Mounting a Pier on an existing shed base

[Mike Read]

I had this wonderful idea of having a greenhouse, and in prep created a large solid 10cm -15cm thick concrete base. Plans changed - we are not green fingered.  I've now bought a pulsar anti vibration pier and later will build "somesort" of ROR Obsy around it.

I'm going to assume the slab is OK "IF" I wanted to bolt the pier directly to it. I've placed a bowl of water on the slab where the pier will go and jumped up and down. There were no ripples 🙂
If I build an obsy with a raise rafter floor - this will raise the floor by 10cm -15cm and would reduce the working height of my pier by the same amount - which I'm guessing I would need to compensate for with a lower wall / roof . 

Here is the question: If cut out a few "plug" holes in the existing floor, added some vertical rebar, and poured a concrete plinth ( 20cm wider than the base of the pier) of 15cm deep  - would this be stable enough for a pier? Running EQ6R and 15Kg of payload. 

[Ouroboros]

If it were me I’d set up my telescope, polar align the mount and see if it is stable enough.  I wouldn’t jump up and down on the concrete. After all who jumps up and down in their observatory?  But I’d walk around etc. and see how stable the mount is over a  period of a few hours, or days if you’ve got a cover.   I’m guessing for observing it’s probably fine. Imaging on the other hand requires you walk around the scope, or be in the observatory at least some of the time. 

I set up and image with my mount on some concrete slabs loosely cemented into some underlying hard core and it’s fine. I sit in my shed 5 metres away, but even so I have to walk on the slabs occasionally. So what you’re describing is more robust than what I’ve got.  But really the proof of the pudding etc etc .... 

[Mike Read]

1 hour ago, Ouroboros said:

If it were me I’d set up my telescope, polar align the mount and see if it is stable enough.  I wouldn’t jump up and down on the concrete. After all who jumps up and down in their observatory?  But I’d walk around etc. and see how stable the mount is over a  period of a few hours, or days if you’ve got a cover.   I’m guessing for observing it’s probably fine. Imaging on the other hand requires you walk around the scope, or be in the observatory at least some of the time. 

I set up and image with my mount on some concrete slabs loosely cemented into some underlying hard core and it’s fine. I sit in my shed 5 metres away, but even so I have to walk on the slabs occasionally. So what you’re describing is more robust than what I’ve got.  But really the proof of the pudding etc etc .... 

Thanks , I'm happy with the slab. The "jumping" test was just to make sure .....
 Its whether a small additional plinth will work . As when I "build" a shed around the pier, I don't want to loose the height

[Ouroboros]

4 minutes ago, Mike Read said:

Thanks , I'm happy with the slab. The "jumping" test was just to make sure .....
 Its whether a small additional plinth will work . As when I "build" a shed around the pier, I don't want to loose the height

Yes, I see. I slightly misread your question. Would a pier extension do the job? 

[Mike Read]

6 minutes ago, Ouroboros said:

Yes, I see. I slightly misread your question. Would a pier extension do the job? 

"occam's razor" - Thank you. That makes perfect sense - and the simplest solution 🙂 I was over thinking and making it more complicated than it needed to me. 

[Ouroboros]

Oh good.  I’m sure a lot more people will be along in a while more than qualified to make it much more complicated. 😉

[Merlin66]

When we moved house I “inherited “ a fully finished 6 x 3 mtr Shed.

This has a substantial concrete floor. I split the roof and made a 3 x 3 mtr observatory for the C11/ NEQ6PRO.

The pier was extra height to give a view over the 2.7mtr walls. This is bolted to the floor slab with 4 x 18mm expanding bolts. It’s been in operation now for over  six years. It works well for me, no issues.

Edited January 25, 2021 by Merlin66

[Mike Read]

9 hours ago, Merlin66 said:

When we moved house I “inherited “ a fully finished 6 x 3 mtr Shed.

This has a substantial concrete floor. I split the roof and made a 3 x 3 mtr observatory for the C11/ NEQ6PRO.

The pier was extra height to give a view over the 2.7mtr walls. This is bolted to the floor slab with 4 x 18mm expanding bolts. It’s been in operation now for over  six years. It works well for me, no issues.

Thank you 

[jacko61]

I'm in a similar situation to you - when I built my shed the pier is now too low - so I've bought extra lengths of threaded bar and some long nuts. I plan to extend the existing pier bolts with bent bar and pour another foot of concrete, sinking a new set of bent pier bolts when it's poured. I had a discussion with a builder friend and he's of the opinion that the bent bars I put in will suffice to keep the new concrete stable although I may take a few three inch chunks out of top of the existing concrete for an extra 'key'. 

graeme 

[Mike Read]

2 hours ago, jacko61 said:

I'm in a similar situation to you - when I built my shed the pier is now too low - so I've bought extra lengths of threaded bar and some long nuts. I plan to extend the existing pier bolts with bent bar and pour another foot of concrete, sinking a new set of bent pier bolts when it's poured. I had a discussion with a builder friend and he's of the opinion that the bent bars I put in will suffice to keep the new concrete stable although I may take a few three inch chunks out of top of the existing concrete for an extra 'key'. 

graeme 

Thanks Graeme, thats what I was originally thinking too , although I might explore the pier extension first - as I don't really want to concrete ...again.

[jacko61]

5 minutes ago, Mike Read said:

Thanks Graeme, thats what I was originally thinking too , although I might explore the pier extension first - as I don't really want to concrete ...again.

I was planning to upgrade from my celestron CG-5 to a skywatcher eq6 R pro mount so I managed to snag a reasonably priced second hand skywatcher eq-6 8" extension for future use. However, it looks like it could be another year before I upgrade and measurements suggest 8 inches won't be enough anyway. A foot of concrete will bring the CG5 up to where I need it and hopefully I'm future-proofed for the upgrade. Just need it to stop snowing long enough to get the mixer out.

Graeme

[JamesF]

Is this for visual use, or imaging?  For the latter I think my first question would be whether the pier height actually needs to be increased at all.

James

[Mike Read]

1 hour ago, JamesF said:

Is this for visual use, or imaging?  For the latter I think my first question would be whether the pier height actually needs to be increased at all.

James

Imaging - Do people look through telescopes? 🙂

[JamesF]

If it's for imaging, do you actually need the pier to be any higher at all?  When I built my observatory I set a mount up on a tripod and looked at wall heights etc. given that I'd probably not want to image anything lower than about 20 degrees and worked from there.  Having a low pier may help with clearance for a roll-off roof.

James

[Mike Read]

18 hours ago, JamesF said:

If it's for imaging, do you actually need the pier to be any higher at all?  When I built my observatory I set a mount up on a tripod and looked at wall heights etc. given that I'd probably not want to image anything lower than about 20 degrees and worked from there.  Having a low pier may help with clearance for a roll-off roof.

James

Valid point, I will give it a check as I might not need to extend.

[Rusted]

Some rambling thoughts on pier isolation:

The "jigsaw" soft flooring must help to reduce a lot of potential problems with footfall transference to the pier.
Haven't tried it myself but it would be in my first thoughts on inheriting a finished slab.

It would be an awful job for an amateur making a large enough hole in concrete and then removing "the plug."
Strictly for "heavy duty" professionals with specialist diamond tipped hole saws and pro hammer drills.
Factory ventilation contractors are good at that sort of thing. They can "drill" concrete up to 60cm or more in Ø.
The problem, even then, is making a suitably large foundation for the pier but isolated from the slab.
I really wouldn't start from there.

I needed a 14' tall pier for my raised observatory. Thought I'd bury a huge, concrete pipe for the foundation. It was beyond hopeless!
Despite its huge mass and being buried full depth in well tamped, self-compacting gravel, it rocked when I added a four x 4"x4" timber pier.