Another pier design to critique

I'm building a 10' tall timber framed structure under a 10' ProDome, which will house a 16" SCT. I'm planning a concrete pier up to just below floor level (roughly 6' above ground level), with a steel pier for the last 30".

The original plan was to use a sonotube, but they're very hard to come by in Ireland in anything larger than 12". So the current plan involves an 18" concrete sewer pipe (with an outside diameter of 22") 8' in length, which will be set 2' deep into a 3' x 3' x 4' footing, and then filled with concrete.

The pipe will have a nicer finish on the outside, and should give me better rigidity (assuming the casting conditions are better controlled in the factory than in my field).

But it does have a smooth inside bore. I'm wondering if the poured core (which has the anchors for the metal pier on top) is going to shrink enough to give a tiny gap between the core and the shell, and therefore not make use of the outer shell?

I also wonder if I really need any rebar inside the core, or if the outer pipe provides plenty of strength.

Any help appreciated.
-- Jeff

I think it will work fine! Just don't pour your mud to wet. The wetter it is the more shrinking it will do. Also you could spray some concrete bonder on the inside of your tube! And you might investigate the use of "Bentonite" in your concrete mix! Good luck with your project! 8)

Jeff,

Have you considered a classic square concrete column using a plywood box form about 18" square? Sonotube might be the preferred forming method because it saves having to construct a wood form, but it's not the easiest thing to fix in place during the pour.
In any case, rebar is a must. Just my 2 cents.

Bob

Here's a picture of the site (the red circle marks the approximate location where the digging etc. will take place).

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Went looking for J-bolts yesterday, and the biggest I could find were 8mm. That clearly wasn't going to work....

So, I ended up with 3 (1m) lengths of 20mm stainless threaded rod. I borrowed the use of a mate's Oxy/Accetylene rig (actually, I think it was Oxy/Propane -- it certainly took longer than expected) and put a bent leg on each.

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I've got about a 100m cable run from the house; the first 40m is under gravel and grass, so that was fairly easy (I burried the armored cable a couple of inches deep). The next 60m is under the field, which needs to go down at least 24". So I'm getting a back-hoe in.

And as long as I've got a back-hoe, a 5' deep pier footing is called for. But I've got serious misgivings about being able to squeeze into a 5' deep, 3' square hole with a 22" concrete pipe and a bunch of rebar in order to tie the rebar together. (I can't tie the rebar first, because I don't have anything to lift the concrete pipe 15' in the air to lower it down over the rebar.) So it seems like the easiest thing to do is dig the pit 4' square to give me a bit more working room.

4x4x5 is 2½ cubic meters of concrete, which is 5 times the mass of the post, which seems like overkill. Granted, I think concrete here is less than than €100/m³ delivered, but I'm still thinking that I might leave the larger outer dimensions, but go back to 4' depth. A 4' cube gives me roughly 1.8 cubic meters, which is still better than 3x the mass of the post, and leaves the CG a healthy 18" below ground surface. Seem sufficient?

Here's a pic of the cable trench w/ cable.

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Like John (see previous post), I was also coming around to a concrete block skinned pier. But I wasn't so happy about it. You see, I had gotten quite attached to the idea of a smooth, circular, pier in the middle of my observatory.

But my local builder's merchant had called back to explain that they got their pipes from Redimix NI, where they don't stock the 2.5m lengths. I was welcome to drive down to Naas, where they do stock them, and pick up a pipe myself, but they couldn't get one to me. Now, Naas is a good 2 hours away during the day, in heavy stop-and-go traffic. Keep in mind that we're talking about towing a 2000lb trailer with a 1800lb pipe in it with a 1.6l Ford Mondeo. Not a pretty sight.

A mate suggested calling a local civil engineer, which I did, and duly explained my predicament. While he didn't have one in his yard, he did offer to call up to the building site he was at. He called back a few hours later and said if I showed up in Dundalk with a trailer, he'd load one up for €80. Now, Dundalk is only 15min away, and there's no traffic in that direction. Plus, my mate had just shown up with his Volvo diesel....

Looks like I'm in business again 8)

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"The Pipe" goes halfway down into the footing block. But it weighs 1800lbs, so I need more than a bit of timber framing to keep it there for the pour. (There will be timber framing as well, but just to keep it vertical.)

I laid some blockwork at either side of the 4'x4'x4' pit, with a pair of 6" pre-stressed concrete lintels to carry the pipe. This will leave the center of the pipe un-cluttered, allowing the rebar cage to extend through the pipe and down into the footing.

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I thought I was getting 8 lengths of 1/2" rebar delivered. But my builder's merchants called to say they didn't have any in stock, and it would be next week before I'd get any. Seeing as I wanted to make some progress over the weekend, that was no good.

So I drove 15min in Drogheda and bought some rebar. Here's where I hit the first snag: they didn't have anything to cut it with (it comes in 20' lenghts -- I was in a Ford Mondeo). I had brought my hacksaw just in case, but I can't say I was looking forward to spending the next 40 minutes getting to know it better.

Turns out, it wasn't that bad. The high-tensile rebar is somewhat brittle -- if you saw about 1/2 way through and bend it, it will break. So, after a bit of sawing, bending, and breaking (and a bit of rubbing rust all over my car seats), I'm away.

I had called around earlier looking for rebar hoops, but no one stocks them in Ireland. The only structural engineering shop I could find that still did them was on holidays for two weeks. Right -- how much trouble can it be to wrap a few rebar around a tree stump?

Quite a lot of trouble, actually. It's both springy, and harder to bend then you might think. But several hours later, I had 8 12" hoops of 1/2" rebar. The bit of a tree in the picture is my form -- I also used a couple of 5' lengths of iron pipe for leverage.

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