in describing my "foundation". Perhaps I should have called it a basement. I also erred in calling the cement cap on this structure a slab. However, your reply highlighted another area of isolation that I didn't think I needed to worry about. I did intend to pour a 2' x 2' x 2' base for the pier, but I didn't think I had to isolate the basement floor from the pier. It's clear that you think that would be a mistake. Given your reply about isolating the base of this structure, I'm sure you would also strongly recommend isolating the cap as well.

 

If I could ask one other question; when making these pours, would it be enough to use standard expansion strips to isolate the pier, or would you recommend a wider gap (perhaps an inch or two of styrofoam)?

 

Emmett



#5 OrionStrider

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Posted 22 May 2020 - 03:59 PM

 

Hello

 

That is some foundation! - by by metric to imperial calc, about 14800 pounds of concrete in the foundation.

 

What is often done is the pier base is poured first, the size is a factor of how big your wallet is, big equals getting a structural engineer to design it, less big is bypass the engineer and over build it, joe average is make it as big as you can afford and hope for the best, I'm in the latter group. My pier base is about 4 by 4 feet by 4 feet deep with a riser coming out of that. 

 

The top of the pier base finishes BELOW grade, when you pour your slab for the obs, you would isolate the pier form the slab, I used a light insulation foam, that is  used on any vertical surface or horizontal surface. 

 

 
 
Hope this helps
 
Ed
 
 
 
 

 

 

Sorry for the confusion Ed. But thank you very much for the photo. I was thinking that a 12" pier would be sufficient, especially if I anchored it to the cement cap of my 10' x 10' x 8' basement (see my reply to Bill). However, both your reply, and Bill's, stress isolating even the base of the pier, so I'm sure you would recommend isolating the cap as well. 

 

This leaves me with one of my original questions unanswered. Is a 12" diameter pier that may be 8' to 10' high (unsupported) sufficiently stable for a mount that would carry a 100lb load? 

 

Thanks

Emmett



#6 speedster

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Posted 22 May 2020 - 04:27 PM

Weight of the load is insignificant.  Your soil bearing capacity will likely be at least 2,500 psf and just skin friction of the underground part can carry that much again.  No need for a spread footing to carry the load and spread footings actually work against you.  There is a PDF file at the to of this thread that tabulates some deflections for steel and concrete piers up to 12":  The entire pier needs to be isolated from everything else.  The size of the gap needs to be big enough to prevent contact when the slab moves in relation to the pier due to soil movement.    An inch should do it.  Expansion joint material can be used in 2 layers to achieve 1" thickness but it's not the ideal material.  You can get a roll of blue foam designed to go under wood walls at any big box building supply store and wrap it to whatever thickness you want.   


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#7 *skyguy*

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Posted 22 May 2020 - 04:49 PM

Conventional wisdom and probably the vast majority of CN members will tell you it's absolutely necessary to isolate the pier from the the slab floor.

 

However, 19 years ago I built a 13' pier using 16"x16"x8" chimney blocks for my garage-top observatory and the pier was placed directly on the garage floor. It is not isolated from the concrete slab. A 12" LX200 scope is installed on the pier, which is used almost exclusively for imaging and I've never had any problems with pier induced vibrations.

 

Back then, the materials for the pier were around $75 USD with the labor supplied by myself and a friend. The performance of this pier went beyond my expectations and IMHO ... it's the easiest and cheapest tall pier you can build. I wouldn't hesitate to build another.

 

Good Luck with your observatory build.

 

observatory pier.jpg

 

OrbitJet Observatory:

 

https://www.flickr.c...57644177074161/


Edited by *skyguy*, 22 May 2020 - 04:50 PM.

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#8 OrionStrider

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Posted 22 May 2020 - 05:30 PM

Conventional wisdom and probably the vast majority of CN members will tell you it's absolutely necessary to isolate the pier from the the slab floor.

 

However, 19 years ago I built a 13' pier using 16"x16"x8" chimney blocks for my garage-top observatory and the pier was placed directly on the garage floor. It is not isolated from the concrete slab. A 12" LX200 scope is installed on the pier, which is used almost exclusively for imaging and I've never had any problems with pier induced vibrations.

 

Back then, the materials for the pier were around $75 USD with the labor supplied by myself and a friend. The performance of this pier went beyond my expectations and IMHO ... it's the easiest and cheapest tall pier you can build. I wouldn't hesitate to build another.

 

Good Luck with your observatory build.

 

attachicon.gifobservatory pier.jpg

 

OrbitJet Observatory:

 

https://www.flickr.c...57644177074161/

Interesting perspective on the normal isolation recommendations. From the picture you provided, it looks like the ceiling is also attached to the pier...and still no vibration issues... I guess I'll have to decide if I want to risk it. 



#9 OrionStrider

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Posted 22 May 2020 - 05:37 PM

Weight of the load is insignificant.  Your soil bearing capacity will likely be at least 2,500 psf and just skin friction of the underground part can carry that much again.  No need for a spread footing to carry the load and spread footings actually work against you.  There is a PDF file at the to of this thread that tabulates some deflections for steel and concrete piers up to 12":  The entire pier needs to be isolated from everything else.  The size of the gap needs to be big enough to prevent contact when the slab moves in relation to the pier due to soil movement.    An inch should do it.  Expansion joint material can be used in 2 layers to achieve 1" thickness but it's not the ideal material.  You can get a roll of blue foam designed to go under wood walls at any big box building supply store and wrap it to whatever thickness you want.   

This entire structure will be sitting on bedrock, so my concern isn't really load capacity but vibrational stability. In fact, it is because all of the concrete of the pier, and surrounding structure, will be on bedrock that I thought perhaps I could just anchor everything together and not worry about isolation at all.



#10 *skyguy*

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Posted 22 May 2020 - 08:01 PM

Interesting perspective on the normal isolation recommendations. From the picture you provided, it looks like the ceiling is also attached to the pier...and still no vibration issues... I guess I'll have to decide if I want to risk it. 

The observatory floor (ceiling) is isolated from the pier with a 1" gap around the pier. I can walk around the observatory during an imaging session without causing any causing any vibration artifacts in a sub-frame. However, I've never tried jumping up and down to see if it would cause any problems. I always run an imaging session from inside my home, so there's no chance I would be the cause of any vibrations at the scope.

 

BTW, The pier was left hollow, except for the bottom 3' that was filled with concrete.The top cap was constructed using a double layer of 3/4" plywood which was bolted to the top block.

 

Here's a shot of NGC4565 taken with my 12" SCT from my  garage rooftop observatory:

 

NGC4565_web_F.jpg


Edited by *skyguy*, 23 May 2020 - 06:04 AM.

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#11 Berny

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Posted 22 May 2020 - 08:12 PM

I don't think your'e going to vibrate bedrock by any significant amount by walking on it. 



#12 Berny

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Posted 22 May 2020 - 08:15 PM

The observatory floor (ceiling) is isolated from the pier with a 1" gap around the pier. I can walk around the observatory during an imaging session without causing any causing any vibration artifacts in a sub-frame. However, I've never tried jumping up and down to see if it would cause any problems. I always run an imaging session from inside my home, so there's no chance from me causing any vibrations at the scope.

 

BTW, The pier was left hollow, except for the bottom 3' that was filled with concrete.The top cap was constructed using a double layer of 3/4" plywood which was bolted to the top block.

 

Here's a shot of NGC4565 taken with my 12" SCT from my  garage rooftop observatory:

 

attachicon.gifNGC4565_web_F.jpg

Fantastic image. One of the best.



#13 OrionStrider

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Posted 22 May 2020 - 11:44 PM

The observatory floor (ceiling) is isolated from the pier with a 1" gap around the pier. I can walk around the observatory during an imaging session without causing any causing any vibration artifacts in a sub-frame. However, I've never tried jumping up and down to see if it would cause any problems. I always run an imaging session from inside my home, so there's no chance from me causing any vibrations at the scope.

 

BTW, The pier was left hollow, except for the bottom 3' that was filled with concrete.The top cap was constructed using a double layer of 3/4" plywood which was bolted to the top block.

 

Here's a shot of NGC4565 taken with my 12" SCT from my  garage rooftop observatory:

 

attachicon.gifNGC4565_web_F.jpg

Not much evidence of any vibration in that Needle. Great image!



#14 appicloudy

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Posted 23 May 2020 - 04:11 PM

Hello Emmett

 

Sorry I had dumb thumbs when I typed the weight of the concrete ( even though it was a solid block ) it should have been 143800 pounds, you could have put a huge telescope on that!.  8-))

 

One bit of advice I would offer is, in the photo you can see some cable conduits, I used 32mm id ( about 1.25" ) thinking it would be fine, drawing low voltage cable through it is ok, USB not so much, if you put any conduit in your slab I would go for 3" or even 4" with the biggest radius elbows you can get, I would also put some draw wires in as I'm installing it.

 

Ed


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#15 Rustynuts

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Posted 23 May 2020 - 08:43 PM

I have a 20 inch concrete pier that is 12 foot tall. i thought 16 inch was too small.

  Also, I dont like things falling/rolling in the 1inch gap. I used the spray foam to close the gap, then used Plasti dip to coat the foam.  well i went a little overboard and coated the pier with Plasti dip as well, it took about 1/2 gallon to coat the 3 foot that  protruded the observatory floor.

best money I've spent.  that stuff really fells great! no more scuffed knuckles on concrete. and if something  hits it, it just bounces off.


Edited by Rustynuts, 23 May 2020 - 08:44 PM.

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#16 OrionStrider

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Posted 24 May 2020 - 02:17 AM

Hello Emmett

 

Sorry I had dumb thumbs when I typed the weight of the concrete ( even though it was a solid block ) it should have been 143800 pounds, you could have put a huge telescope on that!.  8-))

 

One bit of advice I would offer is, in the photo you can see some cable conduits, I used 32mm id ( about 1.25" ) thinking it would be fine, drawing low voltage cable through it is ok, USB not so much, if you put any conduit in your slab I would go for 3" or even 4" with the biggest radius elbows you can get, I would also put some draw wires in as I'm installing it.

 

Ed

Thanks for the tips Ed. I was thinking 1.5"-2" conduit for 12guage mains. I haven't given as much thought to low voltage cable runs as perhaps I should. I have been assuming that wifi would take care of any long-distance communication needs. I can't think of any reason for long USB runs, but maybe ethernet, or coax, or phone (hmmm)?



#17 OrionStrider

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Posted 24 May 2020 - 02:26 AM

I have a 20 inch concrete pier that is 12 foot tall. i thought 16 inch was too small.

  Also, I dont like things falling/rolling in the 1inch gap. I used the spray foam to close the gap, then used Plasti dip to coat the foam.  well i went a little overboard and coated the pier with Plasti dip as well, it took about 1/2 gallon to coat the 3 foot that  protruded the observatory floor.

best money I've spent.  that stuff really fells great! no more scuffed knuckles on concrete. and if something  hits it, it just bounces off.

Thanks for the input Rustynuts. I coated some tool handles with Plasti dip years ago. I never thought about using it to coat foam &. concrete!



#18 Rustynuts

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Posted 24 May 2020 - 02:43 PM

Thanks for the input Rustynuts. I coated some tool handles with Plasti dip years ago. I never thought about using it to coat foam &. concrete!

Yes I love the stuff,  have also covered tables.

   The first time I tried it I thought   what have I gotten myself into!

I use the plasti dip blue for my table that I use with high value items such as optics. best thing is if it gets scuffed up just thin it with naphtha, and brush on more to cover the scuff mark, as you build the layers up over time you build up thin layers only has thicker rubber.

I probably should start a new thread for this if you guys want more info.



#19 Rustynuts

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Posted 24 May 2020 - 10:35 PM

I did not isolate my pier from the 2nd floor where it starts,my second floor is concrete  if i did not ,the pier would have been over 24 foot tall! I did isolate it from the 3rd floor which is the observatory floor. there is no noticeable vibration even when jumping up and down on the 2nd floor.  and no trace when doing the same on the 3rd floor. I guess it would be helpful to know what scope will be used as a large refactor is more sensitive to vibration, hence why I went with a 20 inch pier. for a test I bumped the pier with a 2lb hammer and it vibrated for less than 1 second. I am visual only and do not want any noticeable vibration when focusing.

 

I am using a 9  inch f9 refractor. it weighs 90lbs  without counterweights!

 

  I also tried a C14 even the focal length was 3914 vs 2050 for the refractor it vibrates much less. if you are doing astro photos  with a short /stubby scope the demands will be less than a scope with a long tube. bigger Dia Pier will be better, you may want to do some tests before committing to a diameter.  My personal opinion is I would not hesitate not to isolate from the slab at all. we can only advise what we know or did, you may have higher requirements for your needs than we did.

Jon


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#20 OrionStrider

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Posted 25 May 2020 - 10:59 AM

I did not isolate my pier from the 2nd floor where it starts,my second floor is concrete  if i did not ,the pier would have been over 24 foot tall! I did isolate it from the 3rd floor which is the observatory floor. there is no noticeable vibration even when jumping up and down on the 2nd floor.  and no trace when doing the same on the 3rd floor. I guess it would be helpful to know what scope will be used as a large refactor is more sensitive to vibration, hence why I went with a 20 inch pier. for a test I bumped the pier with a 2lb hammer and it vibrated for less than 1 second. I am visual only and do not want any noticeable vibration when focusing.

 

I am using a 9  inch f9 refractor. it weighs 90lbs  without counterweights!

 

  I also tried a C14 even the focal length was 3914 vs 2050 for the refractor it vibrates much less. if you are doing astro photos  with a short /stubby scope the demands will be less than a scope with a long tube. bigger Dia Pier will be better, you may want to do some tests before committing to a diameter.  My personal opinion is I would not hesitate not to isolate from the slab at all. we can only advise what we know or did, you may have higher requirements for your needs than we did.

Jon

Right now I'm doing astrophotography with a 4" f3.9 refractor. However, I want to step up to a 5"- 6" refractor in the future. I'm being persuaded that I should isolate at least the concrete cap and increase the diameter of my pier. I'm not yet persuaded that isolating a base slab, that will be poured on bedrock along with the base of the pier, will be necessary.

 

Thanks for the input!



#21 appicloudy

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Posted 25 May 2020 - 03:25 PM

Ok,  " I'm not yet persuaded that isolating a base slab, that will be poured on bedrock along with the base of the pier, will be necessary. " 

 

Try this then, you do all the work without isolating the pier from the slab, and then you find you do have some vibration in the pier.

 

Do you try to fix it? how?

 

Or do you spend a very small amount of time and money during construction and go with what seems to work for all who have gone before you?

 

Seems like a no brainer to me.

 

Ed


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#22 OrionStrider

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Posted 25 May 2020 - 03:57 PM

Ok,  " I'm not yet persuaded that isolating a base slab, that will be poured on bedrock along with the base of the pier, will be necessary. " 

 

Try this then, you do all the work without isolating the pier from the slab, and then you find you do have some vibration in the pier.

 

Do you try to fix it? how?

 

Or do you spend a very small amount of time and money during construction and go with what seems to work for all who have gone before you?

 

Seems like a no brainer to me.

 

Ed

It's not clear to me how concrete foundations and slab poured on top of bedrock will be isolated from the base of the pier which will also be poured on bedrock; unless you're suggesting I isolate the entire slab from both the foundation and bedrock (as well as the pier)?



#23 appicloudy

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Posted 26 May 2020 - 02:28 AM

Emmett, you in the end are responsible for how you design / build your pier, if you don't see any need to isolate the slab and pier then it's is your choice to build as you think best.

 

You asked for input, we gave it even though it was the 9th post before you mentioned for the first time about vibration isolation.

 

Personally, I don't care if you go your own way, or you listen to what insight others have offered.

 

I hope it works for you, none of use like to construct anything that does not work 100% the way we thought it would.

 

That is doubly true of a pier that may be impossible to rectify after it is completed.

 

Of course it may be a moot point, apart from when I remove the cover from my telescope and open the dome, I am nowhere near the telescope. that may be the case with you as well so isolating the pier from the slab may not be an issue at all.

 

Ed



#24 Rustynuts

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Posted 26 May 2020 - 03:43 AM

Right now I'm doing astrophotography with a 4" f3.9 refractor. However, I want to step up to a 5"- 6" refractor in the future. I'm being persuaded that I should isolate at least the concrete cap and increase the diameter of my pier. I'm not yet persuaded that isolating a base slab, that will be poured on bedrock along with the base of the pier, will be necessary.

 

Thanks for the input!

Go with a minimum of 16 inch Diameter WITH REBAR.

 

no less than five vertical pieces with at least one round cage every foot of Pier height you will need to wire wrap the vertical pieces to every ring of  rebar and be sure to bend the bottom of your top bolts.  also if you already have a top plate mark north and have the bolts installed on it so  the bolts are all straight wile the concrete sets. give it at least a month before removing the sonotube.

 

Don"t Forget to run a pipe in the middle for power and cables.

 

You dont know what scope you may get in the future. Remember the old saying,  only cry once,

  Don't do anything you can't reverse.

so, isolate it.   your plan is very sound,

 Don't overthink or make it  complicated.

in the end,

 The weak link will be.. The mount.

Jon


Edited by Rustynuts, 26 May 2020 - 04:22 AM.

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#25 mikerepp

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Posted 26 May 2020 - 08:10 AM

Make sure that you add extra conduit in your design.   You do not want to run signal lines and power lines in the same conduit.  This will help avoid crosstalk/interference with the low signal lines.

Make sure that rebar used is not in contact with soil.  This will cause the rebar to rust, better that it is isolated within the concrete.  Good luck with the build.




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