Completely botched pier rebuild

[jetstream]

49 minutes ago, vlaiv said:

was offered hollow / lattice type steel pier as alternative - something like this:

👎

[900SL]

Ok quick estimate deflection of 200 square 6mm thick cantilever approx 0.15mm under 50kg horizontal force. 

My software tomorrow will work out natural frequency mode, but I'd go to 10mm thick and fill it with concrete for damping and high stiffness

[900SL]

Base connection is very important, how will you connect?

[vlaiv]

1 minute ago, 900SL said:

Base connection is very important, how will you connect?

I was thinking of drilling 4 holes in base slab (which is rather big and heavy piece of reinforced concrete - about 1x1x1 in meters) and putting M16 threaded rods in those and using some sort of chemical anchoring agent (there are several options, I'll ask for the best). I think I can easily get at least 20-30cm deep holes for those.

50-60cm square base plate cut out of 10mm steel and pipe welded to that with triangular fin supports at corners.

6 minutes ago, 900SL said:

Ok quick estimate deflection of 200 square 6mm thick cantilever approx 0.15mm under 50kg horizontal force. 

That is way too much. Even if we reduce load by factor of x100 to only 0.5Kg, we will still have 0.0015mm of deflection.

If pier height is 1600mm, then angle of deflection is 0.2"

Well, that is not bad - but that is only 0.5Kg of load.

How much is wind load on 0.2 meters squared?

According to this 6m/s wind speed

https://www.engineeringtoolbox.com/wind-load-d_1775.html

produces 4.32N of load which is equal to 0.44Kg of load.

Ok, I'm sold - that sort of deflection in direct 6m/s wind is acceptable for pier stability.

[900SL]

I think 6mm is too thin Vlaiv. Can you get 10 or 12 mm?

 

 

[vlaiv]

2 minutes ago, 900SL said:

I think 6mm is too thin Vlaiv. Can you get 10 or 12 mm?

 

 

No unless I weld them myself (or someone does it for me) - but I'd rather not mess with that.

200x200mm is largest cross section and 6mm is thickest walls that I have (easy) access to.

According to this calculator :

https://www.engineersedge.com/calculators/section_square_case_4.htm

difference is not that big - less than double deflection between 6mm and 10mm walls

[900SL]

OK can you fill with concrete after fixing base bolts? It will help damping and reduce any vibration

[vlaiv]

Just now, 900SL said:

OK can you fill with concrete after fixing base bolts? It will help damping and reduce any vibration

Could do that, but is it really necessary?

I've heard people using sand for that, and some think it is good idea and some think it's not necessary as frequency is too high.

[vlaiv]

Ok, I re did calculation again from scratch and it checks out.

I got 0.166" for 4.35N load on 200x200mm steel pipe with 6mm thick walls

[ollypenrice]

I think lots of discussion of piers is over complicated.  For around ten years one of my Mesu 200 mounts has been standing on a pretty light weight Mesu steel pier. It has a large diameter main tube made of thin steel and then, at the bottom, has tripod feet which are just threaded bolts with plastic pads at the bottom. I drilled the concrete so I could trap these feet onto the ground with strips of wood just to stop an accidental kick from moving the pier. There is nothing massive or 'industrial' about this setup but it has never been touched or adjusted in all this time. Imaging with dual TEC 140 or dual Tak 106 or 14 inch ODK between 0.6"PP and 3.5"PP, it has never dropped a single sub exposure to tracking error. Seriously.

So if I were you, vlaiv, I would just demolish the failed concrete pier and put a steel one in its place. It would just need three widely spaced feet to miss the hole where the failed one was.

(I also use a really horrible steel pier for my other Mesu mount. 😁  It's one I welded up quickly to carry a cheap visual refractor. When I had the chance to buy a second Mesu without a pier I quickly cut the top of this pier off and welded up a load of steel scrap I had lying around to mount the Mesu on it. My intention was to make a proper one later. However, the horrible pier worked fine so... I still use it!)

I believe many pier designs are solutions looking for a problem.

Olly

 

[Peter Drew]

I would fit angled struts of T section steel from just below the top of the pier down to the concrete base.  This would remove the deflection and vibration issues.  This would suppose a concrete base with suitable constructional integrity.     🙂

[vlaiv]

6 minutes ago, Peter Drew said:

I would fit angled struts of T section steel from just below the top of the pier down to the concrete base.  This would remove the deflection and vibration issues.  This would suppose a concrete base with suitable constructional integrity.     🙂

I have no idea what that is .

T section steel is clear but not sure what would "angled strut" made out of it going from just below top of the pier down to concrete base - look like

[Peter Drew]

A bit like a fixed tripod Vlaiv.    🙂

[900SL]

Like a panther mount. Extra strutting

[Rusted]

Nature abhors a straight pillar. Demand they remove their atrocity immediately.

Nature uses taper to safely carry vast wind loads into their root ball foundations.
Triangulation uses exactly the same principles with much lower weight and material cost.

The Panther and other triangulated piers, avoid the weak spot at the ground.
The triangular braces [legs] bypass the pier itself.
Though most have the severe handicap of needing to be light enough to carry.

You don't have that need. Use your 8x8 pipe and fix triangular "legs" as large as you can live with.
No thin sheet at the base. You don't need it.
Welded "tabs" and steel strip, or pipe, for your triangles will exceed your wildest dreams.
I prefer four triangular feet to three.
The radius to the tipping line, between any two, of four feet, is huge compared to three.

Do a google image search for "telescope pier" for inspiration.

 

Edited November 22, 2021 by Rusted
typo

[vlaiv]

If I'm reading this right - everyone is suggesting to go with tubes instead of fins at the bottom?

Like this:

and not like this:

[900SL]

https://www.cloudynights.com/articles/cat/articles/how-to/pier-design-fundamentals-r1236

[tooth_dr]

15 hours ago, ollypenrice said:

So if I were you, vlaiv, I would just demolish the failed concrete pier and put a steel one in its place.

 

I think this is the best advice.

3 hours ago, vlaiv said:

If I'm reading this right - everyone is suggesting to go with tubes instead of fins at the bottom?

Like this:

and not like this:

I have a finned version, made by a friend to my specifications.  I can see how your first design failed as there was very little to adhere the concrete too.  I have used 10mm pipe with 8mm flanges.  I think you are over theorising it 

[Rusted]

The finned tube design needs bolting down but has a much smaller footprint. Nothing to trip over in the dark.
If you have access to a scrap metal merchant you may be able to find a heavier base plate for a finned pier.
Many scrap metal yards have ox-acetylene cutting torches.
They may cut down a bigger plate if you pay a small sum of money.

A legged pier can stand on hard, level ground without bolting down.
However, the legs can get in the way in a small observatory.

[vlaiv]

Ok, I see where confusion comes from.

There is observing deck and pier will be anchored below it. Observing deck is at about 1 meter of the floor and has hole for pier. Pier is decoupled from the rest of the construction.

I can easily get fins below observing deck and they won't get in the way when on observing deck.

I can't however use legs at all because of that (or can below observing deck - but fins look like adding more rigidity).

[jetstream]

2 hours ago, 900SL said:

https://www.cloudynights.com/articles/cat/articles/how-to/pier-design-fundamentals-r1236

I wonder what force is required to cause the deflection listed for the sched 5 pipe? does a mounted telescope tracking cause similar deflection?

[vlaiv]

13 minutes ago, jetstream said:

I wonder what force is required to cause the deflection listed for the sched 5 pipe? does a mounted telescope tracking cause similar deflection?

We can easily calculate that if you give me exact pipe dimensions.

 

[jetstream]

@vlaivOk I have 8" x .125" x 4ft and 6" x .250 X 4' pipe in the scrap pile. My mount is an AZEq6 used with TSA120 or 90mm Raptor. Gussets needed? It will be welded to a 2x2' base 1/2" or thicker ( I have 1" plate I can use too).

Edited November 22, 2021 by jetstream

[900SL]

57 minutes ago, vlaiv said:

Ok, I see where confusion comes from.

There is observing deck and pier will be anchored below it. Observing deck is at about 1 meter of the floor and has hole for pier. Pier is decoupled from the rest of the construction.

I can easily get fins below observing deck and they won't get in the way when on observing deck.

I can't however use legs at all because of that (or can below observing deck - but fins look like adding more rigidity).

Welded fins will be fine

 

 

 

[vlaiv]

14 minutes ago, jetstream said:

@vlaivOk I have 8" x .125" x 4ft and 6" x .250 X 4' pipe in the scrap pile. My mount is an AZEq6 used with TSA120 or 90mm Raptor. Gussets needed? It will be welded to a 2x2' base 1/2" or thicker ( I have 1" plate I can use too).

So both are 4' long - one us 8" OD and wall thickness is 0.125" and other is 6" and wall thickness is 0.25"

Since they have same length and I'm guessing they are same material - it comes down to one number - that is Area Moment of Inertia for pipe. One that has this number higher - will be less prone to bend under wind load.

8" pipe has: 23.979

6" pipe has: 18.69923

so 8" is better.

We can then go on to calculate actual deflection angle for say 6m/s wind and TSA120 scope.

I'd round area cross section of that scope to 0.15m2

Load will be 3.24N

We have 4' long pipe that is 1.2192m, and above number for meters^4 instead of inches^4 for 8" pipe is 0.00001904107882336

For steel we'll take 210Gpa so 210,000,000,000Pa

Then it's simple 3.24N * 1.2192^3 / ( 3 * 210,000,000,000 * 0.00001904107882336) = 5.87178130931712 / 11424647.294016 meters = ~0.514µm of movement at the top

To calculate angle we use length and movement and do arctan to it - in the end we get ~0.087" of deflection.

Less than 1/10th of arc second.

Btw - that is completely inconsequential for planetary type imaging - it is only important for long exposure imaging if this angle is high enough.