Pier - sand-filled?

[brian nordstrom (As avatar)]

Geoff , what you have is as good as it gets , set it up , use it and enjoy , exactly as AndrewJ say's .
Brian.

ps. I forgot to mention this pier is cammie'd onto a slab about your size and weight , you just cant see it ,,, it's under the carpet .

Originally Posted by Geoff Mc

Hi Andrew,

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.

Cheers,

Geoff Mc

[GeoffMc (Geoff)]

Thanks Brian. Yes, I think I'll leave it hollow. I do like the idea of adding sensors and checking out the facts, however! Unfortunately, like everyone else, it doesn't warrant my time as there's so much else to be done.

Is that a Sirius dome in the background? I'm using a 3.5 m Sirius for this project. How have you found yours?

Cheers,

Geoff Mc

[brian nordstrom (As avatar)]

Yes , Sirius dome , these are great , ain't they .
I would love one in my future .

Brian.

Originally Posted by Geoff Mc

Thanks Brian. Yes, I think I'll leave it hollow. I do like the idea of adding sensors and checking out the facts, however! Unfortunately, like everyone else, it doesn't warrant my time as there's so much else to be done.

Is that a Sirius dome in the background? I'm using a 3.5 m Sirius for this project. How have you found yours?

Cheers,

Geoff Mc

[sheeny (Al)]

Originally Posted by Geoff Mc

I'm beginning to think the solution is behavioural rather than mechanical: don't touch the telescope or pier when in use.

Absolutely.

Al.

[ChrisM]

Geoff,

I have a 11" dia x 6mm wall thickness steel pier, and kept it hollow. As Sheeny said, you want to increase the natural frequency - not decrease it by adding more mass.

See http://www.iceinspace.com.au/forum/showthread.php?t=111597&page=2
for some excellent info on tapered stiffeners for your pier. They work by spreading the vibration energy across multiple frequencies thus reducing the amplitude.

Cheers, Chris

[GeoffMc (Geoff)]

Thanks Chris, I've just read all the exchanges (and will also watch the videos). Some amazing views expressed here by smart people.

Cheers,

Geoff Mc

[speach (Simon)]

what I've read on piers there is no advantage in having it filled there may be a small decrease in it's efficacy.

[AndrewJ]

Gday Chris

They work by spreading the vibration energy across multiple frequencies thus reducing the amplitude.

Thats assuming something is trying to "continually" vibrate the pier, and you are trying to reduce "possible resonance".
If you are just trying to kill off random vibrations quickly, the initial deflection will normally be as a result of a single knock, and will be as big as it gets. ( and lets face it, will be microscopic for a pier of this size )
The effect on damping time is what you are really looking at, as with piers the size we are discussing here, the pier is going to be the smallest source of error. The pier will deflect the same for a given impulse load whether filled with sand or not, but the decay times will be different.
All you need to do is the numbers, to see if the difference is measurable.

Andrew

And as a secondary example, if extra weight / damping is always "bad", why do many tall buildings/structures use massive internal weights as dampers????
https://en.wikipedia.org/wiki/Tuned_mass_damper
The caveat here is the weight and its position has to be designed to suit the structure, but extra mass isnt always bad, and doesnt always increase the resonant amplitudes ;-)
Whilst this doesnt match the example of a pier, it does highlight that you have to design for each case on its merits.
Whilst all good theoretical fun, this is still massive overthink for a pier :-)