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Marples - what do you mean you wish Bolton was here - first time I've ever heard anything like that out of your mouth (usually its - damn Bolton is here).
This turned into a tome do dont bore yourself reading from here on if you arent interested - musing of an old Engineer on piers.
I do note that you have an accomplished Structural Engineer in the discussion (Gday Mr Piggo) and probably others. I agree with Troy that the variables involved make it a very complicated calcultation. Troy and I (and probably others - sry I only know Troy) have finite element packages on our computers in front of us that could model it quite well IF we had time. And as I have said before out at the farm it is no good modelling structiral elements unless I know the loads I going to put on it. What scope - how big - weight - GEM or fork (sry if you've discussed this but im at wrok and dont have time to read all the posts - I have to pretend im intersted in undergrads). i'll read them all tonight.
I agree with Bojan that diffeential flexure is the problem - If I have a scope on one side of the mount on say a GEM at a max distance from the mount (ie max moment at the top of the piers) and im taking a 2 hour image the distance the load is from the centre of the pier is not going to change much in 2 hrs - ok there will be slight difference in distance (a bit like the PDelta effect on columns in reverse Troy I suppose) at least based on my empirical experience. If it is a fork mount there is going to be next to no change in moment. In fact thinking about a correctly balanced frok mount should have bugger all moment at the base or top of a pier.
Anyway when I get time and if you give me the dimensions of the piers you want to compare (leave out gussets Pete - I think I know the answer already and I suspect you will be surprised how low the defelctions are). I will do a simple static analysis like - the pier is sticking out horizontal from the wall with a moment at the end and you can compare deflection of conc and steel like the thread says (and this cantiliver model will be far worse than a real world model) - I do think however at the loads I am thinking about - say a 12 inch Meade on GEM (forget about forks it wont happen) the differential deflection and thus differential flexure is going to be anything at all.
I have a 4 inch water pipe with top and btm plates bolted to the floor of my observatory with no conc pier or anything under it - I have a Astro Physics 5 inch, A Tekevue 4 inch and an Orioin ED80 with 4 large skywacther counterweights, motorised filter wheels, CCD cameras etc etc hanging off it and I get no (ie zero) differential flexure over periods of up to 4 hours of imaging the same object. I do realise that others have far more load however but how much are wetalking about -as much as you weigh maybe sry Pete - no joke here.
Imagine your pier solidly bolted to a wall - now stand on the end of it and measure the height from the floor - note that the moment you induce in the pier from this will be far more than it will ever get from a scope (unless you have a 36 inch RC or something on a GEM) - now decrease the moment by stepping say 300 mm closer to the wall and measure again - the difference in these 2 final measurements is what is important- it wil be extremely low and tranforming this change in deflection to arc seconds in a 'normal' focal length telescope (say 2.5 to 3 m focal length) is what we are talking about. Remember too the pixel scale of your equipment (2.5 to 3 metre f/l) when comparing this - is your set up capable of imaging at 1 or 1.5 arc second resolution? maybe but well thats another variable.
I think we as astonomers get a little carried away in design - is bigger better - yep definately but I have never seen pier flexure causing problems in a normal amateur observatory - I dont doubt it has occurred - just i've never seen in it 30 years. What I see more often is flexure induced by overloading (or incorrect side by side loading ) of GEMs.
My 2c anyway. Im sure some will disagree but I can only comment on what i've seen. If i'm off topic - well im old and I spend my days rambling incoherently.
M
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