Basic pier design

[JohnG (John)]

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.

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 (Al)]

Originally Posted by plasmodium

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......

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

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. Should be rock solid...

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

Just food for thought...

Al.

[Sharnbrook (Mike)]

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 ...

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 ..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

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....

[Lee]

We pour tomorrow!

[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.