New Raised Observatory. Flex problem.

[PKay]

I may have a problem, but not sure.
Advice or thoughts welcome.


I have had to go up to help clear the house and treeline.

The floor, even with bracing will move when climbing the stairs, and when the walls go on wind would cause a problem.

So i installed a solid concrete plinth (250mm diameter).

The footing is 600mm deep & 400mm diameter.

It goes through the floor with a 10mm clearance.

Here is the potential problem:

At the top of the plinth, if I tap it hard with my hand it still flexes (not much < 1mm).

I then went to the scope sitting on the EQ8 and tapped it with my hand, and yes it also moved (flexed).

Resolutions:

1: Leave it as is, I can't think of any forces that would impact the plinth.

2: Put a larger diameter pipe around the bottom section and add more concrete.

[bojan]

1)

[glend]

You have a long pier on a fairly narrow base. Did you install reo bar in the pier form before you poured it? Concrete can flex on its own and reo should be used to provide the necessary stiffness. If there is no reo in the pier you could try bracing the pier (at say 45 degrees) from under the floor to the ground with something like galvanised angle iron , but would need to work out some sort of collar design to weld the steel to. The brace anchored to a ground slab might be fine

[PKay]

Thanks for replying Bojan and Glen.

I did not put reo in the pier, didn't think it would need it.
Quite amazing just how much concrete can flex.

At this stage, I might 'suck it and see'.

If there is an issue, will work on some solution.

I don't want to put angled bracing, since part of the deal (with the other half)
was storage underneath...

Thanks once again.

[xelasnave]

Enclose bottom section in box form work with reo in the "triangular" voids and fill with concrete..you will have a nice square unit upon which you can add small shelves for pot plants...this will make the bottom acceptable...if you are serious in addition tie reo around the top section and get a mold that will fit the reo plus half an inch of concrete beyond it, as a minimum, an inch better still make the concrete a low sand to cement ratio and dry over at least a week to ensure strength...add carbon fibre/ resin overlay to a depth of 3 mm. Personally I would remove all the current unit and saw it up in different lengths so as to make a small sculpture for the garden which ideally should include a pond and a fountain..then rebuild the flexing unit using reo welded in cylindrical fashion with at least three individual cylinders each joined of course then add concrete with a decent amount of blue metal.layer the outter surface with carbon fibre to a thickness of 5 mm..this structure should resist determined blows with a small sledge hammer...better still fill that room below entirely with concrete..that should do it.
Alex

[h0ughy]

Oh no reo, that's not good. Concrete is exceptionally excellent in compression but in tension it will fail. With no reo in your pier it will crack and possibly snap off with the weight of the Mount on top . The steel would have provided the tensile force to keep the column intact. Sorry but I think you should take it down and redo it properly. If that snapped off and fell on you.... If it were my wife she would just dig another hole ;)

You need a caisson cage welded up and made integral in the footings. Your footings will need to be bigger and we'll to support the column weight and leverage. :question:

[Nikolas]

Definitely need a wider footing at the base Even a pillar of sleepers filled with sand will add sturdiness at the base.

[Startrek]

Peter
I agree with David
That pier is way to high without a reo cage or reo rods all the way into the footing encasement. There is no tensile strength in concrete without steel reo especially in that configuration
My piers are only 500mm high out of the ground ( 400mm in the ground ) and 250mm in diameter and I used 6 x M12 reo rods through the pier and into the footings

I know it’s painful, but I would pull it down and start again mate

Also at that height I would use a 300mm diameter pier as a minimum plus a 450 x 450 concrete plinth with reo into the footings , around the base up at least 500mm

Then it ain’t going anywhere !!

Cheers
Martin

[xelasnave]

A rebuild is the sensible thing to do...easier than trying to do a after fix..what is below ground ?
Alex

[bojan]

Guys..
What will be the load on that pier?
100kg max (mount + scope).
No variable side forces, no vibrations.. Floor is isolated so no transmission of vibration to Pier. No freezing of the ground during winter...

As I understand, there will be some sort of housing above (shed)? To prevent wind from affecting the structure during imaging (bars would not help against this anyway)
The only concern may be moving of the whole structure before is settles (after year or two).

Do not over-engineer things...

I have similar structure in the back of my garage in Mt Pleasant.. the hole is 75cm below the top concrete surface, pier is 2.2 m high protruding into the study above the garage. Visibly it didn't move more than 0.5 mm for the year. True, we put 4 steel bars just in case, but I always thought it was overkill.
Yes, when I bump the pier it vibrates for some time.. I don't think I will be doing this while imaging or observing.

Anyway.. see how it goes as is. You can always do something if problem develops.


EDIT: No variable side forces.

[Sunfish]

I agree with Bojan. The movement when you tap is vibration as the pier is long , not flex. To reduce vibration you could add narrow rocket fins or steel plate gussets .at the bottom which would not strictly break the visual code. Or just don’t wack the pier .

[AstroJunk]

Agreed with others.

My sand filled monster pier attached to a tonne of concrete would still vibrate when I thumped it. Fortunately, Thumping the scope hasn't made it to my astrophotography workflow yet...

https://www.youtube.com/watch?v=ArbNmPd_lvI

[PKay]

Firstly I would like to thank you all for commenting.

I did consider everything said, and then chose to phone a friend who happens to be an architect that specialises in very large buildings (high rise, shopping centres).

He came and had a look, tapped and shook the beam and thought about it.

Not a problem, it will work just fine.

Even reo would not stop the movement, since that mostly occurs in the ground.

He called it a 'vertical cantilever', and 3 braces at 45 degrees is the only way to stop any movement. But in this case, unnecessary, since there will be no forces to cause said movement.

The thick walled PVC pipe filled with concrete forms a beam structure in itself, even without reo, and is actually very strong.

So in conclusion, unless a problem becomes evident, will leave it as is.

I will post again after the walls and roof are on.

Once again, thanks heaps.

[bojan]

:thumbsup:
Most likely you will have to check and correct polar alignment after a year or two, when it settles.

[ericwbenson]

No side forces: not true, a GEM or fork mount almost certainly induces a bending moment on the pier. The OTA and counterweights are balanced on the mount of course, however that balance point is usually not over the center of the pier, especially at lower latitudes like in Australia. The fix for that is an offset plate, I used that in my dome but only because of the limited clearance of the CDK20 in the 3m dome.



No vibrations: rare, if the scope is looking at the sky the wind can shake it. Very high ROR walls can reduce it, but that makes the whole building somewhat over engineered (as opposed to just over engineering the pier). A dome can only keep the wind at bay up to 50% of the time!


Regards,

EB

[bojan]

Well, I should have written: "No variable side forces".
Corrected.

[xelasnave]

Like I said earlier it will be fine.
Alex