Concrete vs. metal pier

[pmrid (Peter)]

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/...e/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 (Peter)]

Originally Posted by 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/...e/sunrise.html

Peter

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 (Steve)]

Originally Posted by 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.

after reading it a few times 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 (Peter)]

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 (James)]

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.

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 (Brent)]

Alignment done ..
Got a break in the clouds

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

[steve000 (Steve)]

Originally Posted by ZeroID

Alignment done ..
Got a break in the clouds

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

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 (Peter)]

Originally Posted by 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

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 (Steve)]

Originally Posted by steve000

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

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

[Helo (Peter)]

Originally Posted by 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

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 (David)]

That is very elegant solution indeed. Many thanks

Originally Posted by 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.

[bmitchell82 (Brendan)]

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 (Brent)]

Originally Posted by 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

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

Originally Posted by bmitchell82

. . .
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.

[bmitchell82 (Brendan)]

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 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 (Brendan)]

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.!