Vibrations? Really??

[fatwoul]

Last night, I noticed something a little disheartening. I was on my recently-finished deck, and I noticed that if I stamped my feet, I could see the Moon vibrate in the eyepiece.

I have to take the deckboards up to install power at some point, so I can check that nothing is touching the concrete base of my pier.

Admittedly you have to stamp to create any effect, and once I'm imaging I'll probably be doing it remotely, but still, now I'm worried that my half-tonne concrete block should have been a tonne afterall.

[barkis]

Seems odd. How is the pier attached to the concrete pad. Is it a metal one and hollow. How are the boards supported surrounding the pier itself.?

Just trying to get some clues.

Ron.

[Ad Astra]

If your pier is truly isolated from the deck, this should not happen. I think you would have to stomp awfully hard to transmit vibration through the wood deck - down into the soil - and then up through the pier.

I would check to see if there isn't some point of contact (maybe you have to stamp to make contact???). From an engineering standpoint - this seems more plausible.

Dan

[fatwoul]

I'm taking the deck boards up to check for contact, but the effect also happens if I stamp on the patio, ten feet from the pier. The only point of connection is one leg of the deck, which stands on one corner of the patio. So this suggests to me that the deck may be touching the pier somewhere.

But I know not where...

[Ad Astra]

This sounds very strange.... how deep does the pier footing go into the ground? If it goes down sufficiently far it should be massive enough and isolated enough to solve the problem.

Do you have really hard and rocky soil? That could transmit more vibration.

Is your ground frozen now? A deep layer of frost or ice would, in essence, lock pier, deck, and ground together in one unit. THAT could be a problem!

Sorry I can't be more help without coming over to your place.... (it's a long drive from where I live! )

Dan

[fatwoul]

Hi Dan,

The concrete base wasn't as big as it should have been. It is 20 inches square, and just under three feet deep, but protrudes above the ground making it up to the full 36 inches.

I worked this out to be about half a tonne (500.72Kg, in fact), and I know a lot of pier bases are over a tonne, but considering the scale of telescope that's going on it (the largest will only be my 7.5" Mak-Newt), not to mention that this might not even be the final resting place of the observatory, I decided that should be enough. The pier is also filled with 40Kg of sand.

To be honest, I'm not overly concerned. I don't make a habit of stomping around the patio or deck, and I'm installing fatigue flooring into the obsy when it arrives, which will soften normal footsteps (which aren't noticeable in observing anyway). Once the dome is all set up, I'll be operating it from indoors a lot anyway, so there won't be any footsteps to upset it anyway.

It's just not something I expected, and is a bit of a niggling puzzle as to what causes it.

[Ad Astra]

A 500 kg base should certainly be stable (I'd hate to stub my toe on the darned thing!).

I think the vibration absorbing flooring is brilliant, and should do wonders for you. It is all about absorbing the energy of footfall impact before it gets transmitted to the optical tube, isn't it!

I'm not sure, but I would bet that this is less of a problem when the weather gets warmer and softens the ground some.

Dan

[fatwoul]

A 500 kg base should certainly be stable (I'd hate to stub my toe on the darned thing!).

I think the vibration absorbing flooring is brilliant, and should do wonders for you. It is all about absorbing the energy of footfall impact before it gets transmitted to the optical tube, isn't it!

I'm not sure, but I would bet that this is less of a problem when the weather gets warmer and softens the ground some.

Dan

Cheers Dan, I hope you're right. Funny you should say that about the soil. I was talking to my dad this afternoon, and he said a similar thing; all the soil under the deck is going to dry out, mostly, now that it is covered permanently, and he reckons that as all that volume of water evaporates away, the remaining, drier soil might not transmit vibrations as much. If you/he are right, then once the soil has dried, it should stay pretty dry for the foreseeable.

Of course, that assumes the vibrations are coming from the soil, and not some direct point of contact with the pier.

I've got a few weeks before the POD arrives (I hope it won't be longer!), and I need to install my power cables yet. Weather permitting, I'm going to take up the deck boards around the pier, and check for contact.

Fingers crossed!

[Ad Astra]

You are one lucky lad to have such a setup at home in the back garden, so to speak!

I'm sure your Dad and I are both right (I can tell the old gentleman is a genius... even from here! ) Of course, you could tear in and find that sneaky point of contact and prove us both wrong!

You must post a photo or two as the project comes along, eh?

Good luck!

Dan

[fatwoul]

You are one lucky lad to have such a setup at home in the back garden, so to speak!...

Well, I've had a while to save up for it...I'm 33.

I've already got a thread about it, with some photos. Here it is.

[Ad Astra]

Had a look at the photos - it looks brilliant.

I mostly set up on the gravel in the back garden - nothing permanent!

Dan

[Psychobilly]

What was the ground like at the bottom of the hole? My obs sits on "built" up ground so as well as the deep hole (which I made bell shaped) I drove long lenght of 3" by 1/4" thick angle iron into the bottom of the hole at an angle before casting the foundations (about 750 kg's) around them...

hope you find out whats causing it... I'd leave a wide gap between the decking and the pier base and if you want to fill it with anything use foam pipe insulation...

The foam tile anti fatigue flooring should help a lot and also make the floor more friendly if you happen to drop something...

Billy...