Basic pier design

[Lee]

My observatory is soon to begin construction.... need to build the pier first, then an 1800x2400 roll off roof obs around it.
This is my design for the pier - I am going for a bolt down metal job for portability (relative portability I suppose!).
All steel will be 5mm thick, and will be galvanised after fabrication.

Any comments? - I am making the ribs very long as the pier is long, as I want it a metre off the obs floor for comfort, and since the obs will be on short posts it needs to be higher still.

What size plug of concrete do people think I should attach this thing to.... I was going to go about 600x600 by however deep I could get (600-750 or so).... should that be enough??

Ordered the pipe today! :D

[Astroman]

Pier design looks good. As for the pier pad, deeper the better, Many I have seen have been 1m Cubed.

[Omaroo]

Lee - concrete has an average density of 2.3-2.4. Given this, a cubic meter is around 2.4 tonnes of the slushy stuff - that's about 110 bags of premix. Do you have access to a mixer because if you don't you'll be needing a pretty big wheel barrow, a spade and a good back!

[Lee]

Yes - my fiancees dad has one, if it doesn't work then I'll hire one.... I like my back! If it were in a spot even remotely accessible by a truck I'd readymix it in! Even with a mixer it will be a big job.....

[sheeny]

Lee,

Looking at the sketch from an engineer's perspective (no real pier building experience, but I know what's flexible and what's not...:P) - Can you go any thicker on the baseplate?

A 5mm baseplate will be very flexible as you've drawn it (compared to the rest). I would recommend going to a 10mm or even a 12mm baseplate, and invest in some good non-shrink grout to go underneath it.:thumbsup: What you've got should still be way better than a tripod I reckon, but for the sake of a little thicker plate you could eliminate any chance of a low frequency resonance problem.

Al.

[bloodhound31]

I can tell you from experience mate, that there is no such thing as overkill when it comes to the stability of a pier. Even if you have solid steel, all you have to do is give it a bit of length, add some weight on top of it, and it vibrates and shakes like nothing else. You will really notice it at higher magnifications through your telescope. If you can find bigger and heavier metal, GO FOR IT! Even better, fill it with sand or cement when you're done.

FYI, I started with a telescopic 80mm gal pipe inside a 100mm gal pipe, with 10mm thick steel plates top and bottom. Shook like the Swiss family Robinson's treehouse in a hurricane.

Next, I tried bracing it with 45mm x 45mm x 5mm steel angel welded and bolted at 45 degrees. A bit better.

Finally, I put a cubic metre of concrete under the ground, then built a 300mm x 300mm square reinforced concrete pylon, up 2 metres through the Observatory floor. It doesn't budge a micron!

I also just built a pier for Ron (Tailwag) from steel I beam. It's on this forum too mate under DIY.

Hope this helps.

Baz.:D

[Lee]

It's no problem using thicker baseplate - the cutting place I talked to said they cut plate from 3-300mm thickness :eyepop:
May make some adjustments....

[Lee]

How about this? :D

Same pipe - 165mm OD, 5mm wall, 1500mm long
Base plate - 300mm diam, 12mm thick
Same ribs - 1000mm x 100mm at base
Top plate - 8mm thick - does this really need to be so thick up here??

Should I join the ribs into the base of the top plate??? (Actually should I get the welder to do that....)

I was intending on leveling via double nuts on the base threads - likely M16 or 3/4" threaded rod in the concrete. What play do the holes need in order to be able to provide some degree of adjustment - obviously if the fit is perfect, not much adjustment will be possible - any ideas on play (if any needed), or should I just get them cut to fit the appropriate rod???

Thanks so far guys!

[leon]

If you want a really solid pier that is as solid as a rock go for a 250mm or 300mm sewerage polly pipe cut to the required height you want, dig your 600mm cubed hole, place your polly pipe in the hole, half way up the base cube, get it straight and fill the whole lot with cement, then when your cement is still wet stick in you base plate bolts, attached to a square piece of particle board the same size as you base plate, and let it cure for a week or so and it will never move.

Leon :thumbsup:

[Lee]

Sounds a lot cheaper too..... not portable.... could always jack-hammer it and make a new one if we move though....
More to think about!

Anyone have suggestions about the leveling bolts??

[gbeal]

All good advice. I personally don't use leveling bolts. I got mine close to "correctly aligned" and just used the alt/az adjustments to fine tune the polar alignment. Mind you Monty had a ripper alt/az adjuster in another thread.

[Lee]

To be honest I have concerns about leveling bolts - why go to all the effort with these monolithic blocks of concrete and columns that would support your house, if you then have the whole kit riding on three or four bolts....

[Lee]

I read somewhere last year (cloudynights I think) about using higher cement ratio concretes for this application - is this worth doing, or should we stick to a normal blend....
Any other concrete blend advice, or general concreting advice....???? :D

[sheeny]

Yeah, Lee.

There's no problem going to levelling bolts provided you use a decent non-shrink grout under the baseplate. If you don't use the grout, the whole pier will not be stable enough for a scope. Some people may have gotten away with simply dynabolting the pier to the ground but the result won't be as good as good solid bolts and grout (as far as rigidity and minimal dynamic response is concerned).

I would recommend at least 16mm anchor bolts - 12mm would be pushing the lower limit I think. Overkill in terms of strength, but not for deflection!

For the base plate holes, engineering convention is usually bolt size +4mm. Normal tolerance on other bolted connections is bolt size +2mm. Take your pick... personally I would try to go for +2mm rather than +4mm. I think +4mm an enormous tolerance!:)

Al.

[sheeny]

Top plate - thicker the better. 8 would be a minimum. The top plate and the base plate only have their thickness to provide rigidity, so thicker is better. The pipe is a different story, diameter is what increases the rigidity, so bigger diameter is more important than a thicker wall (usually;) ).

By all means if you can join the stiffeners to the top plate, do it.

See my previous post about the anchor bolts (threaded rod) and base plate holes. If you are worried about placing the anchor bolts and keeping them in position while the concrete goes off you could always opt for chemsets:thumbsup: .

Al.

[sheeny]

I can't see the benefit:shrug: .

Al.

[rsbfoto]

Hi,

Have you thought about using maybe 5 or 7 ribs ? instead of only 4 ?

Theoretically it could be that you could get flexure. Look at the pier and apply force in 45° angle. ¿ Could it be that your pier flexes then ?

[sheeny]

Valid concern, Lee. That's why the non skrink grout is critical. Get it from an engineering supplies shop.

The procedure is:

set your bolts and let them go off (either concrete or chemsets)
Fit your pier and level using double nuts or packers.
If using a free running grout form up around the baseplate (leave about 25-50mm gap from baseplate to formwork)
mix and pour in the grout from one side only until it comes out the opposite side (to force out air bubbles).
let the grout set.
If you are using a plastic (not free running grout) grout, the procedure is the same, but you won't need to form up. You just keep pushing the ground under from the one side till it comes out the other, and then screed iff the edges.

BTW filling the centre of your pier pipe with sand or better yet concrete is good practice. Sand will act as a damper to any vibration and the extra mass with lower the natural frequency of the pier. Concrete adds mass but also adds a lot of stiffness.

:thumbsup:

Al.

[Garyh]

Hi Lee,
your pier sounds very much like the one I finished a few months back for my gm-8. Its only 950mm high made from 150mm x 5mm with 10mm plates top and bottom and also has some ribs out of 10mm steel. I find it plenty strong enough for the newt and gm-8. It to sits on 1/2" leveling bolts set in concrete 450mm x 450mm. If I give it a good rap I can only see a few secs of vibration with high magnification...
With the gm-11 a 600mm block would be pretty stable. To save cement just get yourself a bunch of old bricks to throw in the hole as you shovel in your cement..theres 4 bricks in my block..:thumbsup:.

Like you it would be easy to unbolt and move.. The same with the whole observatory. Just unbolt it of the stirrups and chuck it on a truck!!

Also what might be a good idea is weld 2 pieces of flat bar near the top to hang your gemini/control box from as well. Thats what I have done also..

I reckon just the normal cement mix would be fine, just let it cure over a few weeks keeping it damp, a old bit of carpet kept wet over the top would work well after it has set well.
cheers:thumbsup:

[Lee]

Decisions decisions now - thanks for all the excellent advice guys.... I'm starting to lean more towards the concrete pier (prob 250mm) for a couple of reasons:
1. seems to be better in terms of vibrations etc than similar steel.
2. cheaper - I'm going to need a big plug of concrete regardless - adding the pier whilst pouring won't be too onerous

Obviously my first point of portability comes up - but to move a steel pier I'd still need to pour another big monolith! - my initial 5mm thick pier I calculated would weigh 52kg or so, so with 10-12mm steel and more ribs etc it would likely hit 80kg plus.....

Will keep brainstorming......

[JohnG]

Hi Lee

I am going to go with the cement filled 250mm poly sewer pipe method when I move into a permanent location next year.

I think the sheer mass of the cement base block and pier will dampen any chance of vibrations, I will also be setting the top plate bolts deep into the cement and welding them to the re-bar cage I intend to construct.

All good fun isn't it. :lol:

Cheers

[Shawn]

I see back there you asked about leveling bolts, I used 3/4 UNF, nice fine thread for tweaking. I think I posted my pier DIY here to, heres an older pic of it just installed, very similar dimensions in terms of plate thickness, fill with river sand and vibrated ... Works well , If you give it a thump any vibrition is gone in a second...

Good luck

S

[sheeny]

There's a lot of merit in using the concrete filled pipe, Lee.:thumbsup:

Though I don't have a pier/observatory I have been thinking about it for a while, with the intention that when I move to a site with less light pollution I'll build one. For what it's worth, my preferred solution for a pier at this stage is an 8 to 10" concrete pipe set vertically into a concrete footing at least 600x600x600. The pipe would be cut off at the required height and filled with concrete. Chemsets and grout under the adapter plate for the mount on top. :thumbsup: Should be rock solid...

The concrete filled plastic pipe should nearly as good (I wonder if you could tell the difference in performance?:shrug: ).

Just food for thought...:)

Al.

[Sharnbrook]

Before you buy the poly pipe, check out the costing on the cardboard Formatube, made especially for concrete. It may be considerably cheaper, unless you can score a piece of the poly pipe for free.

[Lee]

Boy this is evolving! Going for a square section concrete pier now.... Going to order a pier plate tomorrow....

Plate will be 20mm steel - 4xM16 rods will hold it onto the pier, central 1/2" hole for Losmandy pier adaptor, and 4 other 1/2" holes to hold a leveling plate if I decide to use one (not planning on it initially though)....

[Lee]

The hole is dug - we pour next weekend, just need to get 40 bags of concrete mix! My back is already aching!

[xelasnave]

Lee can you get to the bed rock?
You may care to consider if you can not... using a system of driving star posts down and have their ends set in the cement block..The bed rock was about 5 foot down in my case so I did not dig all the way down..I have a hernia otherwise I would have:eyepop: ...

but I drove down 3 star posts and wiggled them to get a small tube around them..I drove them as best I could into the bed rock..

I then poured down dry 1 to 1 mix and watered it in and ramed it and led the posts to the larger block region..

I used an old telecom pvc tube with star posts going down past the others and used a 3 to one mix..I did not have any blue metal but if one had it it would be good in the mix...large bolts were set in the cement with wire wrapped aropund them for additional purchase in the cement...

a little slack but it seems firm in fact I reckon it is just so much better than a tripod I dont know why I waited so long:shrug: ..I had a steel bit to put on top of about 8 inch height to set the mount upon....I opted for mostly cemment because I felt it would expand and contract less during exposures ;) than a steel one:D

Your ideas seem ok but dont be afraid of over kill.. thats part of the fun.
Good luck and remember if anything goes wrong its only a proto type;)
alex

[Lee]

We are going to drive down some rods into the hole base before pouring. We are using pre-mix concrete and a mixer, the observatory site is in my backyard - probably a 50m windy walk, up at least 7 metres and 6 steps. I don't relish carting sand and gravel up that - prefer dry mix, convenient too.

The tech specs from the cement co (Blue Circle) state for a "more structural concrete" - add 2kg of cement to each 20kg bag of concrete when mixing, this bumps up compressive strength from 20 to 32MPa. Sounds like what we want.....

At least the rain around here in the past 6 weeks or so has left the ground easy to dig - smooth sailing down to about 450-500mm, then moist sticky clay....

Stay tuned for a new obs building thread.... :D

[Lee]

We pour tomorrow! :D

[Lee]

What a day.... 36 bags of premixed concrete and 4 bags cement later, the pier is poured. 250mm square, 1.5m high, for an above floor height of around 1.1m in the finished obs.
One near catastrophe - one of the pine boards used for formwork cracked through a knot under the pressure once the pier was filled.... all is holding up though - I was worried for a minute or sixty though! - next time I'll use 20-25mm ply instead I think.....
Pier plate was made from 10mm mild steel, cut by a local firm to a square with round corners, drilled out to 1/2" for 12mm stainless threaded rod set in the pier.