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Pouring concrete piers - an alternative approach

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#1 LoveChina61

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Posted 19 August 2021 - 08:10 PM

  I received some suggestions on how I might go about pouring both the cement foundation and the cement sonotube piers at the same time. I am not sure the cement pouring company is going to go for that approach so it looks like I will need to pour the piers first and then call them back again to do a second pouring of the observatory foundation.

  I am running out of time as the observatory construction team will arrive soon. I would appreciate some advice as I am clearly not very experienced or knowledgeable about the procedure for pouring the observatory's cement foundation in conjunction with a cement pier. My previous observatory's foundation was wooden decking so I just had to pour the cement pier base and sonotube but did not need to keep it from mingling with a cement foundation.

 

If I pour the pier base first, what kind of material might I be able to wrap around the protruding part of the cement base before having the cement foundation poured all around it? I imagine that just using some plywood to separate them while doing the foundation pour would not be sufficient as it would probably be really difficult to pull the plywood up out of the floor once the cement had dried. Is there another material that you could suggest? Ideally, it would be stout enough so that it would not be crushed between the pier foundation and pier base when the foundation was being poured, yet easily removable after it dried. Is there a certain kind of foam that might do the trick? I thought of using bubble wrap but knowing my luck the bubble wrap would all burst when pouring the foundation.

 

Would appreciate any ideas from those of you who have more experience than myself.

 

Mike



#2 photomagica

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Posted 19 August 2021 - 08:39 PM

I can appreciate your desire to avoid the significant cost of two concrete pours. This is a really good question and with some careful planning and a conversation or two with your concrete supplier you should be able to do one pour.

 

Just to fill in others, it is necessary to separate the slab from the pier to prevent vibrations in the slab from being transmitted to the pier. Do slabs vibrate? Yes they do - a surprising amount. We did tests on an existing slab at Lowell Observatory to see if we could save money by leaving out some piers at the Giovale Open Deck Observatory. The slab failed the "jump test" of having visitors stomp around and jump a few feet away from telescope while one of the educators observed at medium power. The imaged bounced around. Even someone walking firmly near the telescope was visible. Then we tested it with a telescope where the slab was separate from the pier. Nothing! Needless to say all the telescopes in the Giovale Open Deck Observatory got their own piers.

 

A good option is to build a form out of the same material being used to form the outside of your slab to create this gap. The gap may be covered over later with plywood.

 

I have also seen concrete asphalt expansion joint separation material used. I'm not sure if this material can be bent around your diameter of tube. Your concrete supplier should know about this.

 

Whichever method you choose, pour the pier first, since workers will need to stand in the slab area to do this. When this is done, fix any rebar issues from standing in the slab area and pour the slab. 

 

Think about making up a jig to set on top of the pier and hold anchor bolts for the mount in place while the cement hardens around them.

 

Please post some photos so we can all see how this went.

Best regards,

Bill


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#3 rgsalinger

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Posted 19 August 2021 - 09:43 PM

It depends on the slab and how deep you make it. We have a 6" thick slab (40 years old) and elephants could dance on it and the piers don't move. I've also been to other shared observatories where the piers are not separated - Deep Sky West for example - and again no problems. I think if you can do a pour this thick you're good.

 

If you're trying to get away with 3 or 4 inches then you've got trouble, maybe. The other thing to consider is how the observatory is going to be used. If the intent is to do visual then you have some reason to be in it while using it.If you are imaging go back to the house, car or warming room. 

 

It always pays to be careful but frankly I feel than in a lot of cases people are spending a lot of extra money for nothing. YMMV. 

 

Rgrds-Ross


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#4 LoveChina61

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Posted 18 September 2021 - 02:24 PM

I finally got the concrete foundation poured for my UK Satellite Observatory located in the Carolinas. I dug two deeper holes for the piers and then had them pour the two boxed-off pier foundations flush with the main pad. I will drill holes into the pier bases and use epoxy to glue in threaded bar to attach two steel piers to the floor.

 

I will worry about getting the 2"x4" forming boards out of there later! Maybe a jigsaw???  lol.gif

 

Mike

 

 

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#5 robbieg147

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Posted 18 September 2021 - 02:33 PM

I hope you have reinforcement in those slabs especially around the pier box outs otherwise you will likely get cracking. If it was me I would have lined up the sides of the piers with the main joints.


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#6 LoveChina61

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Posted 21 September 2021 - 10:22 AM

Thanks for mentioning this. They put sufficient rebar in the main slab but I wish they had put a bit more into the pier bases themselves.

 

They thought it would be no problem drilling into them to mount the steel piers. I saw some "non-conclusive debate" back and forth on this issue in another CN thread so thought I would just go with the cement pourer's view on the issue.

 

Hopefully all goes well on that. If not, I will just leave the scopes on their OEM tripods as I will mostly be using the observatory remotely.

 

Thanks,

 

Mike



#7 robbieg147

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Posted 21 September 2021 - 02:41 PM

It's fine to drill and epoxy bolts etc. but I get worried recommending this as then someone down the line may do the same with maybe a much higher pier, or just drill and dowel starters with a short embedment for a tall concrete pier and without knowing the details could be dangerous.



#8 FRANKVSTAR

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Posted 23 September 2021 - 11:02 PM

  I received some suggestions on how I might go about pouring both the cement foundation and the cement sonotube piers at the same time. I am not sure the cement pouring company is going to go for that approach so it looks like I will need to pour the piers first and then call them back again to do a second pouring of the observatory foundation.

  I am running out of time as the observatory construction team will arrive soon. I would appreciate some advice as I am clearly not very experienced or knowledgeable about the procedure for pouring the observatory's cement foundation in conjunction with a cement pier. My previous observatory's foundation was wooden decking so I just had to pour the cement pier base and sonotube but did not need to keep it from mingling with a cement foundation.

 

If I pour the pier base first, what kind of material might I be able to wrap around the protruding part of the cement base before having the cement foundation poured all around it? I imagine that just using some plywood to separate them while doing the foundation pour would not be sufficient as it would probably be really difficult to pull the plywood up out of the floor once the cement had dried. Is there another material that you could suggest? Ideally, it would be stout enough so that it would not be crushed between the pier foundation and pier base when the foundation was being poured, yet easily removable after it dried. Is there a certain kind of foam that might do the trick? I thought of using bubble wrap but knowing my luck the bubble wrap would all burst when pouring the foundation.

 

Would appreciate any ideas from those of you who have more experience than myself.

 

Mike

   I made my homemade observatory and used a 8in dia sonotube. It was poured into a ditch which I hollowed out about 3ft deeper and 3ft around of which I set the sonotube and then filled the tube with concrete and some old metal bed rails as support for the concrete. After this was done I poured the concrete floor for my observatory leaving a 4 Inch gap around the base of the pier to avoid vibration from walking on the floor. This has worked for me, although you still cant jump around the floor because the concrete in the bottom of the pier hole is not 3ooolbs of concrete. From what I have read unless your concrete base is a block of concrete about 8ft deep and the size of a large box that freezer would fit in, there's always a chance of some vibration being that you need the pier to become part of the earth even if the flooring is not touching it. Hard to do, so I dont stomp or jump, I just move slowly when taking photos.  Just my 2 cents worth of talk.  Good luck and wishing you success !!!


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#9 Garyth64

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Posted 26 September 2021 - 10:20 AM

I can appreciate your desire to avoid the significant cost of two concrete pours. This is a really good question and with some careful planning and a conversation or two with your concrete supplier you should be able to do one pour.

 

Just to fill in others, it is necessary to separate the slab from the pier to prevent vibrations in the slab from being transmitted to the pier. Do slabs vibrate? Yes they do - a surprising amount. We did tests on an existing slab at Lowell Observatory to see if we could save money by leaving out some piers at the Giovale Open Deck Observatory. The slab failed the "jump test" of having visitors stomp around and jump a few feet away from telescope while one of the educators observed at medium power. The imaged bounced around. Even someone walking firmly near the telescope was visible. Then we tested it with a telescope where the slab was separate from the pier. Nothing! Needless to say all the telescopes in the Giovale Open Deck Observatory got their own piers.

 

A good option is to build a form out of the same material being used to form the outside of your slab to create this gap. The gap may be covered over later with plywood.

 

I have also seen concrete asphalt expansion joint separation material used. I'm not sure if this material can be bent around your diameter of tube. Your concrete supplier should know about this.

 

Whichever method you choose, pour the pier first, since workers will need to stand in the slab area to do this. When this is done, fix any rebar issues from standing in the slab area and pour the slab. 

 

Think about making up a jig to set on top of the pier and hold anchor bolts for the mount in place while the cement hardens around them.

 

Please post some photos so we can all see how this went.

Best regards,

Bill

Very good idea.  Make a square form around the pier, maybe 2' x 2'.  Pour the slab with the pier, then later after the forms are removed, pour the 2' x 2' section.




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