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Pier footing

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#26 Travellingbears

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Posted 26 September 2020 - 02:49 PM

Pete

 

Just take a look and buy 4 of the ‘rebar chairs’. they’re less than $1 each and wire. If you’ve doing vertical rebar with horizontal ‘circles’ or ‘x’ ensure that vertical rebar don’t extend more than necessary (1”’) to wire-tie with horizontal members on end that’s going into ground. Then wire tie several of the ‘rebar chairs’ under horizontal pieces of the base (so it’s lifted a few inches up). Then cement is definitely ‘encasing’ underneath rebar. As others said don’t drive the rebar into the ground. I dropped rebar cage into ground (rebar chair end) and poured a foot or so. I ‘sticked’ the cement and tapped the rebar with hammer to eliminate bubbles at that point to ensure base was fully encased (knew rebar was up 3” due to use of rebar chairs). Then continued with pour for another foot (repeated ‘sticking’ and tapping rebar), and repeated until reached top. Pic below of my rebar cage when sitting ‘upside down’ in homemade jig. In that position the vertical rebar are extending a bit more than they were when went in hole. You can barely see the thinner gauge wire (rebar chair assembly) going along the visible ‘top’. Sorry for poor photo from our security camera. Didn’t shoot any pics from my iPhone. 

 

I was using 4’ verticals and bending pairs of 2’ for ‘circles’ with 1’ and 2’ used as needed for ‘x’ midpoint to edge or edge to edge.  My finished depth was 42-43 inches (frost line is 30”) and 18x22 wide. Had to bend over the 4’ at ends on top which provided a hook for my L-bolts when template was set. I knew that bolts were in concrete but also had position under 1/2” rebar.
 

Dave

 

Making rebar cage for pier

Edited by Travellingbears, 26 September 2020 - 03:23 PM.

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#27 macdonjh

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Posted 26 September 2020 - 06:59 PM

Can you furnish a link to an astronomical pier that has pulled apart from tension forces?  As you said, not likely.

It's not about saving $30.  The mob think on this forum has promulgated the notion that concrete isn't 'strong' unless rebar is in the mix.  Since most of the people here have never done concrete work they put in the rebar, usually by pounding the rebar into the base of the pier hole and then pouring in the concrete.  How ironic is it that they've assured the eventual cracking of their concrete pier?  Rebar must be fully encased in concrete or else it will rust, expand, and crack the surrounding concrete.  

 

dan k.

If you dig more than a couple of feet down for your footer, there won't be any oxygen down there to make your rebar rust even if you do pound it into the dirt.

 

To answer PeterM's question, generally you want 2"-3" of "cover" for rebar, the same as for anchor bolts.  3" is better than 2".



#28 speedster

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Posted 27 September 2020 - 04:27 PM

Cold weather pouring:  if ice crystals form in the paste, strength is greatly reduced.  Generally, above 40 degrees for 2 days prevents risk of problems.  The hydration process generates a lot of heat so the mix will be heating itself as it cures.  After 2 days, hydration is sufficiently complete to leave very little free water tp cause a problem.  

 

Hunter is spot on that we'd be hard pressed to find a telescope pier that failed purely in tension but we don't have to look far to find problems caused by rebar stabbed into the dirt (if we were to dig some up).  Still, concrete is just not strong in tension but at the diameters we are using, it's strong enough for what we are doing.  Take an unreinforced telescope pier and put it in a parking lot as a bumper and the first car to tap it could knock it right off.  Fortunately for us, with fat piers and hardly any loading, concrete is forgiving of most all bad practices we can come up with or corners we can cut. 

 

Concrete failures in tension are all around us:

 

tension-slab.png



#29 deonb

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Posted 30 September 2020 - 05:39 AM

Instead of using rebar, what are the thoughts of running EMT or Rigid pipe up the center of the pier? That way it can also function as a wire conduit.

 

Would have to come out the bottom with a 90 degree though - not sure if that will be a problem.



#30 macdonjh

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Posted 30 September 2020 - 08:32 AM

deonb,

 

EMT won't do much for you, it's not much more than foil wrapped into a cylindrical shape.  OK, it's not that flimsy, but it isn't strong, either.  Real pipe would work, but as you said, you'd need to use a pipe fitting to bring the "conduit" out of the pier.  Fishing a wire through a piping elbow is tough.  Using those long radius conduit bends is much easier.

 

For my 12" diameter pier I made a cage from #3 rebar and cyclone fence and then ran my plastic conduits through the middle.  I have conduit "sweeps" at below grade where the conduit enters my pier as well as near the top of my pier where the conduit exits.

 

I know the minimal benefit of rebar in this application has been thoroughly discussed, but using a piece of steel pipe as combination rebar/ conduit actually lessens its effectiveness further.  The pipe would likely be placed at the center of the pier where it would have almost no strengthening effect, and not much effect in keeping the concrete together if it does crack.  A few pieces of rebar spaced around the circumference of a circle 2"-3" "inside" the surface of the pier would be better.



#31 PeteM

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Posted 03 October 2020 - 02:55 PM

I want to thank everyone for their support, thoughts, and suggestions on this project.

 

Yesterday and today I got the footing dug (by hand) and poured. Glad I did do it by hand, even if it took a little while longer. I used a post hole digger first to make sure I was not going to hit a piece of the septic system even thought I had all the drawings with dimensions. I did find a pvc pipe portion of it in the first spot that would of been a disaster if I rented a power auger with 18" bit. So after having so discussions, I moved the footing location to the south further and then proceeded to dig the 18" diameter and what eventually ended up being roughly 60" deep (frost depth is 36-48" deep depending on source here). Top soil was the first 24" of the hole and then the rest was sand. I poured to roughly half way before inserting the the rebar (#4 x 48"), making sure I did not hit bottom. I ended up going thru all 15 80lbs bags of concrete, which I am glad I purchased the 15th one...the online calculator I used showed 14 bags would be needed. I actually only made it half way to the top of the form, but I called that good. Smoothed and leveled the top and adjusted my j-bolts down. Below are some pictures.

 

So now I am going to let it sit a couple days before removing the template and form. The SB pier is still 4-6wks out from being deliver so plenty of time to let the footing cure. But I will like start on the observatory around it. smile.gif

 

Pier footing showcase.JPG


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#32 Travellingbears

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Posted 03 October 2020 - 06:53 PM

Pete

 

Congrats on a great start!
 

Dave


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#33 Travellingbears

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Posted 04 October 2020 - 12:09 PM

Pete

 

What size is proposed for the ROR? Is it similar to your last one? 
 

For your AP 1100 to SB pier connection, assuming you’ll use AP’s 119FSA for the direct connection to the 1100. Are you going to directly ‘drill/tap’ the top of the SB ME pier to add the 8 holes required for holding 119FSA or ‘wafer’ with intermediate flat plate arrangement with custom pre-drilled holes (8 for 119FSA and 4 for ME)? Didn’t see ‘off the shelf’ third party flat plates designed with 1100 bolt to SB pattern. I’m lucky with Mach 1 since AP’s ADATRI (ring to hold Mach 1) has three bolt pattern on bottom (3 holes at 5.11” circumference) which fit one of the 3-bolt circular sets on SB’s MyT adapter plate (around $100). The ‘MacGyver solution’ adds less 1” thickness and screws on/off pier (uses 4 MyT holes) when using Mach 1.

 

Dave



#34 PeteM

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Posted 04 October 2020 - 01:06 PM

Dave

 

I am going with a smaller ROR of sorts. To keep the boss happy with a small footprint I am going with a design very close to this: https://www.cloudyni...ng-a-mini-ror/ My old ROR was 8x10 and this new one will be 6x6. But since I am imaging pretty much all the time and remotely (from in my house) the loss of space is no big deal and I have it designed to handle up to a 14" OTA which would be the biggest I would ever likely get.

 

You correct, I have the 119FSA and was planning to mount that directly to the SB pier top. But I do have a 1" thick 12" diameter aluminum plate still I used for my old observatory and could use that as a wafer to save drilling more holes in the top of the SB pier. I will have to mate that plate to the SB pier if I want to use my CGE-Pro mount while waiting for the 1100GTO to come. Now you got me thinking about a dedicated wafer that would be a clean install on both sides..maybe anodized too

 

Pete


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#35 Travellingbears

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Posted 04 October 2020 - 04:26 PM

Pete

 

Sounds like a good plan for ROR. I’d originally assumed that you were waiting to receive 48” pier (similar height to your concrete pier at old ROR). For 6x6 what height are planning for walls. What height is ME pier that’s on-order from Bisque? When does A-P say that you’ll receive the 1100 mount?

 

Dave



#36 PeteM

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Posted 04 October 2020 - 05:26 PM

Dave

 

Right now I am thinking 64-72" walls. I need to get the sub-floor constructed and pier height measured to get an "eyeball" to make sure they will not be clipping too much of the south and to ensure some pesky neighbor lights will be mostly blocked. The ME pier I have on order is 48" high. If my CAD drawings are correct, the floor will be about 9" off the ground, so making the pier in effect, ~39" tall. My last observatory used a raised floor with 2x6' for the trusses and that worked out great to keep seeing good and not have a concrete heat sink to deal with. As for the A-P mount, I ordered it through a dealer back in the end of July. In theory I heard it will be about another month..fingers crossed.

 

Pete




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