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Concrete reinforcement

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#26 Bob Moore

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Posted 20 February 2013 - 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.

#27 JJK

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Posted 21 February 2013 - 12:24 AM

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


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

#28 roscoe

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Posted 23 February 2013 - 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

#29 JJK

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Posted 24 February 2013 - 08:42 AM

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.

#30 Nick Rose

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Posted 27 February 2013 - 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....th 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






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