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JamesF

JamesF's observatory build

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Mind you, having to lift 160kg of concrete over 1.3m high to pour it, now that's hard work.

James

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5 minutes ago, JamesF said:

A 280mm diameter tube 1.3m tall, Kev?  I reckon that's only about 160kg :D

James

Speaking metaphorically of course 😜

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Been thinking a bit more about the drilling of the holes, particularly in the bottom block...

A wise man would probably mark up the intended positions of the holes, possibly from a template, then drill them in each block and make up a new template for any that did wander, using the second template to place the threaded rod in the concrete for the pier foundation.

James

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1 minute ago, Astrokev said:

Speaking metaphorically of course 😜

You never can be sure with observatory concreting 😄

I'd hazard a rough guess that our cement mixer does about 100kg per load and I've done twelve loads so far and perhaps have ten more to do, though it depends how much other stuff I can find to throw into the pier bases.  Today I discovered some offcuts of reinforced concrete beam about 500mm long, left over from another project, so they'll be going in.  I'd only have to take them to the tip otherwise.

James

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Drill a pilot hole with a small masory bit first.   😃

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Yes, I was thinking that starting small and then enlarging the hole would be a good plan.

James

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Little progress this weekend thanks to having quite a few things to do on Saturday and coaching at a swimming meet on Sunday.  I did however manage to drill holes (in both ends) of the lower blocks for each pier:

obsy-build-14.jpg

I've labelled them so I know where they belong once I make up the jigs for placing the threaded rod in the foundations, just in case the holes wandered a bit and they aren't completely identical.

I started with a 5mm pilot hole and opened it up to the required size afterwards.  Initially I just used my corded hammer drill in "non-hammer" mode, but there was a casualty:

obsy-build-15.jpg

That drill had a hardened tip once :D

Actually, there was another casualty and almost a third, as I had "help" from my father-in-law.  He insisted on getting straight in there with the SDS drill with the bit for the full-size hole in hammer mode, failed to drill the hole anywhere near straight and blew a huge piece of the block off the inside as the bit went through making it impossible to bolt down with a nut and washer :(  Just as well I bought enough that there are spares after all.

Once I'd stopped reaching for nearby heavy blunt objects I retrieved the SDS drill, turned off the hammer mode and went back to drilling pilot holes and opening them out again.  That worked nicely and my SDS drill bits remain unscathed (as does my father-in-law, but only just).

This evening I took the shuttering off the pads in the middle of the long sides of the floor and sorted through what I need so I can re-use it for the three pier foundations.  I'd really like to pour all three in one hit if I can, so I'm hoping I can get the new shuttering made up and in place and the jigs and threaded rod for fastening down the lower blocks sorted for Saturday morning.  The best-laid schemes o' mice an' men gang aft agley as the saying goes however, so I'll just have to see how it works out.

James

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I use just single holes on my "Tod" piers but I do use large "washers" made from 8mm aluminium x 75mm diameter to spread the load. The piers are also free standing.

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I did give some thought to the number of bolts/threaded rods to use.  In this particular case where I'm working off sloping ground and have a suspended floor the concrete block foundation seemed most sensible, especially if I want the option to change to steel piers in the future (though I do believe this is unlikely), "just in case" :)  Having them freestanding would have been awkward in terms of ensuring there was no movement at the base and they'd probably need to be four blocks high which brings more stability issues.  Basically the cost of getting this bit wrong is high, so overdoing things in an attempt to reduce the chances of being wrong is less of a problem, particularly when the cost of doing so is relatively low.

Had I gone for a single rod I'd probably have used something like a 16mm or 18mm diameter (I think I even have some 22mm hanging around), but smaller has some advantages (especially if I need to cut them out at a later point) so I chose to go with multiple 10mm rods.  Two seemed insufficient as there is perhaps potential for rocking.  I can't recall why I decided not to go with three, which would have been sufficient.  I ended up at four, anyhow.

In a different situation I'd quite possibly take exactly the same approach as Peter, though if we had one available for guests if the council ever gets around to granting our planning application then I'd probably have it fixed to something hefty at the bottom so there was no chance of it falling on a small child or something equally unpleasant.

James

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On 10/06/2018 at 18:50, Gina said:

How deep is the concrete block with the gravel round it?

50mm.

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I'd be happy with that sort of depth myself in a free-standing environment.  It's going to be exceptionally hard to budge that.

James

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@JamesF. I'm just describing what has worked for my purposes, extra development is worthwhile. I have a 3 1/2 block pier for a 5" F15 APO the base has 5 600x600x50 concrete slabs, 4 in a square with the 5th bolted to the others through its corners but it is still freestanding. 

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Why oh why did I go for a 800mm cube for the base of my pier????  Over engineering in the extreme - but that was what was recommended at the time!

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Mine should work out about 700mm x 500mm x 500mm, but that's mostly to get the required height and then not wanting too narrow a base for that height.  A flat site would have been far easier.

James

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