Old 19-04-2022, 08:11 AM
looking up
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Location: Blue Mountains
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Pier Depths

Hello to all.
Just purchased a nice Sirius observatory and will need to install my heavy
Steel Pier. So given alot of you guys have done this before I thought I would ask the question. What would you think the depths and width of the hole as it would be supporting the big arse Pier, Meade 14inch SCT LX 200 and super wedge.
I would like to get away with the hole minimum size possible. I will attach some pics of the steel pier. Also could someone tell me what is the top section suppose to do of the pier the round bit with the bolts coming through.
Sorry this is very new to me this whole construction thing.
Look forward to your comments
Kind regards Steve
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Last edited by looking up; 19-04-2022 at 08:23 AM.
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Old 19-04-2022, 08:43 AM
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sheeny (Al)
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G'Day Steve.

You're going to get all sorts of advice with a question like that... so lets start.

Without knowing your soil properties and profiles, I'll talk in general principles and explain what I did.

Generally, bigger and deeper is better, but my advice is to keep the top of the footing small (where the pier connects to the footing). My reasoning for this is that just as you don't want to be swinging off the pier itself while you are imaging, you don't want to be changing load on the footing either. If it's big and deep enough it shouldn't matter, but if its not... the disappointment will be palpable!

Isolate the pier footing from the slab or floor of the obs for the same reason.

If you have clay soils the size and depth matters more due to movement. In fact in some cases you may never be able to stop the footing moving (deep clay) but a large footing will at least be stable in the short term, and you should only have to re-polar align the mount periodically.

Here's what I did. I have clay soil. So slippery when wet that clutch starting a car or bike is not possible, and fence strainer posts have spun in their holes when the fence has been impacted or re-tensioned. Having said that we don't get a lot of movement - no cracks in the house, jamming doors etc. I found the reason for this is the ground is shallow.

I planned to make my footing 350 x 350 and 1 metre deep. I didn't get there. At about 750mm down I hit rock. I keyed into the rock about 50mm (that's all I was prepared to go!) and poured the footing. Perfect!

I have a 100mm concrete slab around the footing with 10mm foam concrete expansion joining isolating the slab from the footing.

I hope this helps your decisions.

Oh, the top plate of the pier is where you mount the mount to. The bolts are so you can level the mount. Just a tip: the bolts are (usually) the greatest source of flexibility in the footing, pier mount system so keep them adjusted as short as reasonably practical.


Last edited by sheeny; 19-04-2022 at 08:49 AM. Reason: afterthought.
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Old 19-04-2022, 09:19 AM
glend (Glen)
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There is no standard footing, as Al indicated, it depends on what you find when you start digging. Now honestly I think 350 a side is too small, mine is 600 a side. As far as depth is concerned, I would suggest 600 minimum, and if your on clay or sandy conglomerate type ground consider driving galvanised angle iron into the base in several directions, creating a porcupine type anchor system.
Your anchor bolts are easy, just install as you finish the footing pour, and you need to look no further than Bunnings for foundation J bolts. Obviously a level footing surface is nice but you can shim within reason if you need to.
I will point out that for equatorial type mounts, perfect leveling is not necessary, as your star alignment software will compensate.
Have fun, digging the footing is the worst part of an observatory build, when done by hand.

PS, you will get folks swearing that your footing needs to be a full cubic metre (1000x1000x1000) but they are fairly rare these days.
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Old 21-04-2022, 02:13 PM
appiice (Ed)
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My 2 cents worth

I would also agree that there is no hard and fast rule, but.... each person has their own preference.

My experience is, with reactive clay soils in Bathurst was, I'm not digging any hole deep enough by hand. that being the case using a machine to dig a hole I ended up with about 1.5m by 1.2m by 1m deep, well and truly overkill.

Again, I wasn't mixing concrete by hand so the in-ground mass plus the above ground riser was about a local mini-mix load, to get less than a load would have meant paying a premium for nothing.

So my advice is, go with what you are comfortable with, even a .6 by .6 by .6 mass will weigh just over half a ton, but if it moves and you are unhappy, how much is that worth to you??

My 4.5 tons is never going to move and will never be dug out ( by me anyway ) but I made the decision to do it once, I would rather overkill than something I was less than happy with.
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