Concrete vs. metal pier

[bmitchell82 (Brendan)]

http://www.astropix.co.uk/equipment.html

click on the photo of the telescope, then navigate to picture 4 of 5. this is the effect of thermal mass!

[Tandum (Robin)]

How does that pier expel heat when it's under a roof all day. Where does the heat come from? The concrete under my obs is always cold. Is it sub zero there at night? That would explain it.

[Paul Haese]

Just as a confirmation of what Rally and Brendan have been talking about here.

I poured my concrete footing in direct contact with limestone kunka. After drilling and digging down into it 1m x 1m x 0.8m. I ensured that the footing was 40 mpa (slump at pour) and specified from the concrete company. You can do 35 mpa with bunnings concrete but you need an extra bag or two of concrete powder and be mindful of how much water you add to the mix. 30mpa is readily achievable with the mix itself and water regulation. Too much water means a weaker concrete.

I then had a custom made pier which has a 8" OD with 12mm wall thickness at 1300 long. It has stabilising fins which come up 1/2 of the pipe length. Very stiff configuration. All bolted down with 16mm bolts.

I have tried hitting the pier when viewing at high mag and I did not get any movement whatsoever. It might deflect a bit but I have not seen anything in my images that resulted from pier deflection.

Bottom line is go as big as you can with all that. It saves digging up later and starting fresh. Like I said earlier go thickness and diameter. I was to do this all again I would probably try for a larger diameter pipe with the same wall thickness, just to really ensure stability.

[bmitchell82 (Brendan)]

You have the idea Paul. Straight into bed rock another way that this is done with multi story buildings is drilling down into the soil until they hit bed rock and upon pulling the auger out the back fill with a cementious slurry then connect them with a head of concrete. Also your fins are an attempt to increase the thickness of the section which is the way to go but like you have built it they come right up the section any moment that is taken by the fins takes a path down to the footing which takes the moment and then distributes it into the soil/bedrock. This also decreases the effective length of the un-braced section, if you where to do the same from the top end, this will be in effective because the flex will originate at the end of the fins and there is no where for the moment at the top to be resisted. I would have to draw the pier in 3 space in a program such as auto cad and import it to multi frame or space gass to give you exact figures of what is happening, but as you said you have hit it and at a longish focal length there was no "noticeable" movement. Have you tried this theory while guiding? that will give you a quantitative value of movement not just qualitative.

Although i agree with you that 32/40 MPa is achievable by your average DIY warrior, i doubt that without knowing what they are doing that the concrete will get anywhere near it and once again it comes down to she will be right chuck another 2 or 3 bags in the mix. Im also a tad confused by your 40Mpa (slump at pour) they are two different things ones the characteristic strength 40Mpa and slump is a measure of workability nominally you would get a 80 slump unless you specify differently in which the concrete company will adjust the mix with either plasticisers to give better W/C ratio. How do I know about this stuff? I worked as a concrete batcher for Hanson's for 3 1/2 years, backed with a year and a half of university about concrete as a material, and designing with concrete to Aus standard 3600 (that code is a nightmare!). Its not a dig at you mate not in the slightest I know you have a lot of years in the building industry, I just want this thread for your average fella that most of this has gone over their head to have the right information so they can research it till they understand what it is they are doing and to make it not just another Chinese whisper thread.

Also robin you are right the concrete will try to equalise with ambient temps that means when you drop 15 deg in the night it will release stored energy it almost goes to peter ward's post about big optics suffering local conditions more. The Mylar just traps the heat. In a lot of peoples systems the heat haze will not be noticed as the system is not producing the resolution due to focusing, tracking, flexure and a whole raft of other things. If you really want to see what's happening drop your scope onto a reasonably bright star say Mag 2, defocus until the star is a good 1/4 - 3/4's of your eyepiece and then look critically, you will see heat trapped in the tube if its not at equilibrium, seeing from the atmosphere, different parts of your scope releasing its energy very interesting to see. if you have a DSLR with live view this is even better to see. move your hand under the the tubes aperture you will see it massively swirling away

[Paul Haese]

Yes tried it guiding Brendan and not even a blip on the graph.

[bmitchell82 (Brendan)]

Perfect! so your using a 203x12 CHS with gussets that span half way up. If you where to make another one, ide go the 250x250x9 which would be about the same weight, chuck 4 gussets for each face you would find it would hold a PME + a massive scope without issue! Or work towards my frame system! that will be interesting as I will be building a pier in the future

[Paul Haese]

Not building another one mate. This sucker was over 200kg. Virgs and I could not lift it ourselves. Yes the gussets could have been all the way along but in the end this works well. Wind is more of a problem that I have found. Although not checked this since I worked on the worm gear slop.

With regard to the MPA though. I don't really think it will make much difference. Like Rally said, the extraneous noise in the system is more likely in the weakest point and that is most often not in the pier or footing. Most people over engineer those two things because removal is a problem. Screw fixings and connections is where most of the noise comes from in the end. I specified my MPA but just checked slump for water content. I figured that water content will finally decide what the final mpa will be when cured. Not that I could really do much about it once it was in the hole.

[koputai (Jason)]

Originally Posted by bmitchell82

Straight into bed rock another way that this is done with multi story buildings is drilling down into the soil until they hit bed rock and upon pulling the auger out the back fill with a cementious slurry then connect them with a head of concrete.

That's what I'm currently doing with my pier, though there was no soil
to get things dirty, and I'm using epoxy instead of cement to fix the
pier in the hole.

http://www.iceinspace.com.au/forum/s...ad.php?t=58252

I've now got all the bits to fabricate the pier.



One cylinder has now had its head chopped off, and one of the
flanges welded on. This becomes the post and is epoxied in to
the hole shown in the thread above.

The other cylinder has had the same treatment, then it's flipped
upside down and the two flanges bolted together. The mount
will be fitted to the top of this.

Here is a couple of shots of the trial fitting of the post in the hole.



Oh, and that's a Nagler Type 6 sitting in the flange for reference.

Cheers,
Jason.

[bmitchell82 (Brendan)]

Paul without doing proper tests on the concrete there is no way of finding out the water content. Ide even beg to say that your pier will have varying strengths of concrete through out, this is why i said characteristic strength.

Case in point i could give you 2 samples of concrete one with a super plasticiser and one without, the one with the additive will have about 1/5th amount of water and still retain a 80slump, have a far better water/cement ratio and hence achieve a greater 28 day strength and ultimate strength.

I guess what i'm trying to get though to most people is the idea of over engineering there becomes a point of diminishing returns. Further to that, the actual strength of the materials that people use is significant, with loads that can be carried by a 1m length of sections we are talking about capable of 500+ kN of load in a cantilever configuration to put it in terms for people who don't understand Newtons that is approximately 50 ton. Now you can appreciate when I talk about a 200kg load is not really an issue. Realistically who is buying a PME+20RCOS kit? i wouldn't say there are heaps of them around, once again if your putting more than that on a pier, you should go and get one designed for serviceability, your just about to drop 50-70k in optics, robotics, ccds, whats 1000 dollars in design of a pier?

Back to your test Paul with the hitting the pier at high magnification, when does that happen in real situation apart from if you where a bit clumsy and bumped the mount? You said that the wind was more of a issue but that was before you got rid of the back lash.

My eq6 is running the 10" dob with a lot of gear up there 25 odd Kg, its tuned like no tomorrow and even a bit of wind doesn't affect it too much but my tripod isn't the one that is moving

Jason i like your idea with epoxy, in solid stone like that its the way to go as it is a chemical bond that sinks into the stone and hooks onto the metal. but for the majority of us poor sods who have sand (in wa its just a big old sand pit) concrete is the way to go

[marki]

20/20 is all you need to build a concrete pier. You might consider 30+ MPA with plastisizers if you were building a freeway bridge footing/enbuttment wall/pier/deck etc etc where outright strength and durability is the main concern (remember these structures are often stressed via cables placed in the concrete at exacting angles and the bridge must be able to flex whilst maintaining structural integrity). Main thing is add some reo (not for outright strength but to prevent cracking) and vibrate the concrete well to remove as many air bubbles as possible. Any more is really overkill for the types of loads we place on them.


Mark

[bmitchell82 (Brendan)]

On the money there Marki!