Why a concrete base 1m x 1m x 1m

[iapa]

Overtime I look at pier builds, 90%+ say a cubic meter of concrete.

Can some one explain why?

Not just a 'because', but in terms mass/vibration/ice line etc or other relevant considerations. Or 'because Fred did it that way'.

 

[Knighty2112]

Ever tried moving a cubic meter of concrete? Why go for something less when unless you live in a very waterlogged area that requires piling or something more drastic to stop the ground moving you just know it's not going to budge unless there's a major earthquake? 

[BXRO]

1m^3 of concrete has a mass of around one tonne, this is a solid enough foundation to give you the stability you will need if you intend to do some high precision star tracking (perhaps some photography) for visual observations you could get away with less but I can guarantee in 6 months time you would regret it.

[Tim]

5 minutes ago, BXRO said:

1m^3 of concrete has a mass of around one tonne, 

Actually, it'd be well over two tonnes , when we supply heavy goods in m3 we normally allow 2500Kg for the haulier.

I like to be able to slam my obsy door and not be able to detect any vibration in the pier or picture at even 3 metres focal length. I didn't quite go for a full cube, but did make sure there is well over a tonne of concrete in there.

 

[Bizibilder]

I believe the "cubic yard" (now meter!) originated in the USA where many DIY builders had to contend with very poor boggy ground that would freeze solid in winter.  Others built on clay like soils that would dry out in the hot dry summers.  This had lead to the belief that a cubic yard is standard - but it isn't.  You just need enough to make the mount stable - maybe 50-60cm cubed?  (about two foot on a side).  In UK conditions this is more than enough.

More important is to make sure that the "block" is isolated from any floor that you have - a 1" gap between block and a concrete floor is enough.  this is to stop the transmission of vibrations as you walk about.

[iapa]

Sorry - @Knighty2112 @BXROneither are explaining WHY a cubic meter.

What is the specific empirical reason for this?

Does an AVX with a ED80 (allowing for another 5-6lbs for guide scopes cameras etc) that will come in under 50lb. really need a tonne of concrete?

So, an EQ8 with a 14" SCT?  - its that going to need 2 tonne?

I am looking for specific reasons rather than 'well, it's a big mass so won't move'.

How do I actually calculate the size of this base?

 

 

[Davey-T]

18 minutes ago, iapa said:

Overtime I look at pier builds, 90%+ say a cubic meter of concrete.

Can some one explain why?

Not just a 'because', but in terms mass/vibration/ice line etc or other relevant considerations. Or 'because Fred did it that way'.

 

Because people in the USA on CN  who have extremes of temperature found it necessary to combat their conditions and it transferred around the world.

If your soil is clay / sand / gravel / dry / waterlogged you can use as much or as little concrete as required, much like building a house.

I've got one pier set in 18" of concrete and one just bolted to the patio, neither have shown any sign of moving, my soil is clay.

If you're imaging you don't want to be wandering around the pier no matter how much concrete it's buried in if you get a planet videoing and go six foot away and jump up and down the image will wobble considerably.

Dave

[Paul M]

It's a big chunk of concrete and I suspect often used with little more justification than the fact that a cubic meter is the unit measure for the costing and supply of concrete.

A hole 1 meter square is a nice size to excavate but getting it 1 meter deep is less fun so if I was siting a permanent pier for my NEQ 6 and 10" Newt it would be something less than a cube. My back insists!

I once built a conservatory at my old house and dug out for a poured concrete footing. I started at 2 spades wide and two spades deep. But by the time I'd done it was 18" wide and 18" deep   When the concrete ("spotmix" style) lorry arrived he came to look  at the job and was shocked to find I was only putting a conservatory on it. "Enough footing there to build a 3 story extension! Well, it's you who's paying mate...." he said.

[Uplooker]

Hi Iapa,

I hear what you are saying. I think that a lot of people would rather over engineer things. If you put in too little concrete and find that you get movement or tremors in your imaging kit it will be a lot of hard work to remove the concrete and re-do it.

Depending on what you want to do, imaging or visual, will dictate quite how large you concrete base will need to be. If it is large enough, say 1 cubic metre,  it will future proof you if you go or a larger mount or equipment. Isolating it from any floor joists is probably just as important as overall concrete base size.

I realise that I have not answered your question by giving you a formula to calculate exactly what size of hole you need to fill with concrete. I think that there are too many variables such as people's construction skills, soil types etc.

I hope that helps some.

[johnrt]

I recently poured a concrete base and mounted an Altair pier on top. The concrete base was 40cmx40cm and 60cm deep. I have a railway line 50 feet away from the pier where trains rumble past every 20 mins.

Imaging at under 1 arc sec / pixel at 900mm focal length I can detect no vibration from either myself or the trains. 

I feel on the whole pier concrete bases are hugely over engineered, has anyone ever heard a story of someone who made a concrete base too small and regretted it? I certainly haven't. 

[skybadger]

I have an upside down pyramid of concrete as my base, about a meter on a side at the surface, going down a meter to the clay line and just below.

The pier it supports is 8' high so the intent was to prevent vibration by getting a large surface area and counteract the large fulcrum by having some depth. The load is about 100kg fully laden, so there will be two effects, that of a pendulum with fixed end and column resonance. The Column is 12" diameter so I'm not worried by flexure or resonance, so I just need enough of a damping mass at the end and to resist bending forces.

It has been sufficient ....

Mike

[MeyGray3833]

I guess it is a preventative measure. Most peopl's logic is that it is so big and heavy, it won't move. Having built one obs and now another for astrophotography, the last thing I want is pier movement and vibration. It is easier to over engineer and have everything solid and unmoving rather than trying to shore up something flimsy, which is never really effective.

For both my obs, I used a concrete cube top and a leg style arrangement. My basic thought is that the 600mm^3 block will be the anchoring rock to which my pier is attached and the 250mm x 500mm leg underneath will be enough to stop/prevent/slow any drift of the top block. This equates to approximately 512 kg of concrete. This worked very well in my first obs where I drift aligned once and left it for 3 1/2 years until I pulled it down, only checking once every 12 months.

This is my logic, I'm no engineer, so I could be wrong. YMMV!

[Davey-T]

Let's not forget a lot (possibly the majority ) ? of folk manage quite happily using a tripod.

Dave

[johnrt]

1 hour ago, Davey-T said:

Let's not forget a lot (possibly the majority ) ? of folk manage quite happily using a tripod.

Dave

Quite so, on my tripod I could see visible spikes in my guiding graph when the trains rolled past. Not so with the concrete base.

[pete_l]

It's complete overkill IMHO. Even house foundations don't need to be that deep, or wide.

If you are a visual observer, then all you need is to dampen any vibration that could come up through the pier. Standing on a foam pad is generally sufficient for that (and it's warmer than the ground in winter, too). If you are imaging, then the chances are you won't spend the imaging session jumping up and down, next to your pier (and you couldn't be looking through the 'scope while it's imaging, either - so the point becomes moot.
The key to vibration damping is firstly to remove the sources, then to minimise the transmission of vibrations through the pier.

Unless you happen to live beside a major road or railway, you will have very little ground-bourne vibration so your major source would be the mount when it moves to an object or does a meridian flip. Provided your mount is well balanced and you program a reasonable delay befopre starting to track / image after a movement, there should be no vibrations coming from the mount.

[Shibby]

I'm about a week or two from pouring mine and I'm going for 70x70x70cm. Related question: how important are these rebars I see some people putting in?

[Skipper Billy]

This was my take on it - I will let the guide graph do the talking - digging and concrete are easy and cheap - rectifying built in problems is never easy or cheap.

https://sites.google.com/view/astro-imaging/observatory

 

[Davey-T]

1 hour ago, Skipper Billy said:

This was my take on it - I will let the guide graph do the talking - digging and concrete are easy and cheap - rectifying built in problems is never easy or cheap.

https://sites.google.com/view/astro-imaging/observatory

 

Speak for yourself Billy, after I'd dug down 18" in solid wet clay, having to scrape every shovel full off the spade I decide it was quite deep enough 

Dave

[Alpollo]

Just to add what others have said.

Where I live, you have to put such things 48" down if you don't want them to be pushed about by the frost.  It wouldn't take long to use up a meter if you expand this pier block to also become your observatory floor.

That being said, 1 cubic meter might be overkill, especially if you neglect the pier. All the concrete in the world won't fix a vibrating pier, it might even make it worse.

I don't know if there is a rule of thumb, but 10:1 seems to be a reasonable starting point. This would mean a 100lbs of scope and pier could use ~1000lbs of concrete. In that case I might think that 3915lbs is overkill perhaps underutilized. The way most piers are designed, they won't see any practical benefit from being bolted to 4000lbs vs 1000lbs they vibrate too much on their own.

For those purchasing ready mixed concrete, the minimum is usually 1 cubic yard or meter, so perhaps that is why that volume is used often. Why not it's easy to do if the hole is there.

IMO I would prefer to not have the pier block connected to the observatory floor. I would prefer to use a block 18"-24" wide and go deep. This helps to decouple your footsteps from the telescope. The floor should be a separate ~3" pad if you want one, separated from the pier block with an air gap.  If you are visual only observer, these details wont matter as much.

[Stub Mandrel]

19 hours ago, BXRO said:

1m^3 of concrete has a mass of around one tonne, this is a solid enough foundation to give you the stability you will need if you intend to do some high precision star tracking (perhaps some photography) for visual observations you could get away with less but I can guarantee in 6 months time you would regret it.

Nearer to two and half tonnes!

Here are guidelines for foundations for walls. Anything solid enough to support a wall will support a 60kg scope and mount without difficulty.

http://www.diydata.com/techniques/brickwork/foundations/foundations.php

Depth really isn't that critical for a pad, as much as area. If I was laying one I would make it a meter square but perhaps only 150cm thick on a good base of well tamped down hardcore.

We have the base for a summer house and the concrete is no more than 4" thick (over a sandy soil) and I would defy anyone to bother a scope set up on it with anything short of sledgehammer blows.

[Alpollo]

The data on that website shows wide footings because most of the weight is downward with a brick house, no torque really to speak of.  A wide footing distributes the vertical pressure reducing PSI on the soil, downwards.

For any given amount of concrete, it will resist torque better if it is long and deep vs square.   It just has to be wide enough. Decks around here are supported on ~10" - 12" round sonotubes and they don't sink with 1000s of pounds above, so that should be sufficient surface area for a telescope.

A 12" x 48" deep hole or sonotube with a bit of rebar would only take about 1/10th of a meter of concrete and probably be just as effective in most cases as a big 1m block.

Ultimately I guess it comes down to whether you are digging the hole and mixing the concrete by hand, or having machines do it.

[iapa]

8 hours ago, Davey-T said:

Let's not forget a lot (possibly the majority ) ? of folk manage quite happily using a tripod.

Dave

Indeed, I was looking for something I can leave out all weathers and not workout too much.

I would want full length protection for a tripod - 'cos I'm like that. Even with, a soon to be applied, smear of ACR-50.

Also, want to make it less likely to be moved by a child running into it.

[Adam J]

I am assuming that this has got to be with ultra long focal length scopes in mind because I dont seem to have vibration issues with my setup and I am just plonking it onto a patio that i suspect has not even been set onto hard core.

[ollypenrice]

22 hours ago, iapa said:

Sorry - @Knighty2112 @BXROneither are explaining WHY a cubic meter.

What is the specific empirical reason for this?

Does an AVX with a ED80 (allowing for another 5-6lbs for guide scopes cameras etc) that will come in under 50lb. really need a tonne of concrete?

So, an EQ8 with a 14" SCT?  - its that going to need 2 tonne?

I am looking for specific reasons rather than 'well, it's a big mass so won't move'.

How do I actually calculate the size of this base?

 

 

An EQ8 with a 14 inch SCT is going to need a precision engineer to rebuild the mount. No amount of concrete is going to sort it out.

I think a lot of this concrete business has to do with the land on which the observatory is built and the likely sources of disturbance. You have a train line nearby?? I have imaged without difficulty on a five tonne flat concrete base, but the five tonnes is for a single base which includes warm room. The mount in question, a Mesu, stands on the bog standard Mesu pier with adjustable feet. I have wooden batons screwed over the feet to stop them being nudged but that's that. We did 30 min subs at 0.66 arcsecs per pixel at one time and never dropped a one. I live in the alps on a sub stratum of boulder and clay. It is not considered fit for builing on!!  There is no industry or train line within communicable distance. Our robotic shed houses four mounts, again on a single slab, which weighed in at six tonnes but that, also, carries the large roll off shed.

I can't speak for other parts of the world but I suspect a certain machismo when it comes to concrete tonnage. If you want science, however, I don't have it.

Olly

[iapa]

9 minutes ago, ollypenrice said:

I can't speak for other parts of the world but I suspect a certain machismo when it comes to concrete tonnage. If you want science, however, I don't have it.

Olly

Practical experience also counts

So, conclusion, over all seems to be 1m x 1m x 1m is not necessary, except on soft ground with low frost line.

 

LOL