Foundations for Pier and Shed (I mean observatory)

[Fordos Moon]

Planning to have a 6'x8' construction with a pier all mounted on concrete.

Thinking a 4" thick concrete base for shed.

Please can someone tell me how big a "cube/block" of concrete should be for a three foot pier?

And should I separate the concrete of the pier base from the concrete of the shed base that it is in? This sounds like a good idea to me but I am no engineer!

Many thanks

Bob

[nmoushon]

Yes, you will want to separate the concrete of the pier foundation from the concrete of the floor. You can put some rubber to pad the gap. 4" of concrete for the floor seems about right thought I would make it thicker around the outer edge and the edge around the pier. This is where the most weight is at. This will help prevent cracking. There are a TON of threads about starting a obsy build here. I would look through a read as many as you need till you feel like you are comfortable with your design. Tweaking it if need be. 

[leelee970]

Bob,

 

You need to keep the pier/pillar and Obsy base separated.

 

When I did my pillar I dug a 1 metre cube hole. The pillar tube was a 2 metre length of galvanised heating tube, 10" diameter if i remember correctly.

 

I needed a 1 tonne bag of hard-core and several bags of cement (from wickes)

 

The silly thing was not hiring a concrete mixer. I did it all by hand which was back breaking. It had to be done in one go. I then sank in four 1 metre lengths of M10 threaded rods for the mount base.

It works a treat and will outlast the Pyramids.

The shed – sorry, Obsy was rested on bricks.

[Fordos Moon]

Thanks mate, thankfully father in law has a mixer I can borrow!

[red dwalf]

i know alot of people dig out big holes for there pier foundations and thats ok, but alot depends on the soil, if clay you can get whats called ground heave where when the water freezes the clay can expand and in worst cases can cause problems, i didn`t go to a meter square, i went as deep as i could get my arm down to dig out and about 2 feet square then filled up to just below the joists of the obsy floor and then filled a 8" brown plastic drainage pipe to the top with concrete, took about 10 bags of ballast in total, not had a problem with it, it`s solid.

for the obsy base an easier way and cheaper option than laying an expensive 4" concrete base is again to do what i did, dig out the area and ton bag of hardcore, compact then lay some 2" thick paving slabs on a bed of mortar around the base perimeter and a few across the centers to support the joists, then the pier is totally free from the base of the footings and the base of the obsy floor.

might get some ideas from my build which is in this section of the forum or it`s on my website which has a link to it in my signature below.

all the best with it.

and yes a concrete mixer is a must unless you want a bad back for a week like i had L.O.L.