what's the latest thinking on filling a steel pier with sand to dampen vibrations? I can't seem to find a clear answer. The pier in question is 1.2 m tall, ~800mm diameter, bolted to a 600 mm x 600 mm reinforced concrete plug sitting on bedrock.
I take it that its 80mm in diameter ? , also what is the wall thickness ? anything over 6mm would be good enough without sand but filling it wont hurt .
Just be sure its washed and very dry before you put it in as rust will happen inside very quickly if its salty or damp , I have done a few and its a good idea to fill to within 25mm of the top and put a 10-15mm thick layer of silicon rubber on the sand to stop any water getting in after its set up ( dew or condensation ).
How is it bolted to the base ? , dyna bolts ( good ) , cam sets ( better ) or rods set into the base ? ( best ).
Will you be putting some grout between the base and bottom mounting plate ? .
What mount and scope are going on this pier ? .
Sorry for all the questions but the sum of the whole makes for a good solid pier .
Here is one I made and fitted for a friend here in Perth for his observatory , its 100nb ( 114mm o/d ) x8mm wall thickness and 1100mm tall , its not filled with sand and is camset down with 4 off 16mm x200 threaded rods , it does not move !! , solid !.
yeh, 800 mm diameter doesn't make much sense, does it...? Sorry, I meant to write 300 mm, but if you want to be specific:
273 mm OD, 9.5 mm wall thickness, 1.2 m tall, ~15 mm flanges top and bottom.
Chemset bolts were used to secure it to the plug. (I wanted threaded rods concreted in, but the concreter talked me out of it...) Still, it's pretty solid.
It will be used to support a Paramount ME, holding a Meade 12" SCT.
I'll try and attach a photo of the pier. The plug is isolated from the surrounding 100 mm thick (200 mm at the edges) slab that will support the dome.
We will be grouting the gap between the bottom flange and the plug.
Nice , if it was mine and holding the MX and 12 inch Meade I would fill it as those 2 together are HEAVY !!! , the 'solidity' wont be a problem as I see it but harmonics might be , being that the interior volume is large so I would sand fill it , better be safe than sorry .
Another thread ages ago also recommended using a sand/sump oil slurry as the fill, as the oil creates a good mechanical join to the pipe, and thus helps transmit energy out of the pipe to the slurry much faster.
ie Same theory as using heatsink paste to assist temperature transfer between 2 ( almost) flat surfaces, but in this case its helping the mechanical energy transfer.
Never tried it tho, as my pier is solid concrete.
If you go for the oil/sand mix, don't put the sand in first and try pouring oil on it. It won't sink in and will make a huge mess. Mix it up in small batches and pour the mix in, taking time to stamp the sand down. Then you can adjust the mix too so you don't get it too slushy or dry.
Adding the sand adds mass which lowers the natural frequency of your pier.
For a given vibration energy, a high frequency vibration has a smaller amplitude than a low frequency vibration. Adding the sand gives a larger amplitude of vibration than without it.
There may be anecdotal evidence that sand filling helps to dampen the vibration more quickly, but that's of little value if the first movement is enough to blur your image anyway.
When I did the calcs for my pier (100SHS about 900 tall) the amplitude of vibration increased about 4.5 to 5 times over the empty pier, so it was clear to me it's an old wives tale.
I know there are experts here who know more about engineering than a professional engineer does... so I wish you luck deciding.
Al.
Last edited by sheeny; 18-06-2016 at 02:43 PM.
Reason: typo
on the contrary, I've always held engineers in the highest regard, so your opinion is taken seriously.
What you say makes sense: a larger mass will dampen more quickly: as I understand it, the frequency drops inversely with increasing mass (f = sqrt (k/m)).
I suppose it depends on what is likely to happen in the dome when using the telescope. When I think about it, since this will be a teaching facility for one or two students at a time, the chance of knocking the pier while operating the telescope is pretty remote.
I'm leaning towards an empty pier at this stage, one that can be retrofilled if I can be convinced it's worth it.
Adding the sand adds mass which lowers the natural frequency of your pier.
For a given vibration energy, a high frequency vibration has a smaller amplitude than a low frequency vibration. Adding the sand gives a larger amplitude of vibration than without it.
There may be anecdotal evidence that sand filling helps to dampen the vibration more quickly, but that's of little value if the first movement is enough to blur your image anyway.
When I did the calcs for my pier (100SHS about 900 tall) the amplitude of vibration increased about 4.5 to 5 times over the empty pier, so it was clear to me it's an old wives tale.
I know there are experts here who know more about engineering than a professional engineer does... so I wish you luck deciding.
Al.
G'day Al,
Would filling the pier with grout make the whole thing more rigid and less prone to vibration in the first place? I understand that anything will vibrate if you hit it hard enough or fast enough, but short of hitting it with a truck I'd imaging the pier in question would be pretty solid.
I've been entertaining the idea of building a pier for some time now and couldn't resist the opportunity to pick an engineers brain. Sorry to the OP for chiming in with more Q's.
Gday Geoff
Im also a mech engineer, and vibration analysis is not simple.
The strength and rigidity of the pier itself comes into it as well as the mass.
Just think of 2 tuning forks, one made of tuning fork steel and the other mild steel. Both may be almost identical in mass and shape, but will vibrate and damp very differently.
With a 273mm dia pier with 9.5 walls, you arent going to see much deflection in the pier under any normal circumstances, and as Al mentioned, if the vibration is from a "hit" then nothing will help.
As noted the idea of oil was to speed up the transfer of vibrational energy from the steel to the slurry, not to control the absolute amplitude or resonant freq.
Would filling the pier with grout make the whole thing more rigid and less prone to vibration in the first place? I understand that anything will vibrate if you hit it hard enough or fast enough, but short of hitting it with a truck I'd imaging the pier in question would be pretty solid.
I've been entertaining the idea of building a pier for some time now and couldn't resist the opportunity to pick an engineers brain. Sorry to the OP for chiming in with more Q's.
It's not worth it. The stiffness of the pier is proportional to the cube of the diameter, so you are adding a lot of mass and making very little impression on the stiffness. Keep in mind that steel is typically 10x stiffer than concrete (and any cementitious grout - I haven't done the calcs to epoxies, but I can't imaging them being anywhere near as stiff as steel). So you are getting the maximum stiffness by having the outside of the pier in steel, and are better off with nothing inside to minimise mass, and maximise the natural frequency.
I did the calcs some years ago, which will be in a thread here somewhere, to compare the stiffness of steel and concrete piers. Concrete has to be significantly larger in diameter to achieve the same stiffness, but it then has a significantly higher mass, so it's amplitude of vibration will be higher.
I'm beginning to think the solution is behavioural rather than mechanical: don't touch the telescope or pier when in use. If the decision comes down to such fine points - and the engineers seem to agree here - then I'll go with instruction rather than modification of an already fine pier.
Thats a given,
but remember, you can also get motor vibrations from the mount itself, or ground vibrations from yourself, nearby roads/rail etc.
Random vibrations wont ( normally ) cause resonances, but a constant motor speed might ( highly unlikely but possible ).
ie To properly design for vibration, you need to look at damping vibrations as quick as possible ( which is what the oil slurry is supposed to do ), or preventing vibrations from causing a resonance, and for the latter, you need to know the frequency of the resonator. Again not simple to do properly.
thanks so much for your input. I'm curious just what your final word would be. The site is quiet (Mount Stromlo) with the only movement being grazing kangaroos. The plug is isolated from the concrete slab by a cylinder of rubber down to a depth of 100mm. From there, both the plug and the slab are in contact with the ground. The pier is held down by six chemset bolts. The mount is a Paramount ME, so I'm assuming it's pretty good quality and vibration free. Movement in the dome will also be dampened by marine grade carpet/rubber backed. Do you see, in your opinion as an engineer, the need to fill the pier with sand? I can always retrofit this thing if needed, but it's going to be impossible to take out if I change my mind. The decision is an important one since it is entirely for public (high school/college) education, and hence there is a severe pressure to do a lot with a very limited budget. In other words, I need to get it right.
I'm curious just what your final word would be.
...
Do you see, in your opinion as an engineer, the need to fill the pier with sand?
Based on gut feel, i would probably use sand oil if i had it handy, but wouldnt worry if i didnt. There are plenty of empty steel piers out there that work perfectly well :-)
Also, based on the size of your pier, i dont think there would be much difference between the methods, as it is already massively stronger than required. The only thing that will hurt it is an accidental bump, but people normally bump the mount/CW shaft or OTA, not the pier, and as such the pier "strength/damping" is pretty much irrelevant in that scenario.
The inner geek in me has always thought it would be fun to stick a series of accelerometers onto a mount, and see what really happens, ( esp when manually focussing in visual mode ) but i haven't got up the energy to do it yet.
Quote:
In other words, I need to get it right.
Either method sounds perfectly good for what you describe.
It's not worth it. The stiffness of the pier is proportional to the cube of the diameter, so you are adding a lot of mass and making very little impression on the stiffness. Keep in mind that steel is typically 10x stiffer than concrete (and any cementitious grout - I haven't done the calcs to epoxies, but I can't imaging them being anywhere near as stiff as steel). So you are getting the maximum stiffness by having the outside of the pier in steel, and are better off with nothing inside to minimise mass, and maximise the natural frequency.
I did the calcs some years ago, which will be in a thread here somewhere, to compare the stiffness of steel and concrete piers. Concrete has to be significantly larger in diameter to achieve the same stiffness, but it then has a significantly higher mass, so it's amplitude of vibration will be higher.
I take it that its 80mm in diameter ? , also what is the wall thickness ? anything over 6mm would be good enough without sand but filling it wont hurt . 
.
Nice , if it was mine and holding the MX and 12 inch Meade I would fill it as those 2 together are HEAVY !!! , the 'solidity' wont be a problem as I see it but harmonics might be , being that the interior volume is large so I would sand fill it , better be safe than sorry .
