Piers - how much overkill

[Lee]

I'm planning on a roll-off observatory soon, with a steel pier for the G-11. Looking on cloudynights' pier design article, gives specs for piers that look like an elephant could lean on it whilst imaging at 40 metres f/l and not get a trail....

After some basic investigations I'm thinking of a pier around a metre long, made of 168mm OD steel pipe (4mm thick) welded to a plate of steel (prob 10mm thick). Should I need gussets at the base? Do I really need bigger than this?

Any DIYers have any recommendations/cautions????

[jase]

Piers being permanent things should allow you to mount your current and future astronomy/astrophotography requirements. How big you make it depends on your intentions. Generally speaking, bigger is better, but you also need to take into consideration length - a GEM with a long focal length refractor will require more length than a fork mounted SCT for example. There were some discussions that an all concrete pier will give off thermal heat well into the night. Make the base from concrete, but would suggest you stick to steel for the pier itself. Would recommend gussets, though their effectiveness depends on pier height, steel thickness and OD. J-bolts sunk into wet concrete are great for attaching the metal base of the pier. Easy to get the mount perfectly level by adjusting the nuts. Sand in the pier can also assist in weight and has the added benefit of being a vibration suppressant.

How heavy do you expect your OTA, mount, eyepieces, imaging train will get in the next five or more years?

[74tuc]

Hello Lee,

Can't say much about the suitability of the pier for your application but if you are going to bolt it to the footing you must use large gussets between the base plate and the pier. The main stresses are at the base joint. A lot of bolted on piers do not have adequate sized gussets. Having built a few piers out of different materials I would suggest that the base plate be a disc shape 340mm dia 12.5mm steel and the gussets 80mm at the base extending to a height of 320mm. The base plate being fixed to the footing with 8 x 12.5mm dyna bolts(or chem sets). 8 bolts ensure even forces round the base.

Common falacy is to fill the pier with sand - this is of very little use as the sand has no strength and will do little to damp the low frequency (mode 1) vibrations.

Jerry.:)

[g__day]

Definitely add gussets - the bigger the better. My pier was about 1.3m tall 140 mm diameter 5mm steel on a 45cm square base - all mig welded and bolted to concreted into a 160 kg lump of cement. Surprisingly this had alot of vibration compared to the tripod that came with the CG5. Filling the pier with sand didn't help either so I welded four 10cm by 8mm 1.1 metre tall gussets - now I have zero vibration. It was chalk and cheese once the gussets were added.

[jase]

Agree, a pier filled with sand will only help with high frequency vibrations. Even then something is better than nothing... :)

[Lee]

Gussets it is then.....
Unsure of total weight it may be expected to hold, the largest scope I can imagine myself mounting on it would be a 11-12" SCT I suppose..... For now it will be ED80/G-11 and likely an f/4 Newt in the near future....

[rogerg]

I'll happily admit mine is overkill, it's probably like you mentioned, not only able to hold up an elephant but 4 cars with it. High frequency vibrations was a problem for me, so I filled it with sand to stop it ringing like a bell. I've always thought just a solid concrete pier with reinforcing would be better because of that but duno for sure.

Bigger is better, I'm glad I made it so oversized. Besides, the difference in cost between big and small is not much compared to the overall cost of having someone weld it up and getting the thing stuck in the ground.

Roger.

[Lee]

Your pier is big.....

Now about levelling..... there seems to be two ways - at the bottom plate or a separate levelling top plate.
Doing it via the bottom plate seems much easier to do, but also seems less stable - is it less stable???

[leon]

Same here roger, i have two of the buggers, and, yes i think mine are over kill, they can each carry a dead horse.

Cheers Leon

[Striker]

I'm with Roger..I too have an Pier with overkill but who knows what I plan to mount on it over the years.

[jase]

Tony, that is one mean looking pier you've got. Polish chrome looks fantastic. Could have mistaken it for a SCUD missile.;) Perhap put a NASA sticker vertically down the side. :lol: :thumbsup:

[74tuc]

A couple of tips about piers:

1. The stiffness of the pier varies as the fourth power of the diameter - doubling the diameter increases the stiffness by 16 times ... "diameter is your best friend". ... small increase in diameter gives big gains in stiffness.

2. Most piers will carry huge loads but that is not what one is after. What is wanted is small deflections for a given force applied to the mount - I try to design for a deflection of 0.01mm (at the objective) for a force of 2 KG against the mount - this corresponds to about 1 - 2 arc sec movement.

3. Rule of thumb for short piers (< 1.5 M) make the diameter of the pier equal the diameter of the 'scope dew shield.

Jerry.:)

Re: "So I welded four 10cm by 8mm 1.1 metre tall gussets "

Excellent!! - I like your idea.

[g__day]

If you're going to add gussets, why mess around with piddly things? Go overkill and the amount of vibration supression you'll get is amazing.

PS

Rather than fill it with sand, why not fill it with concrete? Surely that would be the best of all worlds?

[Shawn]

Its here in Micks thread http://www.iceinspace.com.au/forum/showthread.php?t=14820

It bolts to a concrete block thats 900 high, same hight as the false floor in the OBs, hope it helps

S