New observatory in existing shed

[Astrodude]

Hi All,

Just after some input on a shed conversion. I recently moved into a house which has very limited horizons - but there is a big shed out back which I am planning to convert part of into an observatory.

A section of the roof will be converted into roll on - roll off set up, and a platform will be installed isolated from the mount.

My questions at this point relate to the pier's design and construction. The pier is going to need to be about 2.7 metres above ground level. (Access to the ground beneath the shed is not a problem - it is a chipboard/bearer/stump construction). At the moment I'm planning on creating a concrete pillar (using formatube) 300mm (12 in) in diameter reinforced with bar. The foundation for the pillar will be a poured reinforced concrete block about 600mm x 600mm about 1000mm deep.

Do these specs sound suitable?
Can i get away with making a concrete block that is less deep?

Also, I'm a bit confused about the order of construction - does the formatube get set into the foundation block and then filled later, or does the block alone get poured first, allowed to set and then the formatube set on top over the protruding reinforcement bar and filled?

Thanks for any advice. Cheers.

[Tony Leece]

Hi Astrodude, i too am planning to convert my shed/ garage into an obsy.
It unfortunatly has a concrete base i have to cut through first, oh well:(
My point in replying to your post is that the people i have talked to on the subject of pillar foundation blocks is that a 1m x 1m x 1m block is probably the minimum for stability and vibration dampening.
I am sure you will get all the information you need here to build a top class observatory, their are some pretty talented telescope and observatory builders here.
All the best mate...........
Tony.......

[leon]

What you are planning should work but you will have to do the pour all at once, that is the foundation block and pier, other wise a pier sitting on a concrete block will have no strength,

Also use plenty of reo, with the pier structure running into the foundation for maximum strenght

Leon

[Terry B]

My observatory just has a concrete slab for the floor. The pier is a cast iron pillar that is dynabolted to the slab with 8 bolts. It is only 1.2m high but this has absolutely no problem with stability or vibration. I can walk around imaging with no effect. There is very little lateral force on the pier as long as your mount is reasonably balanced.
For a higher pier making a tapered concrete pier would be better. I don't have a pic of it but this (http://www.unentas.armidale.com/history.htm) telescope sits on a pier that is over 2m tall. The pier is about 700mm square at the base and tapers to about 350mm square at the top. The metal base is then bolted to the concrete pier. It is very solid and we can have 40 people wandering around on the floor under it with no obvious effect on the scope.

[ChrisM]

Hi AD,
I also had to create a pier approx. 3m tall. I dug a 2 cubic metre hole (=1.26m/side) and filled it with steel reo and concrete. (I achieved 1% steel by volume). I figured that I could not pour the concrete column at the same time, since the concrete might flow out the bottom of the 457 mm dia formtube. The professional advice that I received was that to get a good bond between the concrete block and the column (apart from using reo bar), roughen up the top surface of the block (where the column will be later poured) and use bondcrete on it as well. I used a wire brush to roughen the surface. I poured the column a week or two later and it is as "solid as". I used 8 off 20mm reo bars in the column, and these were a long "L" shape and fixed into the block both at top level and halfway down the block.

The concrete column stands about 2.2 m above the block, and I have used a steel pipe, approx. 1 metre long, on top of the concrete pier.

I'm happy to supply more details if you wish, and I've attached a few pics.

Cheers, Chris

[ChrisM]

AD,

I've dug up two more pics that might be of interest.

Chris

[Astrodude]

Thanks for all the great replies and advice :)

Good luck with yours Tony - I hope the concrete cutting goes smoothly!

ChrisM - Thanks for the details and the pics, I might take you up on the "helpline" offer as I get more into the project. I figure also that by doing all the concrete at once the concrete from the pier might come spewing out the bottom, I'll use your method - thanks! I see you used 457mm dia tubing, I was planning on using 355mm - did you need the extra diameter for the steel pipe of was it worked out that pillars this high need that diameter?

[ChrisM]

Astrodude (sorry - I don't know your real name), Regarding choice of 457 mm dia former, there wasn't a lot of science; it was a gut feel thing, but I did know that I wanted to allow for a decent size scope in the future, and also that the stiffness of steel pipe varies as the fourth power of diameter. I figured that I might end up using some pipe maybe 10 inches or so in diameter. In fact the scrap that I got was 11" pipe (1/4" wall thickness) and the final result is very good in terms of stiffness. If I kick the pier, the high frequency vibration settles down in under 1 second, and I can't detect any low frequency vibration.

The other consideration with concrete pier diameter is that you don't want the bolts too close to the edge. Let me know if you'd like any other details of the cage or pier.

Cheers,
Chris

[marki]

The main thing with any metal you put in the pier is to keep it 50 - 65mm from any external surface. Concrete is porous for the first 30mm and water + reo in concrete = concrete cancer. These are also the distances that supply the most strength. As has been said here if you intend to pour in two parts you must scabble the surface on which you are joining. When I worked in construction we would leave the concrete for about 3 hours to set then hit it with a high pressure water sprayer. This would leave a very good bonding surface. You can use bondcrete or a mix of wet slurry (cement and water) to prep the joint before pouring the pier. Even wetting the surface helps as you do not want the wet concrete in contact with the dry surface to set too quickly (in general the longer it takes to set, the stronger it will be). Last of all if you really want to make it strong use a concrete vibrator to remove as much of the air from the concrete as possible without sending the aggregate to the bottom. We used to build freeway bridges like this and not one has fallen down.

PS: Chris how big is your scope? 2 cubic meters of 20/20 weighs 4.8 tonne wet :scared:.

Mark

[ChrisM]

Hi Mark,

Glad to hear about your successful bridges!

Whilst my current scope is just an 8", the pier, which is relatively tall, is designed for something bigger down the track a bit. I'm sure that I could have made do with a thinner pier for now, but I didn't want to have to upgrade that plus the scope. The top steel section can be changed relatively easily though.

Chris

[Astrodude]

Thanks for the tips Chris & Mark.

I've got some time off work coming up and am hoping to get a lot of this stuff done. What places sell reo bar? I don't think I've seen it at bunnings - but then I can rarely find what I want there without asking. I imagine junkyards sell stuff that is starting rust. Can you buy it in rods or do you have to buy a mesh and an angle grinder to take it apart?

Also Chris, I can't quite make out from your photos how you connected the bits to make up your cage, did you weld them together or can you use wire?

Thanks again, Trav.

[marki]

Chris I think with a footing like that you will be able to expand up to a 3m mirror no worries :D. I go on holidays in 2 weeks and will be putting my pier in then. I plan to use a 1m x 1m x 0.4m base with a small plinth to mount my steel pier on. This will be filled with sand so it should take care of any vibration. The footing will weigh a little more than a metric tonne, plenty for the LX200 I hope.

[marki]

To connect the reo just use tie wire which can be bought in roles from your steel supplier. Be careful if you weld as the steel is tempered core. Over here we have several suppliers, aquilla, smorgan ARC, Midalia steel etc. Reo will always have a little lite rust (surface) but do not use anything that has obvious deep scale as it will keep on rusting inside the concrete and this will expand and cause cracks. You should be able to buy pre-bent Reo or get them to bend it for you if you are not confident to do it yourself. Reo generally comes in 6m lengths unless the supplier has some short off cuts lying around.

Mark

[ChrisM]

Trav,

I used tie wire to connect all the reo pieces together and I bought the wire at the local builders' hardware store. It's got a slight oily coating and comes in a brown paper wrap.

I looked up the Yellow Pages to find local steel suppliers and got a couple of quotes for the required lengths of reo bar. Cutting to length and bending was included in the going $/kg rate. I prefer using new materials if possible; the cost of the reo bar was not all that significant in the overall scheme of things.

I'm sure that there would have been plenty of discussion previously about whether or not to fill a steel pier with sand. I don't know the consensus view on this, but my approach was to ensure that the diameter of the pipe is generous (stiffness varies as the fourth power) and to keep the weight as low as practical. I would expect that the more mass there is up the pier, the longer it would take for a vibration to dampen. So I used 6 mm wall thickness (instead of something thicker), and fabricated an aluminium wedge (instead of steel). Low thermal mass is also good.

Cheers,
Chris

[Astrodude]

Ta for the great replies.

Procrastination is over, first steps have been made! Bought a circular saw, cut a panel of the floor out and dug a hole (which hopefully isn't filling up with water - pouring in Melb tonight!). I'll get some pics posted tomorrow...but it feels great to have made a start.

Oh..have also ordered my pier plate, should be here in about a fortnight.

:thumbsup::thumbsup::thumbsup:

[Benny L]

When my dad and I built his dome he drilled a 250mm hole down 6m into the clay and bedrock, filled it with rio and has a 500x500mm brick and concrete column coming out of the ground about 1.5 metres.

The domes 170mm thick reinforced concrete floor is independent of the pier which is separated by 10mm of styrofoam to minimize vibration. The dome itself is cemented into place rather than dynabolts etc..

On top of that we have an adjustable leveling platform which can be adjusted whenever necessary and that has 450mm bolts chemically bonded into the pier at 6 points.

As a result of this over engineering I can slam doors and do star-jumps inside the dome with no effect on the image at 3500mm focal length :S

[renormalised]

Better to over engineer the thing and get what you want and need than skimping on things and having problems later on.

[Astrodude]

Hi again all,

Some happy snaps of the work to date:
1st is an external shot of the shed;
2nd is the hole in the floor;
3rd is the hole in the ground;
4th is the roof above.

Next job is the rebar.

Anyone know if there would be any issues later (ie. rust) if I sunk the ends of the vertical rebar rods in the clay for stability until the concrete is poured?

[mithrandir]

Do you know how deep and active the clay is? Our house is on a slab and it moves as the clay under it expands and contracts as the soil moisture level changes.

Not wishing to be a doomsayer, but you might find your pier moves too.

You might want to check before you pour if you need a bigger foundation or tie it to a floor slab to help the pier remain vertical.

[Benny L]

Just posting an image of our pier ;)

http://i197.photobucket.com/albums/aa314/thephotoguy131/Telescope/e9584ed2.jpg

[marki]

Use concrete blocks to stabilise the cage in the hole. You should be able to get them from your steel supplier. Avoid all contact between the ground and reo.

Mark

[Ian Robinson]

For the builder types here :

Every bit of concrete on my property is cracked (most of it is over 30 years old - driveways, footpaths, a couple of slabs (not sure what the prior owner used them for) , and my more recent 6x6m garage slab has also developed a fine crack.

I know much of Newcastle and Lake Macquarie is a mine subsidence area , I also have clay under the soil (which is quite dry now and have been for a few years). There have been road collapses in nearby streets (most recent was about 20 years in Coral Cresent when a big hole opened up in the road).

So how do I tell if the cracking is mine subsidence , or simply due to actve clay ?

This will determine if I go for a slab or an off ground platform for my ROR shed .

[renormalised]

To determine if it was collapse due to mine subsidence, you'd have to get the guys from the NSW Dept of Mines to come along with the old mine maps. They could tell you where the old shafts and/or drives were and whether you were on top of one. If it was active clay slippage, you'd have to get a geotechnical engineer to drill your yard and see if it was that.