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size of pier base

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#1 dmcnally

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Posted 19 November 2017 - 08:48 PM

I'm going to put a pier in my yard so that I can keep my mount mounted, and polar aligned, all the time.  I will be covering it with a Telegizmo 365 cover.  I suspect that I'll bring the ota inside the house most of the time, but it will be nice to leave it mounted when the weather cooperates for more than a day or two.  I'd love to build an observatory, but that's not in the cards right now.

 

I think I read somewhere, and I can't find it now, that there should be a 10:1 weight ratio of the loaded mount (mount + ota + other stuff) to the base (underground) part of the pier.  Using a combined weight of 150lbs for the mount would mean that the base should be 1,500lbs.  That's 25 60 lbs bags of concrete.  Using 0.45 ft^3 for a 60lbs bag of concrete yields 11.25 ft^3, or a cube with 2-1/4' (or 27") sides.

 

Is the 10:1 weight ratio correct?  If not, what should it be?

 

The other question I have is: how long do you let a concrete cube cure before you cover it (with dirt) and put a load on it?

 

FYI: I'm still deciding whether to use concrete or metal for the above ground portion of the pier.  I took a welding class at the local junior college and don't have any real concerns about fabricating a metal column with gussets and flanges (or pouring a concrete column).

 

Clear Skies,

Dave

 



#2 boott

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Posted 19 November 2017 - 09:51 PM

All concrete ,no rust ever!



#3 OleCuss

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Posted 19 November 2017 - 10:16 PM

I've got a really big block of concrete.  My pier should be complete this coming week and is all (except the ball-bearing and the mounting bolts) aluminum and will probably be anodized.  The ball-bearing (unusual design) and the bolts are stainless steel.  No rust anticipated.

 

I don't think that there is a hard-and-fast rule about the ratio of the weight of the block and the weight it supports.  Generally speaking, though, more massive is likely to be better.  I think you should be very happy with a 1,500 pound block.

 

My concrete block is into the over-kill zone.  It was done by a contractor and they couldn't get less than 1 yard delivered.  So my block ended up much bigger than I intended and likely weighs nearly two tons.  I think it is going to be about as stable as one could hope!  But given that my mount will be migrating a bit the bigger block is not a bad thing.

 

If I were in an area where the ground froze I'd want to make sure the block extended to below the frost line.  I don't think that will be a problem in Lompoc!



#4 dmcnally

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Posted 19 November 2017 - 10:32 PM

If I were in an area where the ground froze I'd want to make sure the block extended to below the frost line.  I don't think that will be a problem in Lompoc!

Thanks for the reply.  A hard winter in Lompoc is when you have ice on your windshield more than a dozen times.

 

Clear Skies,

Dave



#5 brave_ulysses

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Posted 20 November 2017 - 01:02 AM

hi dave,

 

i'd double the weight, aim for 4 feet deep and get the footer wider at the bottom

 

here are some concrete strength #s

1 day 16%
3 days 40%
7 days 65%
14 days 90%
28 days 99%

 

good luck!


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#6 dmcnally

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Posted 20 November 2017 - 01:13 AM

i'd double the weight, aim for 4 feet deep and get the footer wider at the bottom

Hi Clay,

Thanks for the info.  That's a lot of concrete.  I bought a HF cement mixer a couple of years ago for I project that I never started.  The mixer is still in it's box, but I guess it'll get some use now.

 

I wish you a happy Thanksgiving and a great holiday season!

 

Clear Skies,

Dave



#7 Messyone

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Posted 20 November 2017 - 04:44 AM

Here is my mount, kept outside for the past year. Post #959.

https://www.cloudyni...e-scope/page-39

 

Never heard of correct proportions. I built mine out of 6mm aluminium sheet and angles riveted together....very solid. Base is 600x600x6mm x3 aluminium sheets...JB weld and bolts holding it together. Basically went for what looked good....250kgs of concrete as the base.

 

Matt


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#8 dmcnally

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Posted 20 November 2017 - 09:13 AM

Hi Matt,

Thanks for the details on your pier.  I followed your mount build and really enjoyed it.  You definitely have a cool, and very unique, setup.

 

Clear Skies,

Dave



#9 brave_ulysses

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Posted 20 November 2017 - 09:59 AM

i think i have the same mixer! this is likely a bigger job than you want to do with that mixer. setup your form so that it takes about a yard of concrete and pay for the delivery. your back, eyes and sinuses will thank you. if you have any other concrete jobs to do at the same time, it will make the delivery charges less painful

 

have a great thanksgiving!

 

 

 

i'd double the weight, aim for 4 feet deep and get the footer wider at the bottom

Hi Clay,

Thanks for the info.  That's a lot of concrete.  I bought a HF cement mixer a couple of years ago for I project that I never started.  The mixer is still in it's box, but I guess it'll get some use now.

 

I wish you a happy Thanksgiving and a great holiday season!

 

Clear Skies,

Dave

 



#10 macdonjh

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Posted 20 November 2017 - 02:12 PM

Concrete weighs about 3400 lb per cubic yard.

 

I used 5x equipment weight as the first estimate for my concrete pier.  I then decided to go deep rather than down-and-wide.  I rented an auger and dug down 6'-9" (original design was nine feet down, but I hit a second layer of sand and quit digging.  That gives me 81" of concrete below grade and approximately 42" above grade for my 12" diameter pier.  It weighs about 880 lb.  It's rock solid for visual-- I haven't tried imaging.

 

If you want to dig, you can get a lot of the benefit of a huge block of concrete by pouring a spread footer at the bottom of your hole.  Just dig a hole, say 36" x 36" x frost depth, and pour an 8" thick slab at the bottom.  Then pour a concrete pillar however high you want on top of the spread footer (be sure the two are connected with some rebar).  Once you backfill the hole, the weight of the soil on top of the spread footer will give you much the same benefit as the big block of concrete, without pouring so much concrete.

 

You're going to love having a permanent pier in your yard.


Edited by macdonjh, 21 November 2017 - 07:48 AM.

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#11 dmcnally

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Posted 20 November 2017 - 05:34 PM

You're going to love having a permanent pier in your yard.

Thank you for posting your pier configuration.  I'm getting older and hauling my stuff out, aligning it, and then hauling it in, every time I wanted to observe or image was becoming a major deterrent to doing any astro stuff.  I'm sure a permanent pier will be a game changer.

 

Clear Skies,

Dave


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#12 Rusted

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Posted 21 November 2017 - 04:28 AM

Any shelter from the wind will help too.

Even a beach style cloth wind break might do.

Pre-drill holes in the ground with plastic pipe liners.

This would allow effortless setting up of the supporting posts.

Taken down again for storage just as easily. 



#13 P26

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Posted 21 November 2017 - 09:34 AM

Hi Dave,

 

One thing that's rarely considered is what to do with a pier 20 years from now when your property is sold or your interest in astronomy has waned.  The pier becomes an eyesore, and removing a massive above-ground concrete pier becomes a major challenge.

 

Since you have welding capability, you may wish to consider bolting a home fabricated steel pier to a concrete base 6" below ground.

 

A secondary advantage to this approach is that you can take the top section with you if you move or wish to reposition.

 

This is the approach that I used years ago on my 1st pier.  Later, when the observatory was built, the upper steel pier section (painted green) was reused:

 

pierstuffs.jpg

 

61022structure1.jpg

 

The 18" diameter reinforced concrete filled plastic culvert pipe extends to the joists under the observatory floor and will be nearly impossible to remove.

 

Best of luck,

 

Pete


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#14 Richard Whalen

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Posted 21 November 2017 - 11:40 AM

My pier is on a 3' square cube of concrete with an additional 8" height 18"X 18" square on top that is about 4" above ground level. Did not want the metal pier sitting in water or staying wet. Also makes it easier to adjust/shim if needed. My pier does not touch the concrete, has anodized 7075 aluminum shims .25" thick about 1-1/2" square it sits on. Pier is 7075 aluminum tubing with conduit running up the middle and filled with sand. Works well for my Parralax mount and 6" f12.



#15 dmcnally

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Posted 25 November 2017 - 01:15 PM

Thanks for all the responses.  I've decided that I'm going with a cubic yard for the base.  I'm still undecided about a metal or concrete column for the above ground portion.

 

I've redone the site survey and where I want to plant the pier.  It will be right in the middle of an existing patio.  The patio is almost 40 years old and the concrete has some major cracks in it.  I've also realized that I'm not ready to do a facelift on the back yard and that's going to postpone the permanent pier.

 

All is not lost because I've decided to repurpose the quadpod and put a short aluminum pier on it.  It won't be perfect, but it's a step in the right direction.   All the materials will be recycled or repurposed.  This should hold me until spring, or summer, when I can do the all the work in the back yard at one time.

 

I've also decided on a name for the temporary pier (Franken-Pier) and I'll start a new thread for it soon.

 

Clear Skies,

Dave



#16 roscoe

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Posted 26 November 2017 - 09:28 AM

If you want to dig, you can get a lot of the benefit of a huge block of concrete by pouring a spread footer at the bottom of your hole.  Just dig a hole, say 36" x 36" x frost depth, and pour an 8" thick slab at the bottom.  Then pour a concrete pillar however high you want on top of the spread footer (be sure the two are connected with some rebar).  Once you backfill the hole, the weight of the soil on top of the spread footer will give you much the same benefit as the big block of concrete, without pouring so much concrete.

 

This is very good advice, many people pour fairly small cross-section and tall footers, and use a lot of concrete, when a wide and thinner base as described here will offer more stability while consuming much less concrete - and time and effort.

 

A pier is sort of a lever, or angle-bracket-shaped structure....if you think of putting a shelf on your wall, you know that an angle-bracket that extends down the wall will support the shelf more than one that doesn't, and the same geometry applies to an upward force on the shelf. 

A t-shaped footing does the same.  As described above, the sideways force needed to lift the edge of that 't' shape is far greater than that needed to move a thin column in the ground.

So, if you have to dig a big hole - with an excavator or whatever, the t-shape is normally the best choice....

if you are hand-digging your hole, the most stable footing is to use the undisturbed soil on the sides of the hole as the form for the bottom foot or two.  That soil, if it's firm enough to hold its shape, is usually far stiffer and resistant to movement than any soil you can backfill with, even if you make a strong attempt to compact it, and the wet concrete flowing into the soil irregularities will make it close-to-impossible to pull out if that time comes, even with major excavating equipment.

In any case, a pyramid-shape - wider at the bottom, is always stiffer than a v-shape - wider at the top.

 

Also, in frost country, you want the top of your footing to be below the frost line, and remember that the frost penetrates deeper in an area under something - like a porch or deck or observatory, because it doesn't have a layer of snow to insulate it from the winter air above.....

 

PS I'm a retired small-town carpenter, I've planted and removed many piers,  and have learned what stays in place, and what doesn't.....



#17 Chrstphrlee

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Posted 26 November 2017 - 01:34 PM

I think your ratio is more than fine as long as you keep your pier and footing (base) isolated from your final walking surface. You can strip the forms the next day, then back-fill the same day. Avoid any big impacts to the concrete while back-filling. The concrete will continue to cure underground. Even in moist soil. Concrete will cure under water if poured correctly. You can load the concrete base with your metal pier later the second day, BUT, the important thing to remember is if your using a metal pier with embedded anchor or J-bolts bolts in the concrete, the same kind used in house construction, you can loosen them or even pull them out in green (uncured) concrete if your really trying to tighten them down. Bad dog. Wait a week. That should be long enough. If your not in a hurry, wait longer. I move 3800 Lb bakery ovens on a two-day old 6" slab as part of my construction business. No worries.




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