42 replies to this topic

[noisejammer]

My version - translational motion is irrelevant in a telescope pier but rotation of the scope's mounting surface about any axis has to be vigorously suppressed.

 

I had a 10' pier that poked through the roof of my garage. The base was a 1.5 x 1.5 x 1.5 (metre) cube of concrete and the pier was a 16" concrete pipe, filled with concrete, rebar and a mediocre attempt at pre-tensioning it. I estimated somethin like 13 tonnes of concrete.. It was a dog.

 

While it was enormously stiff, its loaded resonant frequency was around a Hertz. This meant that oscillation could have large amplitudes and they'd take many cycles to die down. It was capable of supporting different resonances along several axes simultaneously. Lesson learned - a pier is an inverted pendulum. It must be stiff and light. Ideally, it should ring like a bell. On reflection, I think my pier would have been more effective if the tube was empty.

 

Bending modes can be suppressed - and maybe damped - by guy wires and torsional modes can be suppressed by having arms stick out below the deck and applying guys to these.

 

I sold the house 9 years ago; Google Earth still shows it poking up through the deck.

[Oberon]

Professional telescope piers are hollow concrete tubes with a slab on top.

[Oberon]

Basically you want a stiff hollow tube with a ratio of about 3:1, no more than 4:1. For you that means a tube about 1m in diameter.

The professional way to do it is build a steel reinforced concrete tube. 

The economical alternative is to use steel tubes to make a simple truss, just like a truss telescope tube, only heavier. I would be looking at using 100mm steel tube at least, maybe even 150mm. Thinner walled tube with larger diameter is a better use of mass and cost. Either way, lots of welding on the ends, which might get expensive if farmed out to an engineering shop, I don't know what your situation is there. Were I doing it I would make each tube adjustable, either using the turnbuckle principle or at least have threads each end. This is a fairly standard construction technique for modern architectural structures. 

 

If you want to park a car under it then go higher and wider.

Edited by Oberon, 09 December 2016 - 02:20 AM.

[Rusted]

Basically you want a stiff hollow tube with a ratio of about 3:1, no more than 4:1. For you that means a tube about 1m in diameter.

The expensive professional way to do it is build a steel reinforced concrete tube. 

A much more economical alternative is to use steel tubes to make a simple truss, just like a truss telescope tube, only heavier. I would be looking at using 100mm steel tube at least, maybe even 150mm. Thinner walled tube with larger diameter is a better use of mass and cost. Either way, lots of welding on the ends, which might get expensive if farmed out to an engineering shop, I don't know what your situation is there. Were I doing it I would make each tube adjustable, either using the turnbuckle principle or at least have threads each end. This is a fairly standard construction technique for modern architectural structures. 

 

If you want to park a car under it then go higher and wider.

Without wishing to dump on all these great ideas I much prefer to work in wood and bolts for rapid later disassembly.

Each construction unit remains manageable by one elderly worker without outside assistance.

Welding isn't going to happen though I have welded in the past.

I am now retired from that area of equipment, expertise and ready access to affordable materials.

Just obtaining tube and profiles is now difficult and expensive here. The former 'outlets' have closed down.

 

Can't these large diameter concrete tube piers be directly translated into timber with plywood cladding? [Inside and out?}

Lateral stiffness and torque control can be managed by using suitably large dimensions and triangulation.

[Or stressed skin.] Think Pre-WW2 fuselage construction.

Formers, bulkheads, stringers and plywood/canvas cladding held together in vicious dives and rolls.

 

There are hundred-year-old silos and water tanks just made of timber and planks still standing.

A larger diameter multi-post pier is manageable and can be clad for greater resistance to external excitement.

Make it tapered or conical if you like. Square or round is just as easily put together.

 

Even nested steel tube with lathes in between can be immensely stiff if made large enough in diameter.

Paper thin aluminium over cardboard honeycomb is an aerospace material or [humbler] internal door.

 

I do have access to some affordable and manageable, seamed, steel ventilation pipe.

It only comes in maximum 14" diameter and usually only in 6'6" lengths.

Larger stuff could still be manageable with my trailer with the tailboard down.

It can be clamped together, end-to-end and guyed or stayed if necessary.

 

Expanded foam fill with timber or a smaller tubular steel inner reinforcement?

Sandwich or laminated constructions? Who cares if it just takes a bit longer?

The journey is the thing if it leads to useful results.

These are all just ideas I am throwing up in the air to avoid using concrete or concrete blocks.

Doesn't this humble collection of tube [below] stimulate your innate desire to stack one "thing" on top of another?

 

[Oberon]

 

Basically you want a stiff hollow tube with a ratio of about 3:1, no more than 4:1. For you that means a tube about 1m in diameter.

The expensive professional way to do it is build a steel reinforced concrete tube. 

A much more economical alternative is to use steel tubes to make a simple truss, just like a truss telescope tube, only heavier. I would be looking at using 100mm steel tube at least, maybe even 150mm. Thinner walled tube with larger diameter is a better use of mass and cost. Either way, lots of welding on the ends, which might get expensive if farmed out to an engineering shop, I don't know what your situation is there. Were I doing it I would make each tube adjustable, either using the turnbuckle principle or at least have threads each end. This is a fairly standard construction technique for modern architectural structures. 

 

If you want to park a car under it then go higher and wider.

Without wishing to dump on all these great ideas I much prefer to work in wood and bolts for rapid later disassembly.

Each construction unit remains manageable by one elderly worker without outside assistance.

Welding isn't going to happen though I have welded in the past.

I am now retired from that area of equipment, expertise and ready access to affordable materials.

Just obtaining tube and profiles is now difficult and expensive here. The former 'outlets' have closed down.

 

Can't these large diameter concrete tube piers be directly translated into timber with plywood cladding? [Inside and out?}

Lateral stiffness and torque control can be managed by using suitably large dimensions and triangulation.

[Or stressed skin.] Think Pre-WW2 fuselage construction.

Formers, bulkheads, stringers and plywood/canvas cladding held together in vicious dives and rolls.

 

There are hundred-year-old silos and water tanks just made of timber and planks still standing.

A larger diameter multi-post pier is manageable and can be clad for greater resistance to external excitement.

Make it tapered or conical if you like. Square or round is just as easily put together.

 

Even nested steel tube with lathes in between can be immensely stiff if made large enough in diameter.

Paper thin aluminium over cardboard honeycomb is an aerospace material or [humbler] internal door.

 

I do have access to some affordable and manageable, seamed, steel ventilation pipe.

It only comes in maximum 14" diameter and usually only in 6'6" lengths.

Larger stuff could still be manageable with my trailer with the tailboard down.

It can be clamped together, end-to-end and guyed or stayed if necessary.

 

Expanded foam fill with timber or a smaller tubular steel inner reinforcement?

Sandwich or laminated constructions? Who cares if it just takes a bit longer?

The journey is the thing if it leads to useful results.

These are all just ideas I am throwing up in the air to avoid using concrete or concrete blocks.

Doesn't this humble collection of tube [below] stimulate your innate desire to stack one "thing" on top of another?

 

P1250511 rsz dims .jpg

 

Right! Now I get where you're coming from.    

Cool...so you've got access to a supply of ventilation pipe.

Can you do something like this? Do you have enough tube? 

[Rusted]

Right! Now I get where you're coming from.    

Cool...so you've got access to a supply of ventilation pipe.

Can you do something like this? Do you have enough tube? 

 

 

Omg!  Sheer genius! 

Isn't there a kilometer tall tower somewhere that looks vaguely like this? 

I promise I'd be quite happy with only 12'.

That's only two standard lengths of ventilation tube!

Please don't tempt me to take this route!

 

Hmm. Let's see now...

I can't shift the "phase" of the flanged and clamped serial joints randomly.

Tube lengths are strictly WYSIWG unless I strike really lucky in the discarded heap.

It would be an absolute nightmare anchoring it down safely.

Can't I try and find much bigger tube and stuff the smaller pipes inside?

Like a steam boiler?

 

How do you plan that I should join the very thin tubes in series and parallel without flexure? 

Cross studs, re-bar and concrete filling?

 

Sure is really purdy, though, isn't it?

[Oberon]

Ha! The steam boiler is the way to go!

Otherwise...jointing etc...I haven't got that far. Didn't want to get too detailed until I had a better idea what your pipe supply was like.

Another question...can you also get lots of say 8" pipe? I'm thinking of pipes inside pipes then filled in between with liquid polyurethane foam. Could be expensive, so need to know options, what to rule in or rule out.

[Rusted]

Hi Jonathon

 

Thanks for your continuing interest.

 

8" is available in 2m [6'6"] lengths.

10" & 12" in 2m lengths too.

But the 14" and 16" are only in 1.5m or about 5' lengths.

 

The image shows about half of the secondhand stash.

A lot of it is already jointed but I have usually selected only "clean" undamaged examples.

 

Wall thickness is only a fraction of 1mm  [1/30th"?] but the small end flanges help to stiffen it.

It can't easily be dented or made oval by hand.

 

I believe it is available new in much longer lengths.

But, it would have to be ordered through the trade from a wholesaler.

And, probably cost an awful lot more than I have paid in the past for a few odd lengths.

Spiral seamed tube is much more readily available than this, much prettier, straight-seamed stuff.

 

[jpbutler]

How bout this for thinking outside the box.

After you construct whatever pier you decide.

Mount an inertial navigation device on the top plate of the pier and have the x and y axis outputs drive 2 linear actuators that are mounted on the same x and y axis of the pier.

Mount one end of each actuator to the pier column and the other end to the wall of the deck.

This way any detected motion is immediately dampened out by the feedback loop.

 

Of course there is some software development needed to make it all work and a few other small problems to solve.

But, I am just the big picture guy.

 

John

Edited by jpbutler, 09 December 2016 - 02:47 PM.

[macdonjh]

Rusted, since you prefer to work with wood, I like your earlier idea of framing a largish cylinder (say 3 ft - 4 ft diameter) and sheathing it with plywood. Use pressure treated for the columns in contact with dirt. Working with 1/4" plywood in this application would be much easier than 3/4", and if you don't think it's stiff enough you could make multiple laminations. Properly stained/ sealed/ painted, it would likely last nearly forever. No welding required.

With proper forming, you could even cap it with concrete to make attaching the mount or smaller top pier easier.

Edited by macdonjh, 09 December 2016 - 04:59 PM.

[Rusted]

How bout this for thinking outside the box.

After you construct whatever pier you decide.

Mount an inertial navigation device on the top plate of the pier and have the x and y axis outputs drive 2 linear actuators that are mounted on the same x and y axis of the pier.

Mount one end of each actuator to the pier column and the other end to the wall of the deck.

This way any detected motion is immediately dampened out by the feedback loop.

 

Of course there is some software development needed to make it all work and a few other small problems to solve.

But, I am just the big picture guy.

 

John

Hi John

 

Thank you very much for the fascinating suggestion but my few remaining active neurons must remain safely locked in their dusty box.

I'm afraid I missed the Friday afternoon when Electronics and Modern Software were both on the syllabus together.

 

In fact I stopped coding when the Acorn BBC B and home-baked, Basic software and floppy disks were still [almost] in vogue.

I did manage to convert Conrady's treatise into useful optical ray tracing software.

Even up to "mending" Christen's first published APO triplet prescription in S&T.

 

But I haven't done anything very useful since Cmdr. Bill Gates first descended on our Small Blue Planet with his scouting party.

Though I still hope to get my head around the AWR Intelligent Goto drive system presently being assembled for my DIY mounting.

I am relying heavily on its "Intelligent" aspect to save my having to do any more of this hard thinking stuff.

[Rusted]

Rusted, since you prefer to work with wood, I like your earlier idea of framing a largish cylinder (say 3 ft - 4 ft diameter) and sheathing it with plywood. Use pressure treated for the columns in contact with dirt. Working with 1/4" plywood in this application would be much easier than 3/4", and if you don't think it's stiff enough you could make multiple laminations. Properly stained/ sealed/ painted, it would likely last nearly forever. No welding required.

With proper forming, you could even cap it with concrete to make attaching the mount or smaller top pier easier.

Hi Mac

 

I quite like this idea. There is a problem: OTA clearance.

I'd have to match my mounting and very long OTAs to a 4' diameter 'table' at waist height

This would require an added secondary pier just to allow any serious OTA movement.

 

The full cylinder does have some important advantages though:

The top surface could easily become part of the platform floor.

So any commercial pier could be very easily placed on that firm surface.

Provided, of course, one stepped off the central 4' circle during observations or imaging.

 

I like your forethought in allowing additional cladding layers for greater stiffness if it proves necessary.

This would also make bringing the colossus vertical rather easier, perhaps using gin poles.

 

This assumes the pier was built lying down on the ground.

Or on a simple trestle support at each end to allow easy rotation.

I'm thinking about greater safety while working on the ground rather than constantly up on ladders.

 

Not sure about the concrete slab up on top.

I would be aiming for multiple, horizontal layers of screwed and glued plywood.

With through studs [all threads] keeping the 200lb mounting base plate plus 50lb OTA[s]from lifting in a gale.

 

I'd be much happier with a tapered pier reaching almost to full 12' mounting height.

Laying plywood on a conical pier would be much harder than screwing sheet onto a cylinder.

It would require tapered sides to each piece of cladding plywood to avoid a spiral.

I know you suggested a cylinder which would be very much simpler to build and clad.

 

Assuming I did taper the thing:

Is a tall cone any stiffer than a tall, square pyramid of roughly these same dimensions? 

Do our engineering experts like a 12' tapered pier built off concrete ground anchors at 2' radius?

Edited by Rusted, 10 December 2016 - 08:29 AM.

[macdonjh]

[/quote] 
Assuming I did taper the thing:
Is a tall cone any stiffer than a tall, square pyramid of roughly these same dimensions? 
Do our engineering experts like a 12' tapered pier built off concrete ground anchors at 2' radius?
[/quote]

I've never done the are moment calculations for a cone, so I don't know if it would be any stiffer than a cylinder. My days of doing complicated calculations are long over, too. Now I make simplifying assumptions. As you said, it would be more complicated to build, so I'd opt for a shorter cylinder with a smaller pier on top. Of course, the conical pier would have a "wow" factor.

[noisejammer]

How bout this for thinking outside the box.

After you construct whatever pier you decide.

Mount an inertial navigation device on the top plate of the pier and have the x and y axis outputs drive 2 linear actuators that are mounted on the same x and y axis of the pier.

Mount one end of each actuator to the pier column and the other end to the wall of the deck.

This way any detected motion is immediately dampened out by the feedback loop.

 

Of course there is some software development needed to make it all work and a few other small problems to solve.

But, I am just the big picture guy.

 

John

I'm involved with a project that uses an INS to stabilise a gimballed platform. Sensing better that 20 arcsec / second costs about $20k. Getting 1 arcsec / second runs more than a megadollar... It can be done (see the SOFIA telescope) but it ain't inexpensive.

[Rusted]

[/quote] 
Assuming I did taper the thing:
Is a tall cone any stiffer than a tall, square pyramid of roughly these same dimensions? 
Do our engineering experts like a 12' tapered pier built off concrete ground anchors at 2' radius?
[/quote]

I've never done the are moment calculations for a cone, so I don't know if it would be any stiffer than a cylinder. My days of doing complicated calculations are long over, too. Now I make simplifying assumptions. As you said, it would be more complicated to build, so I'd opt for a shorter cylinder with a smaller pier on top. Of course, the conical pier would have a "wow" factor.

Hi again

 

The conical form offers considerable storage space in the hugely oversized lower end.

Without needing the large form factor at the top end where the bending tresses are much reduced.

 

The 'wow' factor might suggest an inland lighthouse to some critics.

Fortunately much of it will be lost under the shade of the raised platform.

Once I had finally dumped the desire for a dual purpose carport the space beneath the platform became irrelevant. 

 

The major advantage of a tall, square pyramid is the rapid speed of build.

It could be literally thrown together, working alone, in only a few hours.

It just needs a circular saw and rechargeable drill for fitting the 3/4"[?] ply cladding.

Four full 4x8 sheets would reach just above platform level for the time 'cost' of only eight slanting cuts.

After that a subsidiary pier could begin or the pyramid simply clad right up to the mounting base plate.

Thinner ply would be easier to handle alone and could be laminated over with more ply with staggered joints.

 

Whether a 1:3 base length to height ratio is adequate for a pyramid I'm still unsure. It should be.

A wider base could provide shelter for the observer at the expense of needing more cladding materials.

Or buying larger sheets: 5'x10' ply is available in some places but incredibly heavy to handle alone.

Birch ply comes in 5'x'5 at considerable extra cost.

Though exterior plywood is probably all it needs given the shelter it enjoys from the platform.

 

An even larger pier could quite literally become the observatory.

Though boxing in the platform around the huge pier makes rather more sense for a "warm room."

A reinforced plate down near ground level could be stacked with paving slabs for extra stability.

Thought that would demand a very firm base to resist the applied loads.

 

The question must remain:

Would such a structure be stiff enough for a 200lb mounting and long OTA for imaging the solar system 12' above the ground?

Edited by Rusted, 11 December 2016 - 04:06 AM.

[555aaa]

I have previously suggested the building inside a building concept where an inner conventionally framed building supports the scope but is isolated from the outer building which has the exterior walls, roof, and observer platform. That retains usable space in the ground floor.

[gregj888]

Rusted,

 

Check out concrete pipe... 48" lengths of 24" diameter are $135 or so.  It will be hard to duplicate that with anything else (except maybe formed and poured concrete).

 

Wood is a great material for a lot of things but is "springy".  You are talking about something ~12 feet tall with a 200# moving mass on top, "springy" is not your friend.  If I had to use wood, I would look at a tapering triangle. 

 

Would be nice to do a real mechanical design with FEA.  I'm guessing a tapered triangle with truss construction on each face could work, but a calculated outcome before building would be nice.  Same concept as the frame for this scope, but probably skinned with plywood, as you said.  Might still want a dampener at the top (earthquake style) and foam or sand in the space (dampening/strength/mass).

http://www.sanluisob...e39207819.html 

Edited by gregj888, 14 December 2016 - 01:40 PM.

[Rusted]

Just when you thought it was safe..

 

I have come up with another [crazy] tall pier idea using timber, with taper and lots of mass at the base. 

 

A massive concrete pipe 1m high x 70 cm in diameter by 60mm thick is half buried in the "dirt".

That's 40" high x 28" diameter x 2.5" thick in Old Money. It weighs 320kg or 650lbs bare.

Concrete could be poured around the base of the pipe for a bit of extra mass and anchorage in the ground.

 

Four 4"x4"s timbers x 12' high are spread apart inside the bottom of the pipe.

Then anchored by cast concrete carport footings inside the bottom of the pipe.

Or even locked in place by poured concrete.

The concrete pipe is then back-filled to the top with self compacting sand & gravel.

This adds another 1400lbs. The filled pipe now weighs well over 1000kg or 2000lbs.

 

The entire area is now raised to the top of the concrete pipe with a 60cm [2'] deep layer of self compacting sand & gravel.

This will help to lock the pipe against rocking in the ground.

 

At the top the timbers are bolted together around a shorter piece of 4x4 with sturdy all-threads [studs.]

The four pole's pyramidal taper will kill any likely flexure.

The poles can be braced or are easily clad in plywood as a stressed skin stiffener if it proves necessary.

 

The main advantages are: Incredibly easy demolition back to bare ground if it doesn't work.

The buried pipe is sacrificed to future archaeologists or concrete filled for slightly more mass.

I can build the whole thing working alone without much expense or effort.

I already own the massive concrete pipe as an oversized "planter."

It just needs to be rolled into place and dropped into a pre-excavated hole in the ground.

What do you think about this one? Will it fly?