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Equipment Discussions >> Observatories

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jazle
super member


Reged: 05/20/10

Loc: California, USA
Re: Concrete reinforcement new [Re: Kunama]
      #5691493 - 02/20/13 06:32 PM

Calculate where the center of mass will be. Add more cubic feet to the base until it's at least a foot below ground level. Then you won't have to worry about it "going anywhere" from being top heavy if your soils are saturated.

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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Concrete reinforcement new [Re: Kunama]
      #5691505 - 02/20/13 06:39 PM

Quote:

Quote:

Nick:

Personally, I think people go a bit "overkill" on the base of these piers.

-Dan




In some cases "overkill" is an understatement, I have spend the last 15 years supervising concrete formwork construction and have built multistorey buildings on footings smaller than some of these pier bases.

The secret is in the base preparation and reinforcement, dig the hole then COMPACT the ground well before laying down plastic, then 'chair up' a sheet of steel mesh about 3" off the ground, another sheet about 3" below the finished concrete level and then fix 6 L-shaped anglebars 1/2" diameter to the bottom mesh arranged so they end up inside the proposed pier with at least 1" of concrete to the face of the pier.

I think a base area of 3' x 3' and 18" deep would be as big as you would ever need. My own pier base will be 2' x 2' x 18".

Just my 2 cents.





Matt, a footer for a building and a foundation for a telescope pier are two different animals. The former has to accept the load from above, whereas the latter has to do that plus not budge one iota. At high magnification, a small displacement at the scope translates to a significant change in the EP.

Also, it is important to get the base of the foundation below frost line (i.e., well below 18" deep).


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Kunama
professor emeritus


Reged: 10/22/12

Re: Concrete reinforcement new [Re: JJK]
      #5691941 - 02/20/13 11:06 PM





Also, it is important to get the base of the foundation below frost line (i.e., well below 18" deep).




My apologies, I did not realise that San Mateo, California suffered frosts or had a frost line.


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Nick Rose
super member


Reged: 11/01/10

Loc: San Mateo, CA
Re: Concrete reinforcement new [Re: Kunama]
      #5691957 - 02/20/13 11:19 PM

San Mateo does not have a frost line, at least as I know of.

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mikey cee
Postmaster
*****

Reged: 01/18/07

Loc: bellevue ne.
Re: Concrete reinforcement new [Re: Nick Rose]
      #5692009 - 02/20/13 11:46 PM

Don't overkill your project like some commercial or government projects. When putting rebar, pencil rod and wire mesh into place make sure you leave enough room to fit in a "little" concrete! Mike

Edited by mikey cee (02/20/13 11:49 PM)


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Bob Moore
Pooh-Bah
*****

Reged: 04/07/06

Loc: New York
Re: Concrete reinforcement new [Re: Nick Rose]
      #5692016 - 02/20/13 11:54 PM

All the piers and pier blocks I do here in the north east are 48"L x 48"W x 54"D, our frost line is just about 48" down. i try to make the hole a little trapezoidal, whats another bag of cement, i have never been lucky enough to have a truck come onto the site. as you can see by the photos in the posted link above the last project took 4 palettes of 80lb bags of Quikrete mixed the a 5 bag mixer, but the kick in the butt was we had no water out at the site. We used a 100gl watering toff in the back of my truck and dipped 5gl buckets into it to get the water.

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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Concrete reinforcement new [Re: Kunama]
      #5692049 - 02/21/13 12:24 AM




My apologies, I did not realise that San Mateo, California suffered frosIs or had a frost line.





Matt, your comments, including the depth of the concrete base, sounded general in nature. And you're right, San Mateo doesn't suffer "frosls".

Edited by JJK (02/21/13 12:27 AM)


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roscoe
curmudgeon
*****

Reged: 02/04/09

Loc: NW Mass, inches from VT
Re: Concrete reinforcement new [Re: JJK]
      #5697300 - 02/23/13 08:52 PM

Hey....be nice! We all make typos sometimes.......

I don't pour concrete for a living, I build things..... including not only houses and decks, but also free-standing radio towers and once a 14-meter radio telescope, and I'm siding with Matt, in that the world of amateur astronomy about single-handedly keeps the concrete industry in business.
But, joking aside, much of the ultimate stability of a pier is determined by the soil composition in which the concrete is placed, and much more is determined by the construction of the pier itself and the attention to detail of the backfill placed around it. Wide and flat bottom surfaces poured on undisturbed subsoil and against undisturbed subsoil walls are always the most stable option. Any removed and replaced fill will be less firm than before it was removed, the best possible packing and compacting of backfill will still leave a free-standing pier built in a dug-out hole able to be moved around a small amount relatively easily. For a house, not a problem - all the load is straight down, and as long as the footing can't push downward into soft fill, all is good. For something like a radio tower with a large wind-induced side load, a cube - even a big one - with dirt shoveled back in around it, will soon indicate the prevailing wind direction. Scopes in a small way replicate the loads on towers. A slab of concrete 5 x 5 x 1 foot containing 25 cubic feet with a pier sticking up from it will offer far more resistance to side forces than a 3 x 3 x 3 foot cube containing 27, with a similar diameter pier, because the leverage forces required to lift an edge are far greater.
If you're wondering, my own pier, built to hold a 6" refractor, is a 2' diameter hole dug about 5' deep in firm gravely clay (liberal use of an electric jackhammer was required) filled with concrete directly into the hole about 3' deep, with a 10" sonotube to ground level, and a steel pier above ground. The pier, and the building's corner-piers, all stop at ground level in case of future removal of the building. A hard kick will vibrate the steel pier a bit, but I don't kick the top of it very often when I'm observing.....
I have noticed, though, that the big town snowplow truck, main and wing down, loaded with several yards of road sand, will vibrate the scope visibly when it passes 1/4 mile away, but that thing's pretty much a rolling earthquake, so that's to be expected, I guess.
Russ


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Concrete reinforcement new [Re: roscoe]
      #5697890 - 02/24/13 08:42 AM

Quote:

Hey....be nice! We all make typos sometimes.......

I don't pour concrete for a living, I build things..... including not only houses and decks, but also free-standing radio towers and once a 14-meter radio telescope, and I'm siding with Matt, in that the world of amateur astronomy about single-handedly keeps the concrete industry in business.
But, joking aside, much of the ultimate stability of a pier is determined by the soil composition in which the concrete is placed, and much more is determined by the construction of the pier itself and the attention to detail of the backfill placed around it. Wide and flat bottom surfaces poured on undisturbed subsoil and against undisturbed subsoil walls are always the most stable option. Any removed and replaced fill will be less firm than before it was removed, the best possible packing and compacting of backfill will still leave a free-standing pier built in a dug-out hole able to be moved around a small amount relatively easily. For a house, not a problem - all the load is straight down, and as long as the footing can't push downward into soft fill, all is good. For something like a radio tower with a large wind-induced side load, a cube - even a big one - with dirt shoveled back in around it, will soon indicate the prevailing wind direction. Scopes in a small way replicate the loads on towers. A slab of concrete 5 x 5 x 1 foot containing 25 cubic feet with a pier sticking up from it will offer far more resistance to side forces than a 3 x 3 x 3 foot cube containing 27, with a similar diameter pier, because the leverage forces required to lift an edge are far greater.
If you're wondering, my own pier, built to hold a 6" refractor, is a 2' diameter hole dug about 5' deep in firm gravely clay (liberal use of an electric jackhammer was required) filled with concrete directly into the hole about 3' deep, with a 10" sonotube to ground level, and a steel pier above ground. The pier, and the building's corner-piers, all stop at ground level in case of future removal of the building. A hard kick will vibrate the steel pier a bit, but I don't kick the top of it very often when I'm observing.....
I have noticed, though, that the big town snowplow truck, main and wing down, loaded with several yards of road sand, will vibrate the scope visibly when it passes 1/4 mile away, but that thing's pretty much a rolling earthquake, so that's to be expected, I guess.
Russ





In general, if someone is located where the ground freezes, they need to get part of the pier foundation below frost line. I agree that digging a hole wider and initially deeper than the foundation is not advisable.


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Nick Rose
super member


Reged: 11/01/10

Loc: San Mateo, CA
Re: Concrete reinforcement new [Re: JJK]
      #5704317 - 02/27/13 09:32 PM

We are having our backyard patio done so we talked to the concrete guy that is doing that to also do the pier for me. I wanted to do concrete footings for the building but he suggested that a slab would be better, since our area is prone to settling. Will the concrete pad affect my local seeing condition or will I really not see any difference in my pictures(The temp in San Mateo rarely gets to 100 in the summer)? Also what kind of gap should there be between the slab and pier?

This is my general location. So since I'm in a area with a lot of houses, concrete drive ways and so forth and guess the slab wont affect the local seeing.
https://maps.google.com/maps?q=1509+South+Grant+Street,+San+Mateo,+CA&hl=en&sll=37.560262,-122.308195&sspn=0.013778,0.027874&oq=1509+S&t=h&hnear=1509+S+Grant+St,+San+Mateo,+California+94402&z=17

Edited by Nick Rose (02/27/13 10:20 PM)


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