Footings for a telescope pier

[Ian Robinson]

Really two questions .

I eventually plan on putting a 14" RF newtonian astrograph on my GEM which will be permanently housed in a ROR shed and the GEM on steel (200mm NB thickwalled) pier 1.2m tall . Will initially be supporting the same GEM and my 10" newtonian astrograph.
I think I'll do a concrete footing first about 0.5m x 0.5m x about that dept into the ground.
I'll then surround it later with a concrete slab (probably 3m x 4m) unless I go to a timber off ground platform in which case I'll initially surround it with smaller slab 1m x 1m and maybe about 6" thick.

1) Apart from metalwork (some offcuts of galvanised steel 100mmx10mm steel flat maybe) to space out and secure the threadbars the pier will be anchore by (4x) that will be imbedded in the concrete , do I need to reinforce the concrete footing with reobar ?

2) Will this be sufficient size for my concrete footing ? (to be stable).

[Barrykgerdes]

What will be stable as a footing for a pier will depend mostly on the type of soil it is set in. For example if the ground is soft and unstable mass is what you will need , about 1/2 a tonne. Reinforcing is adviseable because a reinforcing cage will give you something to bolt onto.

I once built a base in rocky but stable ground (50 years ago) using four bags of sacrete and a reinforced cage made from scrap steel and 1 1/2" angle iron. The builder ran into the extended angle iron with his front end loader and bent the iron at right angles but never moved the pier. It is still there I never used it for its intended purpose. Couldn't get it out so I just cut the protrusions off and covered it with the lawn.

Barry

[Garyh]

I would go bigger than that Ian, if you are planning on putting a 14" newt on top of the gem. Like Barry mentions, depends on soil type etc. Deep loam/clay soils with lots of expansion/contraction would be the worst and would require a larger footing.
If you make up a cage with some scape steel with the threaded rod like you are intending to do, I don`t think any other reinforcing would be necessary. I would go 700 x 700 x 700mm at least.
My footing now is 500 sq and is very solid for the 8" scope.
cheers Gary

[sheeny]

I agree. Go bigger IMO. It does depend on soil type, but even so a .5x.5x.5m footing seems way too small for a 14" Newt on a GEM.

The footing for my pier was 350x350x1000 deep in heavy soil with a good proportion of clay. I never got to 1000 deep as I hit rock at about 700mm so keyed into the rock about 50mm and Robert is most of your near relatives. If you are in sandy soil big is best, if you are in clay, try to go deep and find something stable like rock or even just a more stable soil layer.

Its a good idea to throw a small amount of reo of some sort (could be chicken wire, etc doesn't have to be reo reo!) into the pier footing just to control cracking. It's not carrying significant loads for the size of the concrete so heaps of reo isn't necessary.

Al.

[coldspace]

I got a bobcat in and he drilled me a 600mm hole down 1.25 mtrs deep.
I went down 1.25mtrs as I was going to form the base about 400mm above the ground as my observatory floor is raised above the ground to let storm water pass underneath.
If you are going to stay at ground level then 800mm to 1mtr would be enough and 500mm would be not.
I then made a steel cage from some old gal fencing and placed into hole.
Do use some sort of reinforcing, you don't want to take shortcuts now and have future issues.
I then formed the pier base with form ply and made it with a top 500mm x500mm with the cage coming up through the hole as my pier base was going to be raised 400mm above the ground.
I then fitted a stainless threaded rods welded into a cage type assembly to keep them square with a ply template ontop that was the same as my future steel pier to bolt ontop.

We then filled the whole lot with 25mpa concrete

I have catergory H clay soil and this has proven rock solid.

Regards Matt.

[marki]

I am on sand so I did mine a little differently then those mentioned thus far. I dug a hole 1.2 x 1.2 x 0.7m deep then sunk a 0.5m dia pile down another metre. The mounting bolts run right down to the bottom of the pile (about 50mm above sand) and the whole thing is reinforced with reo. Imagine a block with a round peg driven through it so that 1m sits below and 0.4 m rises above and you have the picture. The block was poured to about 0.5 m thick as I was after the extra weight as well as a stabilising foot but also only wanted a small amount visible above the ground. Solid as a rock.

Mark

[wysiwyg]

Ian,

Having built my own obs I too had this problem, how big is big enough.
Unfortunately I am in clay country so I opted for a 1 cubic meter pier foundation using reo bars as reinforcement.

Also made the bottom of the pier hole larger than the top, so its a square cone shape, giving it a much larger footprint at the bottom.

I have a 2m High, 260mm OD 1 inch wall thick pier attached to this foundation and after two years it has moved.
Just recently with all this rain we have had in Adelaide the pier has slightly shifted. At the top of the pier plate that equated to nearly 1 degree shift.

If you have clay, and you dont want it to move, there is really nothing you can do. Does not matter how big you go you will get movement.
Make sure you have enough adjustemnt at the top of you pier to counter act any movement and you should be OK.

Go as big and as deep as you can, that is my advise.

Mark

[leon]

No Mate Reo is not necessary, trust me.

Leon

[bmitchell82]

From everything that i have lernt in Structural analysis (curtin civil engineering) Reo is only used in concrete to take the tension component, as concrete is poor at withstanding tension but exceptionally good at withstanding compression. unless your pier is withstanding exorbanant amounts of lateral force then the reo is just for your minds ease :)

my 2c worth! and im sure there are a few other civ engineers about that could second or maybe put it to sleep :)

[marki]

Brenden if the footing is underground with long bolts attaching a steel pier then no worries as the temperature stays reasonably constant. If the concrete is brought above ground and used as the pier I would at least use some mesh to reinforce it. It is not that there is likely to be a great amount of stress or strain due to load but it will be subject to larger temp variation which means expansion and contraction which always leads to cracking. The mesh/reinforcing will help in this regard.

Mark

[coldspace]

Got lots of friends in the concreting/formwork and Civil engineering industries. From the engineer to my tradie mates, all advised using reo for my foundations of my pier. If you put footings in without at least a little bit of reo then you are looking for trouble down the track, so put some in and sleep easy.

Concrete is very week without reo. Even in the ground.
Reo acts like the bones of the beast. Without it even in the ground you possibly will get cracking at some stage then this could cause all sorts of hassels. It only sets once.
I would of hated myself in saving a half hr by not putting in my mesh reo which was free and took 30 mins to cut and bend before the cement went in and then spent many thousands and countless Saturdays building my observatory around it, to have it crack or shift.
My 2 cents.

Matt.

[marki]

Yes agree Matt, I also spent a lot of time working in civil construction in my youth. At least in the ground if it cracks it won't go too far :P. For me if you want to do the job properly (i.e ferrari vs hyundai) use reo or some reinforcing and vibrate the concrete which should at least be 25 mpa. If you want the hyundai version mix it in a wheel barrow with lots of water and tip it in the hole :D.

Mark