Concrete vs. metal pier

[Helo]

I am getting back into astronomy (since my youth) and wishing to build a pier and observatory and tossing up what sort of pier to use; fabricate my own 12" (300mm) concrete pier (but where can I get the formatube :shrug:) vs. a 165mm metal pier (available from one of the telescope companies here in Oz). It is to support my CPC1100 and wedge and as I wish to get into astrophotography it needs to be stable so any ideas and experiences with either pier out there? Any help is appreciated. Thx.

[steve000]

DIY...

I looked exactly what you looked into....
I found Concrete was more expensive, almost 1 cubic meter (about $300 from concrete taxi) and about $60 for a formatube..

I got my steel from direct steel at ipswich and total cost was $79
it consisted of
300 x 300 x 20mm mild steel base plate
2of 200 x 200 x 20mm mild steel top plates
165mm OD pipe ( i actually went 150 od pipe as they were out of stock of the 165)

+ bolts total cost is about $100... even if you factor in a cheap bunnings welder + gear at about + $150 or so its still cheaper then concrete and a commercial product.

Stability should be about equal as well, especially if filled with sand. if its out in the middle of the yard on a windy night things might be dodgy. there are members on here who use smaller diameter steel and its still uber stable.

linked is a cad drawing of it.

if you cant DIY find someone who can weld for u.. clean up only needs a cheap angle grinder. IIS members are uber helpful. If you willing to wait a few months i will make mine, i will make you 1 as well.

Pier
http://steve000.dyndns.org/images/200mmpier.pdf
Pier with my scope on it. scope and mount are verrrrrry close to scale
http://steve000.dyndns.org/images/200mmpierscope2.pdf
(http://steve000.dyndns.org/images/200mmpierscope2.pdf)

[ChrisM]

Peter,

Another idea is to incorporate a stubby (ie. not full height) concrete pier with the pier foundation, which typically would also be concrete. Then, a steel pier can be attached to the top of the concrete pier to attain whatever height is needed. Future equipment changes can be accommodated by replacing the relatively cheap steel pier with a shorter or longer one.

I used this concept for my two-storey observatory - refer to some basic details and pictures here: http://www.iceinspace.com.au/forum/showthread.php?t=36222&page=2

I bought a length of Formtube from the supplier in West Heidelberg, Melbourne, and would be surprised if there wasn't an equivalent in your part of the country.

Chris

[Helo]

Thanks Chris and Steve; that gives me some great ideas and suppliers also. The designs are great Steve and I will have a chat to my local suppliers next week. That was a big hole you dug for your pier Chris and what an awesome looking observatory. I'll have to get my pier started as I am putting an astrodome over it once it is in so I can leave my scope permanently setup. Adds so much more to viewing time and then I can get into the photography. Thanks again guys. Peter

[Tandum]

Steve, you might wanna check your maths.
There is no way you'll fit a cubic meter of concrete in a 300mm tube unless it's about 6 meters tall.

[pmrid]

Whether you build some concrete formwork for a full or partial concrete pier, or just do a straight steel number like Steve's, you will still need to lay a solid concrete base for the whole thing. A block of concrete a metre deep and say 400x400 will be less than 1/4 m3. I'd do it with bags of premix. Work on about 15 bags worth. I think they're about $10 a bag these days. And you have the additional option here of getting a longer bit of steel pipe and burying it (with suitable bits of steel welded to the foot to make it solidly anchored in the concrete base - eliminating the baseplate and associated bolts involved in Steve's design. Of course you don't have the option of removing it later on - apart from digging the whole thing up again.
I have one pier that is of this kind and it's rock solid. I also have one that is a store-bought steel one bolted into a slab. Both are rock solid. One cost me $1000 plus the concrete base. The other cost me the same amount of concrete base and the rest was less than $100 in scrap steel pipe. I know which was I'm going next time (there's always a next time).
Peter

[steve000]

LOL fail maths.. ur dead right.. 60cm high and 300 wide is about .42 cubic




This is quite a good idea. Ive always been in the habit of over engineering stuff.

After reading everyone comments on this.
I think I will go 4 Star pickets driven into the ground to reduce change of the ground around the block dissapearing and the block starting to move/shake if pushed.
400 x 400 x 1m block poured on the star pickets reinforced with rebar
4 m16 threaded rods with their ends bent welded to the rebar, these bolts will be used to attach the steel pier
steel from direct for $100


Should i put styrofoam around the sides of it to isolate it from the ground or is just isolating it from the pad ok?

and yes I partly have hijacked Helo (peters) thread. I'm asking the same question as him

[Moon]

I did something similar, here is a picture of what it looked like. The wood held the threaded rod in the right position during the welding and concrete pour.

http://deepspaceplace.com/show.php?id=observatorycube

The pier is a 300mm x300mm steel square section. It was easier to find than a short length of (round) tube/pipe.

James

[pmrid]

You don't isolate the pier from the pad. That would defeat the whole purpose and leave the pier inadequately supporded/rigid.
You isolate the pier/pad as a whole from whatever other floor you decide to put in your obs - so when you're bumping around in the middle of the night, your movements are not translated from the floor to the pad/pier/scope etc.

Peter

[gregbradley]

There was a paper issued by a guy in the US about pier design.
As I recall its the diameter of the tube that is more important than the steel thickness. 250mm diameter tube is a lot more stable than thicker 165mm tube. So you want maximum diameter.

The usual design is a large pier/base that is heavy and supports the steel or concrete pier (as wide as possible) and then a levelling plate so you can get perfectly level mounting for your scope.

You want the pier isolated from the floor if you were planning to do a slab floor (I like pavers instead of slabs, less heat retention, no vibration transmission and easy to lay).

Greg.

[koputai]

I think this is where the design of a lot of (most?) piers falls down. People go to all the trouble of building a fairly rigid pier, then mount their top plate on four long bolts. These four long flexy bolts then become the deciding factor for the rigidity of the whole mount.

If you do have to use a top plate mounted on bolts, these should be kept to the absolute bare minimum length possible to avoid flexure.

Cheers,
Jason.

[steve000]

yea thats what i meant

[Zaps]

Pier Design Fundamentals (http://www.cloudynights.com/item.php?item_id=1275) by Dennis Persyk.

[SkyViking]

Just an idea, but for my pier I built a round tube of decking timber I had lying around. I screwed the timber onto 2 metal bands on the inside, one at the top and another at the bottom, and formed a tube that way. The inside diameter is around 30 cm. And then I filled it up with concrete. It didn't have to be watertight since the concrete doesn't really ooze out unless there is a significant gap.
This was built on top of a proper concrete base into the ground.

It was very simple and yet very decorative in the end, since I could stain the timber. I'm very happy with its performance and it looks great. I would post some pictures of it but they are on another computer. If anyone is interested then I'd be happy to post the pics later.

[ZeroID]

Starting to think seriously about a pier setup now, my EQ6 Pro has arrived :thumbsup: and I just found out how big and heavy it is !! :eyepop:
May go for a trolley setup for a starter but long term I think a pier and 'Ob' makes for better option. I have a suitable location at home. Just got to find all the bits and this is all good info to consider.

[Helo]

I am starting to think the combo pier might be the way to go (concrete base with shorter steel pier so I can adjust in the future - just in case). Does anyone have any tips for exactly aligning south or having some leeway in the mount to adjust for any error when I place a wedge on top?

[steve000]

You probably will need an adapter plate of some sort... just allow it to be moved at different mounting points..

E.g. my plate (attached) will have 6 holes (red) drilled in the outside for bolts, this will allow it to attach to my pier plate and a 7th hole(green) for the mount peg. I will align the peg hole to south including magnetic deviation as accurately as possible.
the attachment bolts will be drilled about 30 degrees either side of the peg hole.

6 bolts are 60 degrees seperate.

The end result is the adapter plate will be aligned for south, where as the pier can be facing anyway as long as the holes match up.

Also i am sick atm so my explanation might not make sence :P

[Paul Haese]

Diameter of the pier is extremely important. Get 200mm and above for diameter. It really does make a huge difference. 150 will be too narrow for astro imaging with bigger gear. This is something you only want to do once in my opinion, so get the largest diameter possible. Wall thickness contributes but not as much as diameter.

[frolinmod]

It's better to use the Solar transit method. It is more accurate than using a magnetic compass and you don't have to worry about magnetic deviation or the influence of things like the rebar cage inside your concrete pier that might throw the compass needle off.

Suspend a plumb bob from a string so that the shadow of the string falls across the top of your pier. Using your favorite planetarium program, look up the time when the Sun transits the meridian on the current date. At that exact time, mark a line where the shadow falls across the top of your pier. For good measure, mark it not just on the top, but on the sides as well. The line runs North-South and is very accurate.

[jenchris]

I think you've overdone the rebar James.
But that's ok -
You've used about the same amount of reinforcing as for a pontoon gangway mounting block. There's very little stress on the block and the rebar is only for crack stopping. F62 mesh would be more than adequate two pieces about 600mm apart vertically

[pmrid]

I go along with Ernie. The solar noon method is spot on. There is a very good Solar Noon calculator maintained on the US National Oceanic and Atmospheric Administration that allows you to input your local Lat and Long etc and get a precise Solar noon time for your spot. Go to:
http://www.srrb.noaa.gov/highlights/sunrise/sunrise.html

Peter

[gregbradley]

If you have your mount on a portable tripod then drift align it near where your pier is.

Then transfer that angle to the new pier.

Then the steel pier has slots in its base rather than a hole.

Adjust it so its close.

The mount will have the ability to rotate a bit for polar alignment and you should be within the range of that rotation.

That worked for me.

Greg.

[Helo]

Solar align method sounds so simple yet elegant. Now I just have to convince the sun to come out long enough ;). Thank you all for your help.

[steve000]

after reading it a few times :eyepop: what a fantastic idea!.. so gonna do

its funny.. ive always had issues getting around polar alignment..

over the last few weeks I have discovered
for RA use pumb bob..
for DEC get protractor and cut out piece of wood and align next 2 mount.
for leveling steel ball bearing on pier top..

IIS community you are fantastic

[Helo]

I have a welder who is happy to construct a steel pier from my design and I want the larger diameter as suggested for rigidity (trying to source 250mm) but any suggestions on where to obtain pipe (from 200mm up to 300mm) otherwise all I can locate comes in 6m lengths? Thx again IIS.

[Moon]

I also found it difficult to find pipe of the right dimensions. Most places are reluctant to buy an entire length for a small project like a pier and have the remaining piece sitting in their workshop after the job is done. My fabricator had some leftover 300x300 square section, the same that is used for road signs on freeways. It worked out well for me because it's the same size and shape as the base of the mount. Structurally there is no issue going with a square section - it doesn't have to be round.
James

[ZeroID]

Alignment done .. :thumbsup:
Got a break in the clouds

Watch out for SKYSLAB ....coming your way.

[steve000]

nice one mate, you lucky bugger.. its been cloudy here for what seems like the last millenium.

what was your method on aligning it

[Helo]

Thanks James; I'm waiting on the fabricator getting back to me with what he can get but just in case, can you give me the details of your fabricator so I can source it from them or even get them to make it? Then I have to factor in the transport cost to 'sunny'? Qld. Also what was your design and for what type of scope (I have a CPC 1100). Thx, Peter

[ChrisM]

Peter,

Try your local scrap steel yard. It doesn't matter if the pipe is not shiny new. I got a 1.6 m scrap length of 275 mm dia x 6 mm pipe, cut it to the length I needed, and after a few coats of paint, it looks great. More importantly though, it's quite stiff. You might also try a large engineering works or steel fabricator to see if they have any scrap.

BTW, the local noon shadow method worked a treat for me.

Chris

[steve000]

I have found the iphone app angle (free) to be reasonably accurate as acting as a inclometer.
I got a 0 deg 90 deg and 25deg peice of wood cut using a protractor.
I used the iphone on the edge of it with a small peice of plastic under the iphone to shim the gap between the volume buttons and side of the phone.
the iphone was dead on 0 and 90 and was flickering 24 - 25 degrees.
Id say it was very close.. enough to use to get the dec angle of an EQ mount close to proper.

Combine that with the plumb bob method and id imagine you will be within 1 deg of aligned! im going to confirm this next sunny weekend.. I will report back in 97 years :P

[Helo]

Thanks Chris, I tried some scrap yards and then thought of trying a friend who makes trailers (so uses a lot of steel) and he was immediately able to source some pipe for me - problem solved. Have all the pieces and design now so just need to have some sunlight to align and to lay a slab!

[DavidNg]

That is very elegant solution indeed. Many thanks

[bmitchell82]

Just for those who love the science and the facts behind the shield of Arm Chair Experts S (ACES)
I know the Ixx (second moment of inertia) or rigidity formula follows

Sum of ((b*d^3)/12)+(A*h^2)) for rectangular sections

for a circular section

(Pi*(do^4)-(di^4))/64 where do is Diameter Outer and di Diameter inner

Unit is generally in the form of XX*10^6 mm^4

there is a lot more going into this than saying hey yeah i have a good value of stiffness. I should do a simple space gass load run down for the "average" pier

As has already been said thickness of the material doesn't really matter for "stiffness" but i beg to disagree, the amount of material away from the NA (neutral axis) hence the thickness is paramount for stiffness in a rectangular section (Look at a I beam for instance, thick top and bottom flanges thin web the web resists shear and gives the section depth (stiffness) and the flanges give moment resistance for compression/tension coupled with the resistance of buckling of the web), also for a circular section it does with the formula subtracting the inner from the outer diameters hence the thickness of the material. For the circular section the geometry leads to good torque (twisting) control as the shear flows around the section.

Just out of pure interest a little while back i did do some calculations on a 200x5 SHS at approximately 900mm in height it required 50kgs of horizontal force at the top of the section to produce approximately 5 micron of movement. Now i don't know about you but that is a bucket load of force for something that tracks with the earth and isn't flying around all over the shop. You are more likely to get larger errors within your mount.

Yes a larger diameter is stiffer, but unless your mounting 16"+ scopes with PME's that are in the 100+kg bracket and the like 90% of the time its over kill *100.

[ZeroID]

Good ol' Solar. I got a break in the clouds as I said.

Next is to plan how I want to progress.
And for a pier mount base might buy an EQ6 extension for $83 and make a short concrete pier for it. I like the idea of a tapering concrete base because my slab is already down and stuck to the lava rock of the local volcano so I would drill into the slab, fix steel rods into that and pour my pier base on that. Wider the better in that case. Rockcrete will secure the rods easily.

[rally]

Brendan,

I just calculated the deflection of a 50kg load on a round 200mm dia 6mm wall thickness steel pipe (not high tensile or Square section)
A 50kg lateral load deflects the pier 11.5 arc secs

Whereas a 3.5kg load (more typical of the sorts of loads for cable drag plus eccentric loads due to camera rotation and out of balance due meridian flips and maybe a bit of light breeze etc) deflects just under 1 arc sec which is more in the realm of where you want to be
Preferably well below your image scale so its lost in the other noise of your system.

Some of the deflection is dynamic - eg breeze, knocks, vibration of fans and motors etc some is more static in nature - eg a complex rig with guide scope and camera with filter wheel can be difficult if not impossible to balance perfectly on both sides of the meridian especially with the rotation of say a 8 x 50mm filter wheel and camera and an OAG etc
So stability will affect pointing and it will affect the quality of the image - small vibrations and movement simply translate into blurred detail.

I think the trick is to cost effectively ensure that the pier becomes the least cause of unwanted noise in your system - so bigger (using that philosophy) is better, especially since scrap steel pipe only costs $100/m or less at the local junk yard
Its very cheap insurance !

To the OPs orginal thread - Steel is usually easier to work with and modify and can be potentially cheaper than concrete.
But you need to be able to weld it !
Solid concrete for the same OD is going to be more rigid, but there isnt much in it.

Rally

[bmitchell82]

Im not up on the rhelms of Arc seconds from deflection that isn't my area, though i have put a blog on my website with actual deflection information pulled from Multiframe which is one of many industry standard structural analysis programs.

Like i say in the blog, everybody keeps on going on about section size and i found some interesting things in relation to this that a SHS is not the best section at all! with a 329*12mm CHS Grade 350 having deflection with 50kg (500N) point load at the top of a 900mm pier deflecting .006mm this load is unrealistic and far greater magnitude than you would ever see. A 219.1*12.7 with the same configuration will be deflecting .02 which is even worse :(

the largest section that is avalible in my library of AS sections was 250x250x9 giving .01mm which is .004mm difference the economics of the two sections will be a lot in terms of pure weight of steel. and the fact that you wont find a 330mm OD piece of scrap tube hanging about readily but I know that there are plenty of shops around that will carry SHS in 250^2 size.

though at the end of the day it comes back to the connection of the "adjustment" plate to the pier and to the mount head, it doesn't matter how big the section is if you run small bolts in comparison you would loose everything you just gained.. very counter productive

As for wind and what not, if your imaging in wind your seeing is most likely going to be shocking and if you can see 1 arc second vibration then wow! not that the pier will vibrate unless you have a sinusoidal laoding pattern. For cable drag and what not? how does this happen unless your setup is messy and has cables draging off the scope and not run in neat bundles? alright for testing but get it all sorted. Mine runs either side of the 10"and out though the Dec axis and vertically down. I have no issues. Did you take into account the backlash in the mount system? Flex in the OTA? These are big areas of movement.

I guess what im trying to say is I keep hearing people saying that this section is the best to use or this material is far more benificial, without them even knowing the implications or what the formulae that govern the outcomes. Ive kept very quiet untill now for a good year and a half but with the level of knowledge and skills that i have now i feel i can input onto the subject without being a A.C.E.

Furthering to the comment that a similar size concrete pier will be more rigid isn't correct, at 300mm 32mpa concrete with the same loading will deflect in the order of 0.017mm, unless reinforcing steel is added in which case will make it more expensive and a proper design will need to be preformed to stop cracking and premature failure of the concrete though corrosion and the cracked section being reduced hence rendering the 300mm OD pier with even less stiffness.

I would say that steel is far easier to work with because of the avaliblilty of small fabrication shops that are avalible. who knows what a carton of beer on a friday will produce.

As a realistic test I might try and get a hold of a spring balance and on the pier that i designed and built for a friend grahame (IOTW before mikes dragons) and utilise maximdl5 and guiding to see what a 50kg load will produce in the way of Arcsecond movement. because when you let go of the load once the guide star has recentered the image you will see the errors changing!

I would love to work with you in regards to giving the mass's a No B.S. No bias analysis of Piers and materials in relation to X force at Y distance will approximately equate to Z arc seconds if you would like to please send me a PM.

Hear (http://brendanmitchell.net/?p=410) is the analysis output and blog

Sorry for semi hijacking the thread but i do belive that this is relevant to the topic and will be benificial to all. If somebody from a structural/mechanical engineering back ground can go over my calculations to cross check/referance that would be great as I myself being the hater of miss information would hate to be the propergator of said information!

Brendan

[rally]

Brendan,

I appreciate your concerns about people overspending/overengineering a pier, but in my opinion a good approach would be to reinforce that the rigidity of the pier in a high end astrophotography setup or tracking rig where pointing accuracy (especially at long focal length with a small but sensitive camera) is more important than you dismiss.

For Visual it just doesnt matter at all and much the same for Planetary where best frame selection and high shutter speeds will also make it much less important.

Large astro cameras are often the biggest balance culprits where no matter what you do there will always be an out of balance or substantial change in balance situation at some point in the sky or camera rotation angle.

Its not the strength of the pier that is important its the rigidity.
Problem is, to get rigidity the cheapest and easiest way you get excessive strength as a byproduct.

If in fact that is the sort of requirement that you have to start with ?
Since many dont need it and may never need it, it is therefore not essential but its a 'horses for courses' requirement.

If the mount is not up to the grade then the mount is going to be the weakest link, but if the mount is good with PEC, Tpoint modelling and say Protrack or similar in operation then the pier and other fixtures and adapters will start to become the next weakest link producing noise and errors.

If one is never going above say an EQ6 for example then one could argue why bother and I dont disagree, but the cost difference is only pennies and you never know what you might decide to do in the future.

To a degree Tpoint can help model this out - but Tpoint does not understand or recognise camera rotation in any way whatsoever, so it cannot correct for this type of changing deflection.

I just did the calcs for a 200mm OD x 900mm concrete pier (unreinforced) and the deflection was 1.2 arc secs compared to just under 1 arc secs for the steel one.
But I cant imagine anyone doing one without at least some reo inside (reo is dirt cheap) !
I am also guessing that 32mpa concrete used in your example isnt something the average Joe is going to do at home with a wheelbarrow and shovel !

But certainly concrete is a very viable option but I am still not sure that its as cheap as salvaged steel pipe (after formwork, reo, mount adapter bolts, conduit for cable etc) and I would think much more work, although labour is free and concrete is still needed for the footing in any case, but on a different day !
I bought some pipe with 12.7mm wall thickness around 320mm OD (12.75"= 12" nominal bore) and it cost $100 per metre (smaller diameter with lesser wall thickness was much cheaper)

As far as steel pipe availability goes, either recycled gas pipe or structural salvage pipe is readily available in most capital cities in wall thicknesses from 4mm up to 16+mm and diameters from 5" to 14" nominal bore.

A large number of piers used within wooden floored observatories are longer than 900mm so wall thickness becomes more important as the height increases - ie deflection is proportional to height squared
I have calculated a few and some that I have the figures for were 1220, 1900mm and 2700mm !

Cheers

Rally

[ChrisM]

Good discussion Brendan and Rally,

Another consideration may be thermal mass. The concrete pier's mass could be up to about three times that of the steel pier (@ 6 mm wall thickness).

Chris

[bmitchell82]

All extremely valid points, and no i do not discount that precision is paramount. I would love to know how you are actually modelling the arc second errors as i see so many variables that simple calculations would only give a very rough ball park figure.

I have also stated from the start that wall thickness is 100% important, logic will dictate that without a issue paper thin walls the thing is going to flex like a bugger, thicker walls its going to hold still! Also height of the pier will change the over all design that is normal.

You are also correct in stating that the 32MPa concrete isn't your back yard bunnings derived varient :)

Im guessing that you have some form of technical training as calculating deflections of beams and sections isn't just a oh i felt like learning it kinda thing. My main issue with everything is that with a small amount of weight and lets face it 10kgs off set is a small amount of weight in the big picture when your talking about possibly a pier (even at 900mm) weighing a good 30kgs a mount head thats packing in somewhere around 30kgs and the OTA system which can be balanced to within a bees fart of perfect. how much weight is it going to take to really pull the pier over to a point of noticing? So it comes back to my initial argument in relation to OTA construction, if you have a 7kg camera system how is that going to go on the OTA flexing? Mikes new scope has some massive stiffening around the focuser and don't quote me but i think there is still a tiny bit of flex in that... I guess to figure out the total moment at the pier head its just out of balance force x distance.

My poxy eq6 is capable of landing targets back on the sensor with a meridian flip (no im not using a maxpoint, Tpoint modelling just plain old EQmod) and its only on the poxy eq6 tripod! So i believe that its more a case of polar alignment and a little bit to do with your OTA orthoganality.

Another thing people neglect when building these beasties is the footing. sure its great to drop 1m^3 down a hole that will weigh 2.4 ton, but thats a pip squeek to the soils undernieth. Whats the properties of the soil, has it been compacted is it loose is it reactive? this will move your scope more than 1 arc second!

If i was to build a pier and I am talking it will have to be for a big scope 14-18" mirror kind of size ide look at placing 3 100mm SHS's and forming a frame system, it will preform far better than any section alone!

The other thing to think about with pier design is look at Strongman mike's kit. It isn't that big and well we can all agree that he is getting very good guiding with the NJP & Newt/Starfire.

The only time i would start really thinking to beef it up more would be at really really long FL's eg F10+ but at that end of the stick seeing will be your enemy in any case.

So no im not discounting it and yes people over engineer because they don't know better, thats why most engineers are paid well because they design things that are safe, work and are economical to construct.

My offer still stands to put some real scientific knowledge together so its not just speculation and bum fluff that is just B.S and a waste of time, but real findings on real systems with real figures. Test numerous piers with the same method and come to an agreement on what is the worst possible case for loadings.

Im up for it.!:thumbsup:

[bmitchell82]

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]

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]

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 :D

[Paul Haese]

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

[bmitchell82]

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]

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/showthread.php?t=58252

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

94624 94625

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.

94626 94627

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

Cheers,
Jason.

[bmitchell82]

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 :D

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]

On the money there Marki! ;):thumbsup: