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#1 Ziggy943

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Posted 22 July 2013 - 11:52 PM

Made out of concrete, in general, is it better to sink a narrow base for a pier deeper or to wider and shallower?

Is there a general principle of base width (or depth) vs pier height?

#2 Midnight Dan

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Posted 23 July 2013 - 08:43 AM

A lot depends on your ground and environmental conditions. The depth will often be dictated by your frost line, or for some people it will be limited by rock layers in the soil. You're really looking for a certain amount of mass, rather than a particular depth vs width.

Things to consider:

1) Frost line. The depth should be whatever the spec is in your area for foundations. In my case, the frost line is 4' deep so that's how far I went down.

2) Soil conditions. A sandy soil will require more width to be stable and support the weight of the pier. You can look on the web to find charts of soil types and how many pounds per square foot they'll support. An area with large slabs of rock may limit how far you can go down and may require you to drill holes in the rock so you can use rebar to tie in the rock layer with your pier concrete.

3) Mass of the base. If you have a fairly deep pier due to frostline, you will have a lot of mass in the concrete just to get to the right depth, so you won't need quite as large a base. In my case, I have a 1' diameter pier and used a 2' diameter hole for the base. If you go shallower due to rocks or if you don't have a deep frost line, then you would need a larger width for the base to get enough mass.

4) Uplift resistance. In northern climates you need to be concerned about frost heaves. This is where the soil near the surface becomes saturated with water in the fall, and freezes in the winter. Since water expands when frozen, it will lift the soil and anything in it ... like your pier. This usually causes tilting as well. To avoid this, the base needs to be wider than the top (at the soil surface) so that it locks the base into the ground below the frost line. In other words, don't dig a 4' deep, 2' diameter hole and just fill it up to ground level. Rather, fill a foot or two at the bottom with the full diameter, then use a sonotube above that for a 1' diameter pier the rest of the way up to the surface and beyond.

-Dan

#3 stmguy

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Posted 23 July 2013 - 09:28 AM

You can also use something called a "bigfoot" at the bottom of the hole like I did, just make sure it fits your sonatube
Norm

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#4 Ziggy943

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Posted 23 July 2013 - 06:06 PM

Thanks Dan

#5 Ziggy943

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Posted 23 July 2013 - 06:07 PM

Thanks Norm. Good advice both.

#6 roscoe

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Posted 25 July 2013 - 06:18 AM

All good advice above. In general, bigger at the bottom, smooth sides if you're in frost country, and dig your hole so it is flat on the bottom, so your pier won't tend to rock around on a round-bottom. Pouring the concrete right into the bottom of the hole, if you have soil conditions that let you dig a hole with stable sides, will be the most stable, because the concrete will flow into the irregularities in the walls, and anchor itself to the subsoil better, then after the first foot or two, switch to sonotube or the like.

Two things to look for: There are one-piece concrete forms that look like a bigfoot and sonotube all in one, that taper all the way up, made of sturdy black plastic,
and I just saw at my local lumber dealer precast concrete pier parts that are a footing piece, and round sections that look like 5-gallon buckets, that all stack, and are held together by a thick threaded rod. I assume you would mortar or epoxy them together, but this could be an excellent product for smaller scopes....dig hole, insert, backfill, no mixing required!!
R

#7 roscoe

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Posted 25 July 2013 - 06:25 AM

I just did a quick search, didn't find them.....will photo the system next time I'm in town....

#8 roscoe

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Posted 25 July 2013 - 06:35 AM

And, in re-reading the OP's question....you want your pier to look more like an upside-down 'T' than just a round tube, because when you push on a tube, it can move sideways easier than a T, because side thrust tries to push one edge of the T into the ground, and lift the other edge, which is much more difficult. All this is dependent on soil conditions.....and a backfilled hole, no matter how well you compact it while refilling, will always be softer than undisturbed soil.
R

#9 Ziggy943

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Posted 26 July 2013 - 11:03 PM

Thank you.

We're looking at two piers into a base. The outside of the observatory is to be 16'. The piers will be equally spaced on the diagonal. We're looking at a single base for the two piers. The piers will be about 5 1/3 ft apart. What are your thoughts on the size of the base? 8 ft square? 10 ft square? The frostline is at 30". How thick should the base be. If we go down to 36"... is that enough depth?

#10 Midnight Dan

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Posted 27 July 2013 - 09:34 AM

Hi Ziggy:

36" is plenty deep if your frostline is 30".

Is there some reason you want to use a single base for the 2 piers? I don't see an advantage to that. Connecting the 2 piers means that you may transmit vibrations from one to the other. If you're imaging on one, while you're working on or setting up the other, that could be an issue. I'm not sure if vibrations would travel that far through concrete buried in the ground, but why take the chance?

What will your pier diameter be?

-Dan

#11 mikey cee

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Posted 27 July 2013 - 10:30 AM

Hi Zig. If we understand you correctly you are combining the piers to a common base? What's the point except more concrete in a trench between the two?! :confused:Standard house footings are a minimum of 8" thick. So I would put the total depth at around 38"-40" minimum. Also is that 5-1/3' on center or between the walking perimeters?? Remember you'll lose roughly 8" overall on your inside dimension with the wall thicknesses. Seems a little on the tight side to me but that's your call. ;) Mike

#12 Gastrol

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Posted 27 July 2013 - 10:39 AM

I don't image so I did an inverted T, 10" sonotube on 36" square 6" tall base, completely above ground on paved surface.

#13 Ziggy943

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Posted 29 July 2013 - 11:52 PM

Mike, the current telescopes are a C11 and a TEC 160. Neither has a huge swing. The purpose is to have mass in the base. The size would keep any wiggle room to a minimum. They would be equally spaced on the interior diagonal, i.e., roughly 5'.

Dan, we don't know yet. My preliminary drawing puts the floor bottom about 7½' above ground level. I figure the floor thickness to be close to 1' more. Up to the floor I am thinking 24" pier diameter. Through the floor and up maybe 12". The floor would be built tight around the piers.

I like the 36-40" deep suggestion. How thick?

#14 Midnight Dan

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Posted 30 July 2013 - 06:37 AM

Hi Ziggy:

At 7.5" above ground, plus another foot for floor thickness, plus maybe another 3 feet above that for the 12" pier, that puts the total height above the ground at around 12 feet. That's a lot of weight hanging out of the ground a long way, so that would lead me towards a larger base for stability.

I'd probably start with at least a 3'x3' base for each pier. I think I'd go 40" deep and pour it 12 to 18" thick, with rebar for reinforcement. Also put some rebar in the middle sticking up so you'll be able to tie in the rest of the pier solidly. For a 24" column, I'd use 4 pieces of rebar. Using a 24" column up to the floor and then 12" above that should work well.

-Dan

#15 Fortune07

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Posted 31 July 2013 - 07:08 AM

Ziggy: be careful about putting your floor tight to the pier, any vibration in the floor will be transfered to the pier. Fiberglass insulation between the floor and the pier will dampen the vibration






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