whats wrong with doing a fully integrated concrete floor for an observatory?

[h0ughy]

ok i am having a huge think here, snakes and moisture, but, and like me its a big butt:question:, can i get away with a fully integrated slab which would be 800mm thick in the middle for 1.8m2 and then the overall thickness would be 200mm on the outer edges for a total size of 3x3m. the 2.3m dome would sit on this and the pier mounted in the middle? would this be stable enough?:help::thanx:

[multiweb]

You still need your pier separate from the slab you walk on.

[h0ughy]

sure:question:? i would have thought there was enough mass not to affect it? ok what would you suggest? all ears:question:

[multiweb]

I don't have an obs but I have a pier with about 1.5 cubic metres of concrete and reo under it in a pyramidal shape with a base of 600x600 about 1m deep. When I walk around the pier in the grass, it's enough to throw the guiding. So no matter how big your slab here, I'm pretty sure it will transfer to the pier when you walk around if it's integral to the slab. I'd make the pier footing thinner and deeper, with a gap in between its edges and the slab.

[Bassnut]

Although I did isolate in my previous OBs, and intuatively think thats best, itelescopes OBS is one giant slab and is no prob at all. If you wont be in the OBs during imaging, or tippy toe when you are, I suspect youll get away with it.

[h0ughy]

i found with the old doghouse that even with a separate pier walking around transferred anyway to the guiding so i can confirm that, similar size to yours. i even picked up trucks passing over the hydrant cover in the road about 50m away

the mass of the new proposed structure would be around 9 154kg or 9 ton:question:

[h0ughy]

ok initially i will be to focus and target and start of the guiding but after that no so much until the end. probably for my solar work i will be in the observatory the whole time, except when setting up for a time-lapse grab session.

the real reason is the ground i am on is very soft, crappy soil and there are only so many ways to dress a tu....

[sheeny]

It'll probably come down to what your soil under the obs is like, Dave. Hopefully you don't have any reactive clay layers?:shrug:

I would probably still separate the pier footing and the slab. Depth is key for stability for the pier footing. The deeper you go the more pressure there is holding the footing. Mine was only 350 x 350 in area but the plan was to go 1000 deep, but I hit bedrock at about 700 or so, so chipped into that about 50mm to key it. Put some 10mm foam expansion jointing around the footing before you pour the slab. 100mm slab with 1 layer of F62 mesh on 50mm chairs will be fine (cracking control).

Al.

[multiweb]

I guess the heavier the structure, the less chance of rocking it, but for a small size obs I'd still separate pier from slab. Or have a separate floating deck.

[h0ughy]

well then i go back to my original idea? maybe i could get the outer part poured leaving a bell shaped are for the pier to be poured. so if i wrapped the black compose bin in the foam would that be enough buffer? therefore it would be done in two pours:shrug:

[h0ughy]

high reactive soils with a high moisture content :sadeyes:

[glend]

David I hope you did not get hit with that massive storm that swept through here heading towards Newcastle. A real observatory weather test, high winds, heavy rain, and hail. It would fill up your pit pretty quick. Not a day to concrete.

[thunderchildobs]

I have a single 4" concete floor for my observatory, with the pier bolted to it. The problem is the concreate sits on ground. Where i live, there is black soil that moves with the weather, eg wet / dry. I have a high quality bubble level on the the pier. In a week or two the pier moves. I need to constantly relevel the pier and adjust the polar alignment.

Since I am only in the observatory to setup / packup and image remotely, moving around in the observatory is not an issue.

Brendan

[AstroJunk]

I have a single slab. I did that because I have no way of digging down to bed rock to give a solid foundation for a pier. And I've heard of plenty of wobbly hyper sensitive isolated piers in concrete blocks in my type of soil.

The slab isn't ideal though and whilst I don't get the same sort of issue that Brendan talks of (I thought I might, but got lucky) I do notice on occasion that the rather crude dome rotation of the Sirius can put a small vibration into the scope. I noticed it much more with the LX200 forks, my EQ8 is so much more robust.

So my ideal set up would to still use a wide slab to give it stability on the surface, but mount the Observatory on a floating Deck. Thankfully - I can still do that in retrospect if I can ever be bothered. I can't...

[coldlegs]

David
One possibility is to pour it in one go then use a concrete saw to cut a one meter square in the middle and fill the cut with expanding foam. Should isolate it enough to work.
Cheers
Stephen

[ZeroID]

Yes, but the central 1 meter would have insufficient mass as a slab, needs to have a solid block beneath it for stabilty.

Skyslab sits on a single big slab, because it was already there when we bought the house. But I have an isolated floor from the pier although iy is sitting on the same slab with the corner blocks well away from the pier base. I guess I'm lucky in that the slab is stuck to the solid lava flow rock of our local wee volcano ( extinct, or so they say :P )

I'd say in shifty soil having a deep isolated block for the pier is even more important. As has been mentioned earlier the deeper the better to find a more stable subsoil base. I'd be more inclined to pour a block and pier setup then build an Ob around that with a raised wooden floor above the soil line and forget the big slab idea.

[RB]

David, you've seen and used my slab.
I'm not going to bother isolating the mount when I build my obs.
I reckon you can get away with it too.

:D

[h0ughy]

here is an example of what i have to work with. the concrete boys turned up got bogged and left. in their words they will return when it dries out. that may be never:eyepop::help:

also shows the soil - crud:sadeyes:

i think i am going to risk it and do the big massive pour. it will either sink or swim:question: need a big floating raft that is massive enough not to deflect. i think the titan will dampen things out for me. the whole setup also wobbled over at the other house? agggrrrhhhhhhh i feel like SHREK living in a swamp

[multiweb]

I'd get the 'concrete boys' back asap to clean out the mess they left. Unreal...

[h0ughy]

i have and its not wee slab either. hey been to Camden twice over the weekend - its like little italy there, as opposed to mangrove Greece:P all i have done is be the chauffeur ;)

[h0ughy]

that bit is to be covered in concrete:confused2::rolleyes:

[RB]

Speaking of Mangrove Greece and my slab, don't forget the RB Private Star Party (RBPSP) which will be held at a time most convenient to all attendees.
The 150 needs to air.

:D

[h0ughy]

maybe the april lunar eclipse?

[ZeroID]

Woah ! that is soft !!!
In that case pour your slab big and deep with plenty of rebo. I'd also dig deeper holes at the corners to try and get some stability.

Do you get flooded there ?

I think I'd also be getting my slab top surface above the general ground level just to stop any surface water intrusion.

[RB]

You're on!

:party::party::party:

[h0ughy]

i will RSVP to the RBPSP closer to the event:P

[h0ughy]

well we would get a great view of the 1:100 but the PMF is a lilo event indoors:rofl:


um the PMF is the Probable Maximun Flood (http://www.newcastle.nsw.gov.au/environment/flooding_and_waterways/flooding_faq):cloudy::cloudy::cloud y::sadeyes:

[ZeroID]

!!!! not good ... make your pier very TALL ... gives the term 'floating floor' a whole new meaning.. :rolleyes:

[Terry B]

My pier is just bolted to the slab of my observatory. It is about 3m x 3m. My soil is pretty hard granite soil though. I sit on a chair next to the scope and have never noticed any movement in images if I move my chair.

[h0ughy]

:thanx:and that may be the difference:question:

[multiweb]

Ah ok, I was referring to the tire marks. I had a similar experience years ago. The concrete spindle reversed into my driveway and fubar the paving. Too heavy.

[pmrid]

David, my experience of slabs and piers tells me that:

1. Those people telling you to separate the pier from the slab are right.
2. Those telling you the key to pier stability is depth are right.
3. Those telling you that the slab only needs to be 100 mm of concrete are right.

I would also encourage to to consider laying your mesh (before you pour the slab) on a base of capped soft drink bottles of the 1 litre kind. That will greatly reduce the thermal mass of the slab without reducing its stability.


Peter

[Bassnut]

Ha, that sounds so clever :lol:

[gregbradley]

I do a separate centre section 900 deep x 1 x 1 on top then I leave a slight gap and then pavers for the floor. Works well.
Depends how large a scope you intend to mount as far as how large the central concrete will be. The larger/heavier the scope the bigger the concrete pier should be.

Pavers probably cool off a bit faster than a slab as well.

Greg.

[h0ughy]

because of the soil type I cant separate the slab. Basically I am building in a swamp - soft alluvial soil with very high clay content

the shed started today for forming up the slab- after the shed is done they will start on the dome

[h0ughy]

project update:
stairs to be built to access dome site.

then preparation of base of dome slab ready for pouring

[h0ughy]

the stair sub project - almost finished thanks to my brother in law and my son - easier access to the dome location:thumbsup:;)

[bkm2304]

Thou know lad, ah built first 'servatry and it felt into swamp, so ah built second and it burned down then fell into swamp, and a third and it blew over, burned down and then fell into swamp, BOOOT, lad, the fourth one stayed oop!!

[DarkArts]

... an' noo singin'! :P

The obsy is coming along nicely. :thumbsup:

[h0ughy]

:lol:

[h0ughy]

thanks

[pfitzgerald]

Is it being built on a huuuuge tract of land? ;)

[h0ughy]

:question: no

[glend]

So how is it coming along David? Have not heard anything in awhile, all ok?

[pfitzgerald]

Hi David

I hope that all is progressing nicely with your observatory build.

My 'obscure' reference to 'huuuuge tracts of land' follows on from DarkArts '... an' noo singin'!' reference - both of which come from the Monty Python film - The Holy Grail. You'll understand both references once you've seen the movie - incidentally (from memory) both references come from the same scene in the movie (about a castle that was built in a swamp).

Enjoy!

Paul

[h0ughy]

been very busy with family stuff

[h0ughy]

now ready to move forward with this - its raining

[h0ughy]

well obviously i did it - thread closed