Permanent Piers

MAPUG-Astronomy Topical Archive     AstroDesigns    MAPUG-Astronomy.net

 

Also see Observatory Plans  and  Wedge Modifications

rule

Subject: List of Resource Links for Piers & Wedges Top Button

From: Mohib Ebrahim <mohib_ebrahim.smza_tattglobal.net> Date: March 2005 (links verified)

I thought I share the links to some of the best info I could find about piers, wedges, and the topic in general. (A couple of the links are from the MAPUG site itself, the rest are elsewhere).

Vibrations and how to dampen them -- info I've not see elsewhere: (read the whole thread to understand what's really needed or you can be led easily astray while the discussion hammers out the issues):
  <http://astro.umsystem.edu/atm/ARCHIVES/AUG01/threads.html#00243>
  <http://astro.umsystem.edu/atm/ARCHIVES/AUG01/threads.html#00457>

And In particular:
  <http://astro.umsystem.edu/atm/ARCHIVES/AUG01/msg00624.html>

Tips on piers and wedges in general:
  <http://www.MAPUG-Astronomy.net/AstroDesigns/MAPUG/PermanentPiers/PermantPier.htm> This page!
  <http://www.MAPUG-Astronomy.net/AstroDesigns/MAPUG/PermanentPiers/Pier-plates.htm>
  <http://www.astronomysite.com/mapug/16/msg16796.htm>
  <http://www.astronomysite.com/mapug/16/msg16688.htm>
  <http://astro.umsystem.edu/atm/ARCHIVES/APR01/msg00926.html>
  <http://astro.umsystem.edu/atm/ARCHIVES/AUG95/0413.html>
  <http://www.homedome.com/chpt6b.htm>
  <http://easyweb.easynet.co.uk/~chrish/lx_pier.htm>
  <http://home.earthlink.net/~mtnviews/just_astro.html#Fundamentals_of_Permanent_Pier_Construction>

Cantelevered wedges:
  <http://www.astroman.fsnet.co.uk/pier.htm>
  <http://www.ameritech.net/users/wc_observatory/telescope.htm>
  <http://www.amtsgym-sdbg.dk/as/piercut.gif>
  <http://www.amtsgym-sdbg.dk/as/pier.gif>

A few installations:
  <http://obs.nineplanets.org/obs/obslist.html>
  <http://astro.umsystem.edu/apml/ARCHIVES/FEB98/threads.html#00303>
  <http://www.tucsonastronomy.org/pier.htm>
  <http://www.robertreeves.com/mount2.htm>
  <http://www.arizonausa.com/sky/instruments.htm> Note: scroll down to pier description
  <http://spazioweb.inwind.it/gpasi/tech/pier/pier.htm>
  <http://www.noomoon.com/noomainastroOH.htm>

rule

Subject: Tall Pier Design   Top Button

CAUTION: read this statement before pursuing a large/tall pier.

From: Dave Roady <DMROADYa_tcharleston.net>

I've been reading with some dismay the recent posts about tall pier construction. I caution the group that it may not be wise to proceed with plans given over the Internet without the advice or the design services of a registered engineer or architect.

What one man does in his back yard is his own business, but when those ideas are given as advice publicly, liability issues arise. The situation in Florida (5x5x4' footing with another 12' of pier) is special in that he has saturated soil. Such conditions may lead to settlement and possibly overturning.

Each area of our country has adopted at least one building code to go by. Building such a structure without adequate design may be a violation of that code.

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Subject: Tall Pier Design  Top Button

From: Timothy L. Gettelfinger <ro-techa_tworldnet.att.net>

Following is a bitmap of a tall pier design proposal. For a 10' tall pier the above ground portion weighs approximately 5,000 lbs. The 5'x5'x2' footing weighs approximately 7,500 lbs. Taking in consideration the displaced soil the footing bearing pressure due to gravity loads is approximately 300 psf. Lateral loads at the top of the pier will alter the soil bearing pressure considerably but an average pressure in this range is considered small. Typical allowable soil bearing pressures range between 1,500 psf to 3,000 psf for good soil. Soft material with organic matter or any disturbed soil would not be considered good.

Several things must be taken into account before a final design is reached. Is the footing in a heated space? If not the bottom of the footing should be below the frost line. If the pier is exposed to freezing and thawing cycles. If so a higher strength concrete may be considered (3,500 - 4,000 psi).

Under no circumstance should the footing bear on disturbed soil or loose fill. Long term consolidation of the soil will take place which may cause the pier to lean and/or settle. Obviously the weight of the instrument is a consideration but under most circumstances this will not be very significant.

These are just some considerations that come to mind, if anyone has any specifics they would like to discuss of if you would like the pier detail in a different format just let me know.


    pier design graphic

rule

Subject: Constructing a Permanent Pier  Top Button

From: Clifford Peterson <cpetersona_tweber.edu>

I had the following fabricated by our local steel service company --Boman & Kemp. The pier is a 13 foot length of 6" inside diameter steel pipe with a .28 wall thickness. I had one end of this pipe welded to the center of a 1" thick steel plate that was 14" square. I then had them cut out a 5" hole in the center of the plate so any moisture that might find its way into the pipe had a place to drain out the bottom. I also had 1 and 1/8" inch holes bored in each corner of the plate so it could be attached to four 1" bolts that would come up through the concrete slab it would be attached to. I also had them weld 1/4" thick 4" wide "wings" that ran the entire length of the pipe from the bottom plate to the top of the pipe. Four of these "wings" were placed at 90 degree intervals and were welded to the bottom plate and the pipe and placed so they joined the square bottom plate on the flat sides midway between the corners. The top of each "wing" was tapered from 4 inches to 1 and 1/2 inches towards the pipe for the last 18" and a 1 and 1/2 inch tab bent 90 degrees at the top of the wing so the tab is parallel and even with the top of the pipe. This gives me a place to bolt my mounting hardware for the scope on top. I had them fabricate a second 14" square plate that was!

Only 1/2" thick with a 10" long 1" diameter bolt stock in each corner that were matched to the holes in the plate on the bottom of the pier. This was really only a way of getting accurate spacing in the bolts that were coming up through the concrete so the pier bottom plate holes would line up. I had 4 large holes cut through this plate so concrete could flow through and not leave voids above or below the plate holding the mounting bolts. (See diagram in post above)

I expanded my balcony to accommodate the pier area so I had to extend the concrete slab below the original balcony. when I extended the slab I dug out a large area under the place the pier would go so it was 2 feet deep and 5 feet in diameter under where the pier would mount. while the concrete was wet I sunk and positioned the piece that the bolts were mounted in so 4 inches of the bolts stuck above the concrete. After the concrete had dried for 10 days I placed a 1" nut on each bolt and screwed them down near the top so very little of the thread was exposed when I put the pier on top so I wouldn't damage the threads. With the pier held by several neighbors I used a wrench to screw the nuts and the pier down towards the bottom of the bolts.

As soon as enough thread was showing I put a second nut on each bolt so one nut was on the bottom of the plate holding the weight of the pier and a second was on top of the plate to lock it in place. By adjusting the nuts on the corners I could then adjust the pier so it was perfectly plum and vertical. I placed a heavy walled 20" Sona tube that was 12 foot in length over the pier and epoxied it to the cement around the pier base leaving a small area on the low side un epoxied so any moisture could escape there also. !

I filled the pier pipe with fine dry sand up to the top and then filled the Sona tube with sand also. This killed all vibration in the pier. I had cut the hole in the balcony to be 1 and * inch larger than the Sona tube so nothing could touch the Sona tube or pier within. I had the local sheet metal shop make me a flashing that resembled a large top hat with the top cut out. The brim portion fastened to the balcony floor and the side walls rising up along side the Sona tube for 10 " with a 1" space open all around the Sona tube. My balcony floor is solid so I didn't want snow and rain to be able to go down the balcony hole around the Sona tube. I bought a bicycle tire inner tube and placed it over the Sona tube and partially down in the flashing. When some air pressure was put in the tube it filled the 1" opening around the Sona tube and inside of the flashing and made that water tight but didn't couple the balcony vibrations to the pier. I built a shed under the new balcony and around the pier area so I could store lawn mowers and tool the Sona tube that might cause any movement or deterioration of the Sona tube. from below.

Hope this gives you an idea of my setup. Sorry about the long winded explanation, --the reason I didn't want to post it in the first place. This could probably be shortened with a few pictures so descriptions didn't have to be so long. If you have further questions I will try to help if you aren't afraid of receiving another book.

rule

Subject: Permanent Concrete Pier--Part 1 of 3  Top Button

From: Richard Plasencia <RZP1a_taol.com> Date: Aug., 1998

I see all the construction details, your and others, you are making it way too complicated and expensive.

All it takes is 8 or preferably 10 inch sonotube that goes down a few inches below the frost line ---if you live in that part of the country-- otherwise about 4 feet down. Let that project about 18 inches above ground. Fill with Sakrete or other bag ready mix and go EASY on the water. Mix outside the hole and then dump. The mixture should be grainy and somewhat dry. This makes increadibly tough concrete. Before filling the hole put in two pieces of 1/2 inch rebar spaced about 3 inches-- do not allow the rebar to show outside the concrete; top or bottom.

Place a pipe flange with machine screws into the top of the concrete and level the assembly. Stainless steel is a waste of money for this job and may not adhere to concrete as well as plain steel. On the pipe flange will be screwed a length of pipe to the height you desire and at the top another pipe flange on which you attach the wedge. This arrangement is cleaner looking, more durable than all concrete and and and! allows you to change pier height as your needs change. I.E. Tall for zenith observations and short for horizon--etc. The pipe should be 3 or 4 inch diameter and can be found at any plumbing supplier that handles fire sprinklers and steam pipes.

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Subject: Permanent Concrete Pier--Part 2 

From: Joshua Kent <jdkenta_tworldnet.att.net>

Go here <http://www.homedome.com/> They have lot's of info and ideas....
Note: should open a new browser window over this one.

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Subject: Permanent Concrete Pier--Part 3 of 3 

From: Michael Hart

> Sounds OK to me. About the same type soil as here in Iowa. You are down 4
> feet and that's below frost for you---great. Go with it. Make sure you put
> the two pieces of 1/2 inch rebar as I said before. No need to use more.
> Forget the rock packing and all the other crap. Nothing is going to move that
> pier. The builder is thinking in terms of a support column for a building.
> He has no experience with a short concrete column used to support not more
> than a 100 pounds.
> As for the gap around the column and the the pad-- 1/2 inch is OK and 1 inch
> may verge on too much. If you get frost you have to pack the joint with
> tarred felt to prevent the ice from breaking things. Maybe not this year but
> it will eventually. That's why the gap should not be overly large.
> Don't worry about your pad floating around. No matter what you do it will
> float unless it has 4 foot footings--ridiculous

Six to twelve inches of large aggregate is enormously helpful at minimizing or preventing concrete pad movement- this is common practice for highway construction. The aggregate allows space for moisture and frost to expand and move away from the underside of the concrete with little to no concrete movement. It is not a practice for sidewalk and driveway construction, which is unfortunate. Much aggravation and premature failures are produced by concrete slabs rising and moving about which could be minimized or prevented by a few dollars of rock under the concrete. I have seen newer concrete slabs with full reinforcing rebar fail when laid directly on soil, while older un-reinforced aggregate supported concrete remained intact.

Dick is quite right about builders thinking in terms of supporting large loads and over-building the pier. The purpose for the rock under the concrete is to redistribute the load in the soft ground. This prevents settling and allows a heavy structure to reside below the frost-line and above bedrock with little to no movement. In soft soils, this is arguably more important than rebar.

Rebar is useful to support shear, shock, and vibration loading. It also increases the tensile strength of concrete. A typical short pier used for a telescope has predictable loading. Typical concrete has a compressive strength of 2500 to 5000 psi and I suspect this is quite adequate to support 100 pounds as evidenced by several piers in excess of 5 years old with no signs of concrete failure.

rule

Subject: Opinions on Permanent Pier Design --part 1 of 8  Top Button

From: John Hopper <JohnLX200a_taol.com>

<dave23sch22a_thome.com> writes:
<< I saw some 1/4" flat bar stock 3" wide at the hardware store. I may get a section of it and cut it into a bunch of flat, square, washers (with holes in the center) and space 3 or 4 out on each threaded rod (with nuts on each side to keep them in place) and then imbed it in the concrete. >>

That's a good idea if you're using the bolt-down method and don't pre-stretch the rod before setting the concrete, except that hopefully only the top-most one will come into play even if the concrete around the uppermost threads fails.

My latest thinking is that if you don't pre-stretch, then either the J or a big washer, or better yet, a plate with holes matching the flange, should be fairly far down in the concrete, and you should wrap a piece of cardboard around each rod so that the threads aren't intended to hold in the concrete. Then all the force is pulling up on the bottom plate, which puts all the concrete above the plate in compression.

Also, rather than use a Sonotube, I always just dig a hole and pour the concrete into the irregular hole for better holding power, but I'll qualify this by saying that this is very debatable above the frostline, and that ideally you should leave the hole irregular below frostline and smooth above frostline. Details details....

BTW, the most easily made really good, vibration-free pier is big, few-year-old, non-rotting tree stump, preferably cut in the plane of the celestial equator and having a few 1.25" and 2" holes in it.

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Subject: Opinions on Permanent Pier Design --part 2   Top Button

From: Tony Floyd <tony.floydea_tpsion.net>

I am posting this on list so that others can see the design concept of a steel pier i am having made up. And of course you comments. I have also been discussing the pier off list with another Mapugger who may use the same design.

Seeing your last reply I figured that I would check out the design with some one who at least appears to know what he is talking about, so here goes:

The base plate of the Pier 400mm diameter 15mm steel with five 13mm holes drilled 25mm from the edge set at 72 deg. from each other. Welded to the centre is a 165 external Diameter steel tube with 10mm walls this section of tube is 1050mm long. Welded to the top is the top plate which is to take an adjustable leveling system for the wedge. This plate is 300mm diameter 15mm steel in the center is a 50mm hole so that the pier can be filled with sand after installation and then sealed.

There are 4 Vanes made from 10mm steel welded to the Tube, bottom plate & top plate, these vanes are 115mm at the base tapering to 50mm at the top they are set at 90 degrees from each other.

The pier will be fixed to a concrete slab 600mm X 600mm X 600mm using 12mm X 75mm HILTI Bolts or 12mm stainless steel threaded rod fixed with HILTI Chemical Fix buried 200mm into the concrete.

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Subject: Opinions on Permanent Pier Design --part 3   Top Button

From: <stephejla_tsongs.sce.com>

Tony, Your pier design is slightly shorter and carrying less weight than the pier that I looked at for Dave. Without redoing the calculations, I can say that your natural frequency will be higher than his, probably in the 100 Hz range. You also have vanes on the outside that will add stiffness and raise the natural frequency. Any deflection by unbalanced loads will be negligible. Your 5 mounting bolts should be sufficient and if you use the HILTI recommended procedures for their fasteners I have confidence that they will hold . Make sure you read my other posts on how to mount your pier to the bolts. BTW, as I was walking past our switchyard yesterday, I noted that the giant towers that hold our 220kv transmission lines are mounted just as I recommended yesterday for piers, with the nut on top and bottom of the flange. Some smaller towers holding only a small capacitive transformer or something light were mounted by bolting directly to the concrete. All our safety-related, seismology qualified pipe supports are double bolted.

You pier sounds great to me. As with most piers we've been discussing here, you'll probably find that the weak link is the wedge.

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Subject: Opinions on Permanent Pier Design --part 4   Top Button

From: stephejla_tsongs.sce.com

If I can interject some of John Hopper's comments here on this thread ... he brought up the issue of the concrete base and imbedding the threaded rod. Instead of bending the end of the threaded rod into an "L", I saw some 1/4" flat bar stock 3" wide at the hardware store. I may get a section of it and cut it into a bunch of flat, square, washers (with holes in the center) and space 3 or 4 out on each threaded rod (with nuts on each side to keep them in place) and then imbed it in the concrete. I'll use 4 - 36" long pieces with 6" protruding above the top of the concrete base to attach the pier. I'll leave the 6" protruding up because that will give me enough stock to first mount the base plate in a sandwiched between two bolts configuration. If that proves to be unacceptable from a vibration standpoint, I can remount it directly on the concrete and see how well that works.

I think it is great that you are using such a deep pier with long rods and I think that bending a J in it would be adequate. What I've seen at construction sites is bending a zig zag with 4 or 5 bends into it. That would be more work. So your large washer plan sounds good to me. The downside is that the first washer will probably take 90% of the load so I would put it down at least 8-12". As far as wrapping the rod in cardboard to isolate it from the concrete, my gut feeling is not a good idea. What you are saying makes perfect sense from a tensioning standpoint but the second thing that the concrete does for you is stability. If you Isolate the rod from the concrete you basically have your pier sitting on top of a tall cage made from threaded rod. You want the concrete pushing on the sides of the rod for stability. If you are concerned about the threads, then I would step up to 3/4" rods. The heavier rods would not stretch as much and thus you have less movement inside the concrete. Remember, the steel is much stronger than the concrete so you will be stretching the steel very little. The concrete will actually be experiencing a little strain (movement) at the location where most of the force is applied.

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Subject: Opinions on Permanent Pier Design --part 5   Top Button

From: stephejla_tsongs.sce.com

Maarten, The first place I would look is here:

Is your bottom flange bolted directly to the concrete base or is it raised up off the concrete with a nut on either side of the flange? - directly bolted to

Concrete generally isn't perfectly flat and a high center or low corner could cause you problems. Any minor problem here would be amplified at the scope. Do you have room to put a nut above and below the lower flange? If not, put a large washer on each flange under each stud and then shim on top of the washer to get them all in the right plane. This shimming process can be simplified by using U shaped shims that you can slide in without removing the pier. These are used industrially to shim the mounting for pumps and other large rotating equipment but I don't have a source for them.

Resist the urge to just tighten this joint as the concrete is the weakest link and you will likely pull your anchor loose eventually.

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Subject: Opinions on Permanent Pier Design --part 6   Top Button

From: Brad Bradfield <b_bradfielda_tyahoo.com>

I've been following some of the ongoing thread on pier design, and in particular the offshoot on bolting an adapter plate to the top of a pier. McMaster-Carr and others sell a great device made just for this type application called a Belleville washer. A Belleville washer, also called a disc spring, is a concave shaped steel washer designed to be placed into compression without permanent deformation. McMaster-Carr shows them in sizes from 0.093 to 2.00 ID. To look at their selection and how they're used, go to the Raw Materials and Springs section on McMaster-Carr's main page <http://www.mcmaster.com/> and select the very last item, which is Springs. Select Disc Springs when the Springs page comes up.

Belleville washers are ideal for stud mounting an item that must be aligned rather than just bolted down. To give you an example of where I've used them, I used to work on an RF anechoic chamber with a feed antenna system in one end that had to be precision aligned. The antenna assembly was mounted to a flat plate, which was in turn mounted to the top of a very rigid frame. Threaded 3/4" studs protruded out of the top of the frame, and on each stud was a stack of six or eight Belleville washers, with the dish on each washer alternating from that on the adjacent washer. These washers allowed the antenna plate to be leveled extremely accurately, by torquing down the corners as required.

Just as an approximate cost, McMaster-Carr sells 1/2" Belleville washers in a package of 12 for $10.50, which isn't bad. Of course I don't know what their minimum purchase is either. You also might be able to buy them locally from an industrial machine shop or other industrial hardware supplier.

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Subject: Opinions on Permanent Pier Design --part 7

From: Darren Carlisle <darrencarlislea_thotmail.com>

If you are using a double top plate to level off your wedge I would personally bolt the pier directly to the concrete. Float the concrete off as level as possible, it will not matter if the pier is not quite plumb, as you will level off the wedge with the top plate. I would not use stainless steel if I were you, they are very hit and miss, there is less give with stainless (stretch). I would use M12 x 175 or M16 x 175 Throubolts with a passivated coating, there are many different manufactures but they all do the same job, I checked the HILTI web for you and they call them HSA Expansion Anchors. Before they paint the pier ask them to make sure the base is perfectly level.

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Subject: Opinions on Permanent Pier Design --part 8 of 8   Top

From: John Hopper <JohnLX200a_taol.com>

Doug writes:

<< I started to wonder as I read through all the discussion about this if there might be an alternative that hadn't been mentioned or thought of.. What about using 4" box steel tube, welding 4 pieces together to form a larger box? The inner "+" pattern of the tube walls would add a significant amount of rigidity to avoid flexure and possibly eliminate any harmonic resonance. A center notch-cut in each piece would allow the welding of a piece of threaded rod to the tubes which could pass through a top plate and interface directly into a wedge base.>>

Having the additional places to bolt or weld the end plates to would be convenient. But other than that, it's a losing deal compared to a simple pipe or box section.

A square box section isn't much worse than a cylindrical pipe pound for pound, but your solution puts half the mass inside the outer perimeter, which is much worse than either a pipe or square box section. Both of those have all the mass in the outer perimeter where it does the most good.

So all the mechanical properties of the pier itself are not as good as putting that extra mass out at the edge instead of near the center. That is true even if the whole shape were extruded as a continuous, single cross section. Having to fabricate it by welding makes things even worse, as it's not practical to weld the entire surfaces together, and any rounding of the corners of the individual boxes cuts into the critical outer perimeter thickness regarding torsional stiffness, only partly made up for if welded well.

So it was a good try, but putting 50% of the mass in the interior of the hollow section hurts rather than helps both the torsional stiffness and bending stiffness. In bending, it's the equivalent of adding wings in the center of an I beam, like typing a "-" on top of an "I" or making it a tri-plane instead of a biplane. Unfortunately, the center of an I beam does absolutely nothing to resist bending, which is why the shape of an I is used to begin with, with the center of the I just thick enough to resist shear and buckling.

rule

Subject: Permanent Pier Construction --part 1 of 7 Top Button

From: Roger Hamlett <ttelmaha_tntlworld.com>

I did a pier in my garden (lighter than needed for the current scheme being discussed here), but the design has proved successful. My 'starting' point, was the pier I wanted at the top. I decided I wanted about 6" diameter tube, so talked to a steel stockholder. I went for a 'high' specification, I asked for Stainless tube, with a wall thickness of at least 3/8", and an outside diameter of about 6". After some 'shopping around', they found me the perfect tube. It ended up as a cast Stainless tube, in a grade that would resist ground water corrosion (again I had asked for this), with a wall thickness of 5/8", and the actual OD, of 6.5". In a 7'6" length, it massed at over three hundredweight!...

This stuff was made for super high pressure pipes for the oil industry, and a similar pipe to this, would be a fabulous starting point. The price was not as bad as I feered (including 'dropping off' in my front garden). The actual depth and size needed for the base, will depend massively on the local subsoil. If (for instance) you have peat, or clay to depth, then you should consider effectively a 'raft', comprising a reasonable 'area' of concrete with reinforcing mesh set in it. However if you are lucky, and there is some nice solid soil, a much smaller base can be used. A small 'test' hole, or some questioning of local builders, should help to identify the conditions. It is possible that the reasonably small size proposed by the original designer, reflects 'local knowledge.'

In my case, 3' 'down,' there was clay, but this is a thin layer only about 18" thick. Immediately below this was a lovely firm layer of alluvial gravel, so the hole was dug to this layer. As others have said, try to design, so that the telescope base and column, are isolated from the surrounding floor, and structure. It is surprising how sensitive a scope is, as a 'seismograph,' for local vibrations!...

A tube can be 'mass damped,' to remove it's natural tendency to ring, by backfilling with various densities of material in layers. Steel on it's own, moves far more than you might expect (remember that a normal 'I' beam, in a building, will typically deflect by something in the order of 1/200th the span under normal loadings - on a 3' length of beam, this gives an 'angle' of about 20 arc minutes!...). Remember that whatever you use, will tend to 'move' when concrete is poured around it, so you either have to build a strong frame to support it while pouring, arrange alignment marks, so that it can be 'tweaked,' or provide adjustment at the top to compensate.

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Subject: Permanent Pier Construction --part 2 Top Button

From: Vic <zigmaa_tiinet.net.au>

I live in Western Australia. I built my conventional observatory about 4 years ago from RHS. My local government authority allows the construction of a 10ft x 10ft enclosure without the need of a building permit. The dome is 10 ft in diameter and is 10 ft to the top of the dome itself. It can be rotated 360 degrees - is not automated.

In short form a description of my pier is as follows.

  1. Firstly I drilled a 14" diameter hole to a depth of 10 feet - from the surface.
  2. I then centred 4 x 5/8" x 20ft reinforcing rods in the hole and filled the hole with concrete.
  3. Once the concrete had set, I lifted a 4ft x 3 ft well liner over the top of the exposed reo rods - centred and levelled it on the ground, then mixed a rich blend of cement, sand and aggregate and filled the liner. This gave me a strong base from which I could construct the base for my telescope.
  4. The base was built from normal clay house bricks to the desired height. The dimensions of the base are 6 ft H x 24" L x 16" W. With the reo rods centred in the cavity of the base I then filled it with concrete.
  5. At this point only about 1ft of reinforcing rods protruded. I cut them off.
  6. Next I placed polystyrene foam over then entire area of the liner and also up the side of the base 3".
  7. 3" is the thickness of the concrete on the dome floor. The polystyrene acts as a vibration dampener and isolates the concrete floor completely from the pier itself.

I have operated my permanently mounted Meade 10" LX200 for three years now and any one who enters the dome can literally slam the door when I am guiding and there is absolutely no sign of vibration. It is very very solid in construction. Even giving the pier a heavy bump will not cause any noticeable vibration while viewing at high powers.

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Subject: Permanent Pier Construction --part 3 Top Button

From: Doc G

Bob Moorman wrote:
> I WAS intending to install a 10" pier, 3' in the ground into a cubic
> yard of concrete. Using a 10' X 10" 1/2" steel pipe for the form of
> the pier. The concrete block will be sitting on top of blue clay. Any
> thoughts as to this installation? Doc?

I normally recommend that the length of the pier into the ground be about the same at that above the ground. Of course for two story piers or something like that you have to compromise. In all cases you should go at least a foot or two below the frost line. If you are going into bed rock, these suggestions change. It is wise to put in a footing plate at least 2 to 3 feet in size.

I also recommend a pier about as large as the aperture of the scope. Larger is ok, but not smaller. When you have frost to contend with, it is important to have the base of the pier in the ground larger at the bottom than at ground level.

I also recommend a concrete pier, with re-bars, since it is strong, stiff and will not ring. It is also inexpensive.

These are rules of thumb as it were and there will be reasons to deviate from them or go to a steel pier or something more complex. But for a permanent pier these are good rules.

Your pier sounds reasonable good to me.

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Subject: Permanent Pier Construction --part 4 Top Button

From: Doc G

Doug Carroll wrote:
> I know this is not perfect but seem to be working quite well.
> The owner of the place I rent wouldn't let me dig up the back for a pier but
> did ok drilling 4 holes into the patio behind the house. I sunk
> 4 - 4" anchors into the concrete. Then some washers and nuts to lock them down.
> slide a 3/4" aluminum plate over than and more washers and nuts' mounted 4
> 1/2" 13TPI all thread then length of a Sonotube pier. Filled it with concrete.
> Mounted another 3/4 plate on top then another 3/4" plate with the bolts
> to act as levers. But the wedge on top and have a semi-permanent pier than
> is very strong. Rings dies in about 1-second, if hit with a hard whack.
> You can see it here at:
> <http://www.ccdguy.com/observatory/homepier.htm>
> Total cost was $45. The plates were off eBay and are a
> regular item. The rest of the hardware was from Lowes. Anyway it works
> great and may be of use to other in the same situation as myself.

That pier looks quite beautiful. The original pad looks very large and stable. Does the telescope move when someone walks around on the pad. We have found that with 6 foot pads about 1 foot thick, the telescope moved quite a bit when people walked on the pad. That is, the pad was tipping. OK for viewing, but no good for imaging.

Another test is to stomp around on the pad when viewing to see if the image jiggles. With a pier mounted on the pad it is usually required for everyone to stay still or away from the pad. I have seen cases where a person jumping on the ground 10 feet from the pier actually jiggled the telescope too much for high quality imaging.

You have to just do the stomping test for yourself. If it is stable you know right away. With a good pier you should see no movement in the eyepiece at all.

--------------------------------------------------

Subject: Permanent Pier Construction --part 5 Top Button

From: Jerry Stephenson <stephejla_tsongs.sce.com>

As an engineer, I have several qualitative comments that might be useful concerning the recent discussion on piers:

If you are using steel, then circular is the stiffest cross section. If you get a good deal on a really heavy i-beam or some other shape, it will probably work but is not designed to be equally stiff in all directions. As I discuss later about the value of material at the edge being greatest, other tubular cross sections would be my second choice. Square tube steel is used extensively in heavy industry because it is easier to weld things to than circular.

Larger diameter contributes more to stiffness than thick walls (within reasonable limits). The concrete corollary is that larger diameters are stiffer and you could even put a tube in the middle to make it hollow if you want to save material or make it easier to remove later ( a stick or two of dynamite at the bottom would really help in breaking it up. The steel I discuss later will be a significant impediment to removal.

When we build a nuclear power plant, we make it look like concrete from the outside but the inside is about 50% steel. If you build a concrete pier, put plenty of steel in it (no, I'm not recommending 50% steel, your pier doesn't have to withstand an airplane crash). The farther from the center, the more stiffness it will add. Wire you steel together in a cage that is a couple of inches smaller than your form.

Try to pour it all at once from the underground to the top. Adding a joint could introduce some flexure as concrete isn't glue and doesn't automatically bond to the layer below it. Rebar helps but without the concrete continuity you introduce a stress riser that could cause the rebar to strain at this point.

If you pour a pier of concrete, use very long bolts either bent in a J, or better yet with cross bars welded to them, and include them in the original pour. A good design would be to weld reinforcing rod from bolt to bolt so that they are pre-aligned relative to each other. If you are using stainless bolts, use filler material for welding that is designed for stainless and you should have no problem welding to the carbon steel rebar. I believe this attachment here is the source of many problems that people attribute to the pier being too small. The other significant source of problems is the attachment of the steel pier to the concrete footing using inadequate HILTI bolts or inadequate embedded bolts.

If you use steel, then a strong design with either anchor bolts in concrete or inserting several feet of the steel pier into concrete is a must.

Of course you want your floor to sit on the ground and have a gap between it and the pier. If you need to seal this gap use something very flexible like a rubber camp mat. I think your can buy foam insulation that would work well. RTV would probably work but I would prefer something a little less dense.

If I were building a pier that I wanted to be qualified for all scopes I might ever put on it, I would probably pour a 16 to 18" pier that penetrates about 8 feet into the ground. I would probably avoid a big footing because then you end up walking on it or setting your floor on it. Such a massive structure probably wouldn't vibrate anyway but I like the idea of having your pier totally isolated from your floor. Perhaps an inverted mushroom head a couple of feet below ground would be good. Of course I would probably do some quantitative evaluations at that time too.

As always, these are just my opinions. I've been told my writing style makes me sound like either an expert or an opionated oaf. I'm not either one. I consider myself a jack of all trades but a master of none. Good luck.

---------------------------------------

Subject: Permanent Pier Construction --part 6 Top Button

From: Dave Graham <hardwarea_tdatasolaz.com>

Following up on Jerry Stephenson's excellent post, a few thoughts on the practicalities of actual construction...

As background, I've had to drill a 24" hole into pure sand 200' deep to reach limestone, bore out a socket, and then set and grout an 18" steel casing from the base of the socket back to the surface. I lost two holes, a $20,000 drill bit and almost lost a drill rig before we figured out how to get it done.

Assuming this is to be at least somewhat self done, and not left to the contractor(s) to do, I offer a few items of concern.

  1. Safety. Any deep excavation, especially a narrow one, must be adequately shored to prevent cave-in. It's too easy to get killed in there, especially if the soil is at all unstable. The shoring must extend down to the point where the top of the footing will be, but below that, you want a clean excavation so you will pour the concrete directly against undisturbed soil.
  2. Getting a mono pour can be a challenge. The weight of the concrete poured into the top of the pier form once the footing is poured and consolidated will try to push its way right down the pipe and displace the concrete already in the footing if you pour all at once without something to stop it. There are a lot of approaches to this problem, but most of the time we compromise and pour the footing with plenty of rebar bent so it extends laterally in the footing and up the pipe in a radial pattern, wait for it to set enough to hold the weight of the concrete to come in the pipe, and then pour the pipe. In the case of 18" .188" or .250" wall thickness (no real advantage to thickness, just availability) well casing assumed as the pier form, 8 #5 rebars set radially will offer an adequate continuity against lateral displacement of the bottom of the pier and a good degree of stiffness to the connection. In addition, as Jerry mentioned, some rings of #4 or even #3 rebar 6-8" or so apart up the column welded into place will give good rigidity and grip for the concrete to hold to the rebar even though the concrete may well form a curing crack at the joint. Jerry is very right, you want the rebar at least a couple of inches inside the pipe. Has to do with bending moments with one side in compression while the other is in tension, but that's is for a later beer.

Jerry is also quite right in that a mono pour *is prefereabe* to achieve the very best connection at the base of the pier. One way to achieve this is to pour and consolidate the footing, then immediately backfill over the footing so the weight of the soil will push down on the concrete and prevent the concrete coming down the pipe from displacing it. This is easier said than done, since the area will be confined, and you will need to compact the soil overlying the footing as you go on the backfill. Lots of extra labor can be a big help. It can also be in the way... One thing that helps is that the concrete already poured will be stiffening as you start the backfill, making it harder for the concrete coming down the pipe to displace it. Working against this will be the vibrations of tamping the backfill which will tend to keep 'breaking the klink' of the concrete already poured in the footing, in effect postponing its stiffening enough to resist the pressure of the weight coming down the pipe. It's a balance. Sometimes a frustrating ($$$$$$$) one.

In reality this usually becomes two or three pours. First the footing and a couple of inches of the pipe, then go like h... to backfill and compact enough soil (might take three feet or so depending on temperatures and cure state of the concrete in the footing) while a second (or maybe even a third) pour of very freshly mixed concrete (and a cold day helps) is conducted slowly with lots of 'rodding' with spare rebar to break up any pouring joints and consolidate the concrete in the pipe.

Sounds complicated? It is. If you have the ($) and a contractor, and trust him to get it right with a good connection, I'd rather punt than run with the ball.

One design point. You don't want the bend in the rebar supporting the pipe. The rebar must (should, just ask some of the contractors around here) complete its bend to vertical well inside the wall of the pipe and at least a couple of inches below it. Nah... that's getting nit-picking on stresses that probably won't matter at all in this type of application...... time to shut up.

---------------------------------------

Subject: Permanent Pier Construction --part 7 of 7

From: Mike Dodd <mdodda_tmindspring.com>

>Of course you want your floor to sit on the ground and have a gap between
>it and the pier. If you need to seal this gap use something very flexible
>like a rubber camp mat. I think your can buy foam insulation that would work well.

Or don't seal it. I decided to provide for ventilation. Heck, the roll-off roof already has plenty of air gaps (plus we included a ridge vent), and there's nothing but sky when it's rolled off. To get cooler air from under the observatory, I affixed plastic window screen between the pier and the floor cutout, using construction adhesive all around. I also added screened floor vents in three corners of the floor.

rule

Subject: Permanent Pier Design   Top

From: Gordon Lupien <glupiena_tnetsville.net>

The most solid pier design, which happens to be inexpensive and simple!

I just got in from the first use of my new pier! Very cool! I wanted to share it with you folks, because sometimes simple works best. A link to some pictures is at the bottom. This pier design is as simple as possible, while being exceptionally impervious to vibration (based on Doc's info, thanks!!!) and very easy to align to a high level of accuracy with none of the problems I've read about with the wedge designs! I have not heard or read about this design, aside from seeing a very basic picture of a representative pier, so here goes! It's a no-brainer when you start to think about it a bit. What you want in a pier is a rigid, high mass structure, deep in the ground, and the most solid connection possible, direct from pier to telescope.

The pier is concrete all the way up, and it IS removable. It is 16" in diameter and goes into the ground 4 feet. I used a 16" Sonotube that I stuck into the hole about 2 feet. The rest is just the auger hole filled with concrete. The pier stands at about 5 feet above ground level, so that I can make a platform and later an observatory. If you are worried about movement, then do the 3 foot by 3 foot block at the bottom of the pier and make it a 24" Sonotube. The more concrete, the better. You want mass! The way I did it, you can rent a scoop and take the pier out of the ground easily if you had to (I had to, I'll explain later). Put a block down there, or make it too big and it's game-over on ever taking it out with ease. It is very stable as I designed it.

It has a set angle in it so that the top face is at an angle equal to that required by my latitude. (Thanks for the help with that question, folks, I appreciate it!) I made the angle by cutting ellipses out of two pieces of plywood and gluing them to the Sonotube. I then cut the 16" Sonotube at an angle equal to half the expected value by sawing in between the two plywood ellipses and then swinging the end part around 180 degrees and bolting it together again.

I then cut a 16" circle out of particle board and bolted five pieces of 2 foot, 5/8 threaded rod into it in a pattern that would allow for the base of my 12" LX200. Two on either side at the top, two on either side close to the bottom, and one right at the bottom all with about 2.5" sticking out of the 3/4" particle board.. Note that you end up having a nut in the concrete for each threaded rod, which is a good thing. Use two nuts and a large washer on the end that will be placed in the concrete for each rod. No need to bend the rods and such, just tighten the washer between the two nuts and sink that into the concrete. Note that you could get away with three rods, but for a positive mount, you need five.

The Sonotube assembly was put into the ground and the face was pointed north (corrected for magnetic declination, don't forget! Also make sure you use the magnetic declination correctly!). If I were to do this again, I would have made an assembly that fit on the surface of the Sonotube that had a finder scope. I would then simply adjust the face of the Sonotube until the finder scope was looking at Polaris. If you do this, and your finder scope assembly is done right (90 degrees to the face of the end of the Sonotube) then you should be all set. I did this later, after I found I was pointing the wrong way and I had to excavate the pier and turn it 18 degrees. (ouch) I used the finder scope assembly to verify the new angle of the pier. Note that excavating a pier and then back-filling it makes the pier weaker. It is still fine, though!

Fill the hole and tube with concrete and rebar. My pier took 20 80 lb. bags of concrete. The last step in filling the Sonotube (and hole) with concrete was to get those threaded rods into the concrete. Tack the particle board circle into the face of the Sonotube with the rods inside the Sonotube (so that it makes the mounting surface with the rods sticking out) and cut a flap on the top to completely fill the tube behind the particle board. Tamping concrete is a pain, but worth it, by the way! Tamp it after each bag and tamp all around the threaded rods. You will regret it if you don't spend the time to tamp!

While I was mixing, cutting, and pouring, a friend of mine was preparing a 1-1/4" thick aluminum plate that had holes to match the pattern of the threaded rods coming out of the pier. The holes in the aluminum plate are 3/4" holes, so there is slack between the holes and the threaded rod (5/8") when the plate is installed. A small amount of gasket takes up the space to center the holes on the rods, and is optional. A nut on the bottom and a nut on the top of the plate for each rod allows the plate to be adjusted +/- about 15 degrees in any direction for a total of 30+ degrees of swing in all directions.

A pattern is also cut into the plate with a slot at the top of the plate and two holes, one left and one right, that match up to the bolt pattern on the LX200 base. Put a bolt in the top hole of the scope, slide it into the slot and attach the scope firmly to the aluminum plate with all three bolts. The scope base is now aligned with the plate.

Once the scope is mounted on the aluminum plate, which was already on the threaded rods, you can adjust the angles of the plate very easily using a wrench and the top and bottom nuts on the threaded rods. You leave one pair of left/right rod nuts loose (top and bottom nuts away from the plate) and use the other set to adjust the horizontal angle, which should be very close if you used the finder on Polaris before pouring concrete. The bottom single rod/nut assembly adjusts the vertical angle. The scope has a tendency to want to fall off the threaded rods depending on your latitude, so you can use that to your advantage and leave the top two rods holding the plate on with nuts on top, while the bottom rod has a nut on the bottom of the plate. (all other nuts are in place, but those three nuts are the only ones touching the plate during adjustment, don't let your scope fall off!!!) You can then adjust the plate very easily using the three nuts described. A quick trial and error and you are on your way!

I drift aligned the assembly VERY EASILY (first try tonight and it took me 2 hours to get used to it and align it) and the LX200 centerED all objects I chose to slew to in polar mode on the first try! It was much more precise than my previous ALT-AZ attempts on a tripod by a long shot! WAY COOL, especially after reading about the nightmares some of you have had with polar alignment. I truly thought I was in for a nightmare! The nuts on the threaded rods allow for exceptionally accurate angle setting. It takes quite a bit of turning on a nut to get the angles to change, so you can tweak to your heart's content without having any problems with the amount of accuracy. Adjustment is totally smooth as silk.

Once the plate is set at the right orientation, simply tighten all nuts very tight to the plate and the plate becomes one with the concrete pier. NO JOKE! Nothing moves at all, except for the suddenly not-so-awesome fork design of the 12" LX200. I believe you need to make sure your soil is proper (not sandy), or make sure you do the extra concrete thing if you have any doubt.

Total cost of all materials obtained at your friendly hardware store (except aluminum plate):

Sonotube, 12' 16" diameter = $76
Concrete, 20 bags a_t $2.50 each = $50
Wood for Sonotube construction = $6
1-1/4" thick aluminum plate, 16" X 12" = $120 (buy it surplus if possible)
Rebar = $12
5/8" threaded rod, LX200 base 3/8" bolts, washers and 5/8" nuts = $30
One-man hydraulic auger, 14" bit, 1 day rental = $75

Total: $369 INSTALLED! Sounds like the price of a Superwedge.

You can also use ALT/AZ mode on this pier by adding an adapter plate. The adapter plate is simply the angle required to make your plate horizontal. You screw it onto the plate using the LX200 bolt pattern and mount the scope with a single bolt through the top plate. You can then turn this over and put it on your tripod as a wedge, if you design it right. Just make it slightly adjustable and you have a dual function AltAz adapter/polar tripod wedge for less than $200! I'll show you that when I get it done. It'll basically be four pieces of aluminum and a hand full of bolts.

EVERYTHING can be done using a run-of-the-mill band saw (For circular cuts) or table saw (for straight cuts) and a drill press. Aluminum can be cut with a wood cutting blade! (please talk to someone in-the-know before starting, there are hazards) and can be drilled with common drill bits. Use lubricant while cutting/drilling. Aside from that, all you need is a shovel, wheel barrow, hoe and some elbow grease.

rule

Subject: Telephone Pole as Permanent Pier? --part 1 of 3 Top Button

From: Doug Bennett <d_bennetta_tmweb.co.za> Date: Oct 2002

>John Ruthroff wrote
>I'm considering using part of a wooden telephone pole for a 'permanent'
>pier mount for my 12" LX200. This post is to invite comments from those
>that may have suggestions and corrections to make to my ideas and
>assumptions.

Gene Horr wrote :
>This should work quite well, if you are willing to live with a
>couple of limitations. The primary one is that you will get
>movement over time, requiring you to be occasionally adjusting
>PA (assuming that you are wanting to do imaging). The second
>is dealing with wood itself - moisture prevention, some sort.

At the time my house was being built, my architect took me on a site tour of one of his other projects where he had used telephone poles to support an external roofing structure. The weather on the Transvaal Highfeld is notoriously rough on exposed timber and I've always thought the method he devised to prevent moisture / weathering damage to the poles was both simple and ingenious.

Seemingly, most timber damage occurs due to water moving between the telephone pole and concrete footing. This leads to water pooling in the base of the footing and results in rot. In order to prevent this, his footings were constructed with a rough aggregate stone with a thin coating of concrete slurry. This results in a porous structure which allows water to drain away from the wood into the sub-soil.

Another (theoretical) advantage to this construction method is that the large spaces between the particles of aggregate would allow the footing to be demolished easily.

----------------------------------

Subject: Telephone Pole as Permanent Pier? --part 2

From: John Hilliard <messier27a_tearthlink.net>

Try the link below. I received this on another group and also a post from the "fabricator". He is very happy with it and uses it with a wedge for CCD imaging. We were looking at the wooden pier for exactly the same reasons as you. The generator has been very satisfied with it, has built a couple of them, and uses it inside an observatory.
<http://pages.cthome.net/astroleo/bequip.html>

----------------------------------

Subject: Telephone Pole as Permanent Pier? --part 3 of 3 Top

From: Clif Ashcraft <WA2GUFa_tnetscape.net>

I have used a wooden pier for my 10" LX50 for about 5 years now with completely satisfactory results. I made mine by joining four 8' long pressure treated 4x4s together with structural adhesive (liquid nails) and 6" long lag screws to make a sturdy composite beam with a 7" square cross section. I set it in concrete poured into a 12" diameter, 3' deep hole in the sandy, well drained soil of my backyard. I sawed the top of the beam off at an angle equal to my latitude and oriented to approximately face the pole, and to this angled surface I attached a plate made from two laminated thicknesses of 3/4" oak plywood. The telescope is attached to this plate with three bolts going into the threaded holes in the base of the mounting and three push screws held in the oak plate with tee nuts. Adjustment of the push-pull screws allows precise adjustment of the orientation of the polar axis, albeit not so convenient as some of the super wedges I have read about in this discussion group. Once adjusted it has been stable.

The pier passes through the floor of my tilt-off roof observatory, not touching it at any point. Vibration isolation is excellent: no image movement is detectable when someone walks or even jumps on the floor while I'm observing. Damping of vibrations resulting from accidentally bumping the pier itself is also rapid. If I were to try to improve the pier, I would probably try to make the polar axis adjustment a bit easier than using push pull screws to do the job, but since I don't schlepp the telescope around much, I don't have to make the polar axis adjustment very often. I have not regretted making the pier out of wood.

rule

Subject: Pier Design?--Another Viewpoint--Use a Tripod

From: Mark de Regt <deregta_tearthlink.net>

I have pondered this question ad nauseam in my own case. My difficulty is that my best viewing/imaging place is in my front yard, and a permanent pier is not an attractive option for one's front yard. On the other hand, if I have to polar align from scratch each time, I simply won't ever do it. As an effort to cut the Gordian Knot, I bought a used Meade giant tripod on Astromart for $375 (my scope is a 10" LX200); I put my Milburn Wedge on that, and I leave that outside almost all the time, suitably covered. During our Seattle monsoon season (October through May or June), I put the scope on the tripod/wedge on the first decent night around the third quarter moon (if there is a decent night), and leave it on until just after the first quarter moon. The giant tripod is a true monster, and does not seem to move when I remove and replace the scope, since polar alignment is a 15 minute tweak. It is rock-steady, as my images show, allowing me to image at f/12 with my LX200 (with an AO-7), and, if necessary, I can have a clean front yard on short notice. I am very happy with this arrangement, which may work for some others. Also see this post in the Superwedge topic.

----------------------------------------------------------

Subject: Pier Design?--Another Viewpoint--Use Wood  Top Button

From: Bill Arnett <billa_tnineplanets.org> Date: Dec 2002

Clif Ashcraft wrote:
> Or just use wood... ÍMy wooden pier of 4 glued and screwed together
> pressure treated 4x4s of yellow pine has worked very well for several
> years (set into concrete).

I did a similar thing with redwood before I built my observatory. The truth is the 4 cheap 4x4s worked just as well as the fancy custom steel pipe.

rule

Subject: Proper Material to Fill Steel Pier --part 1 of 4  Top Button

From: Doc G

> IMO, the damping characteristics of the material used, is more important
> than the mass. But the best person to ask would be Doc G.

I appreciate the thought, but I am mostly a physicist and electrical engineer. Of course I have spent 40 years in solving vibration problems as a consultant in industry. But that admitted. I have to say that these seemingly simple mechanical systems are really quite complex.

The characteristics of a mass on a post are the resonant frequencies of the structure and the Q of the resonances. One balances these by means of the mass, the stiffens and the damping of the materials. For a telescope on a post, it is reasonable to think of the system as first order. That is a mass on the end of a lateral spring. The mass is the telescope, wedge and that stuff. The spring is bendability of the post.

The resonant frequencies are also very important. Most telescopes, especially the LX fork types tend to resonate at frequencies in the 2 to 20 Hz range. This means it is essential to keep the pier from resonating at these frequencies and feeding energy into the telescope.

Much confusion comes about when you kick or pound on the pier. The audible frequencies are above 20 Hz. Thus if the post rings audibly, the motion that causes this ringing probably is so small that it does not affect the image at all. But, the audible sound is an indicator of the amount of damping in the system. This might have a strong affect at all frequencies.

More mass lowers the resonant frequency and more stiffness raises the resonant frequency. So when you make the pier stiffer you also make it heavier and it is hard to predict what will happen to the resonant frequency. This adds confusion to what you need to do to make a "solid" pier.

In the case of a concrete pier the mass is large, the stiffness is large and the damping is also large. That is why I always recommend a big concrete pier. These piers are almost always perfect as long as they are stuck well into the ground.

Steel piers are a much more difficult problem to evaluate and solve. Steel generally has poor damping. That is why it rings when you strike it. But when filled with concrete, they become well damped and no longer ring. There again it is the concrete that gives the damping. So this is a good idea.

Both steel tubes and concrete columns generally have enough stiffness in themselves to support a telescope. Assuming they are not more than a few feet long and of at lease 6" diameter. A principle problem with longer piers is that they are not stuck into the ground far enough. If 1/3 of their length is in the ground and the ground is reasonably stiff and stable, the pier will be quite solid. But here again, sticking the pipe into a block of concrete is a good idea.

A major design problem is with steel pipes that are mounted on top of a concrete slab. In this case it is very difficult to get the flange strong enough so that it does not create springiness in the structure. A large welded flange with welded side supports is almost always required to get this type of mount strong at the base. Also this resulting frequency of vibration is at a very low frequency which might pump energy into the natural resonances of the telescope structure. This is really bad news.

If the pier sways from side to side, no amount of stuff inside the pipe will fix the pier. It might sound better but it will still sway back and forth at some low frequency which is damaging to viewing.

The way to test for the low frequency problems is to give the pier a sharp push and watch the image. In the final analysis it is very easy to test the pier and scope for vibration. Look at a star and have a friend batter the structure. If the star moves, it moves and you need to add stiffness. If it move for a long time you need to add damping as well.

It is very hard to add enough damping outside of the basic material. And the lower the frequency, the poorer damping works. Again I recommend using materials that have built in damping. Use heavy strong stuff that does not by itself ring. Concrete is nice! But sand is a total illusion. It has no strength and contributes nothing to the low frequency characteristics of a pier. (generally a hollow steel pipe.) The sand damps the sonic (audible frequencies) and makes the tube "sound" dead. While this is a gratifying result, it is a placebo as far as the important mechanical characteristics are concerned.

These are just the briefest basics of vibrating systems. Fortunately many solutions both simple and elaborate work. In 95 percent of the cases I have seen, the pier and the wedge are not the problem. The mount and its bearings are.

----------------------------------

> I'm wondering about the base of my pier, though. It is a square plate of
> 3/4" steel attached with big gussets to the tube. It attaches with three 1"
> bolts firmly set into the concrete footing. I have it a couple inches above
> the concrete captured between 3 nuts on each bolt. I'm wondering if the
> bolts might be vibrating. Would it help to sandwich a piece of wood between
> the concrete and the steel pier base plate? Perhaps also with some more
> sorbothane?

The base is where the forces are greatest. Your arrangement is excellent for leveling, but is very likely a weak spot for flexing. I suggest that you put some blocks of hard wood and some steel wedges under the base plate in several places and hammer them in place this will stiffen up the structure quite well. You can also tell if the base is strong since if the wedging moves the pier at all the base is the flexure point.

I had a case somewhat like this where a large home made equatorial mount was set on three bolts. (this on a 16" scope) The mount was made of 3/4 in thick aluminum plates, with cross pieces and lots of 1/2 inch bolts. Astonishingly when slewing, the scope could be seen to wobble from side to side. Steel wedges, buffered with a thin sheet of hard wood, driven under the sides, between the bottom aluminum plate and the concrete pier solved the problem.

Never underestimate what the large forces at the stress point can do to seemingly strong structures. We are talking about arc second flexure in scope mounts.

---------------------------------------

Subject: Proper Material to Fill Steel Pier --part 2   Top Button

From: Radu Corlan <rcorlana_tprofis.ro>

> I'm going to put in a pier this spring, and for reasons I won't get into,
> I'm left with one option, a large concrete base with a locally made steel
> tube pier bolted to it. The concrete base and a few inches of the steel pier
> will be below ground.
> To help keep the vibrations down, I had planned on taking a rectangular
> (8x36") vinyl bladder (the ones normally filled with water and used to hold
> winter pool covers down), putting it inside the tube and filling the bladder
> about 3/4 full of a 50/50 water-antifreeze mix. I figure it would last
> several years before it would need to be inspected and possibly replaced.
> I'm certainly not an engineer but I assumed that water would be a relatively
> decent medium to absorb any low frequency vibrations. Does that sound right
> or does it sound more like a waste of effort?

I made a steel pier some time ago and thought a lot about it - that's why I can't resist not adding to this thread.

A reasonably thick steel tube is very rigid. I think that a tube that has a length-diameter ratio less than 6-7 and is not very thin-walled is essentially rigid (compared to the rest of the system). So, filling the tube doesn't help very much. It helps damping sonic vibrations (which don't matter too much anyway). From a structural point of view, filling the pier, will increase the mass more than it increases the stiffness, so the net result is that it pushes the resonant frequency down (and that is not what you want).

The most important vibration mode of a pier-telescope system is it's first order - the one when the pier and tube all arch in the same direction at one time. It's important to imagine where flexure can appear in this mode. I would bet most of it appears where the tube connects to the flange. Imagine the structure built of cardboard - most of the flexure will manifest itself by a warp in the base flange. You need a thick flange, and as many fillets as practical - rely on the stiffness of the flange to the minimal extent possible.

My pier has a 40'' high tube that is 7'' in diameter (and has 3/8'' thick walls). The base flange is 20'' square, made of 1/2'' thick steel plate. There are 8 fillets that go all the way to the edge of the flange. The fillets are made also of 1/2'' plate.

I didn't mount the thing yet, but it looks very rigid. Since the pier is a little short (i couldn't make it longer because i didn't have the right diameter of tube), i plan to make the concrete base higher than the floor level.

The concrete base needs to be buried down to below the freezing depth (1.10m in my case). I will pour a slab around the base, with some space between, in the hope that it will isolate the pier from the movement of people around.

The pier rings a little when you knock it. I plan to wrap it in a polyurethane blanket, not as much for damping, but to avoid it being kicked in the first place. I don't intend to fill the pier with anything.

---------------------------------------

Subject: Proper Material to Fill Steel Pier --part 3   Top Button

From: John Hopper <JohnLX200a_taol.com>

Pier-makers can use filling in their pier to attack even that more basic pier-design problem. At some point, weight and cost do come into the picture for piers, or we'd all make them with 2" thick steel walls and so forth. For a given weight (hence price) of metal, a larger-diameter pier is stiffer despite the correspondingly thinner walls. At you get to large-diameter, thin-walled piers/tubes/pipes/whatever, the stiffness is getting better but you start looking at other problems coming into play, including sound vibrations and buckling. I defy you to actually bend a 55-gallon drum with your bare hands (or by almost any other means without it first denting or buckling) but it's pretty easy to make it vibrate "like a drum" very badly, and it's very possible to dent/buckle it by kicking it, hitting it with a hammer, or whatever. Good luck buckling it if it's reinforced internally with plywood rings or ribs, much less if it's tightly filled with concrete, wood scraps hammered into place, solid cork, or whatever. It won't make the same booming drum sound either.

Filled with sand, it will be more difficult to kick a dent into it toward the bottom, but still pretty easy toward the top. The water or sand methods rely upon gravity, so they create their full effect only at the bottom, and zero effect at the very top.

If you don't have a 55 gallon drum handy, try the experiment with a metal garbage can, metal wastebasket, or large coffee can. Short ones won't demonstrate the effect of sand or water very well.

Bottom line: put in as heavy a pier as you can tolerate, spread it out to as large a diameter as you can tolerate, being sure not to make it so large/thin that it will dent or buckle, then internally reinforce it with (preferably lighter) strong stuff that doesn't bounce very high when you drop it, or bulge in one direction when you squeeze it in another direction.

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Subject: Proper Material to Fill Steel Pier --part 4 of 4   Top Button

From: Jim Slay <JimSlayNTSa_taol.com>

The footing is the key element I believe. I used a 16" diameter steel road culvert 12.5' long (cost $150) supported in a 60" by 60" footing by 60 to 65" deep (a lot of digging and concrete over 10 tons) I also used 3/4" rebar 10' long with most of it pounded into ground at various angles, total of 200#lbs. The culvert was set 5.5' deep or to the bottom of footing and had six sections of rebar set inside, the culvert was filled to with in 1.5' of top and later a four post 3'x3' structural steel leveling pad was added that added another 4' of height to pier.

In use, the only vibration observed has been from a heavy semi going down the street but very rarely in the night, I can walk and move around observatory and see no vibration in eyepiece (12 mm with 3X Barlow = 762X used for drift alignment) or when dome turns 1 deg. every 4 minutes. Pier is isolated from observatory 100%, if I pull on pier or kick pier, I will see some movement but very little. For details and pictures of Jim's pier click here.

rule

Subject: Typical Dimension Pier Mounted 12" LX200   Top Button

From: Rod Cook <Cookroda_tgurulink.com>

With my 12" OTA on the tripod and superwedge, the centerline (cL) of the Dec. axis is 65" above the ground. This puts the eyepiece too high for me at 5'9" for all viewing without rotating the diagonal. From the bottom of the wedge to the Dec. cL is 26" on my 12" LX at 40 N. Your height of 34" puts the Dec. cL at 60". I have seen another reference that shows the Dec. cL at 54". As the position of the scope is lowered, there is a tradeoff between the ability to see lower to horizons versus the height of the observatory sidewalls. Somewhere is this range is an optimum height that I will need decide upon, but obvious is a compromise.

Bert Katzung wrote:

I'm not sure what the height of my LX OTA is above the floor, but the top of my pier, on which I put a Meade Superwedge and then the scope, is 34" above the floor. You can see it at

<http://www.astronomy-images.com/observatory/pier_&_mounting_plates.htm>

and that 34" is to the top of the pier mounting plates, not the top of the concrete base. Of course, when you add the wedge and the scope base, the eyepiece is going to be higher than 34". This works pretty well for me (I'm about 6 foot but don't mind bending over).

rule

Subject: Permanent Pier--Dampening Vs. Stiffness   Top Button

From: Doc G, Date: May 2001

Doug S. wrote:
>> Doc G writes:
>> Concrete damps just as well as sand. Concrete adds stiffness. In a
>> pier, stiffness is important while damping is more cosmetic. >>

> I'm not clear what you mean by "cosmetic", Doc, but I agree that the pier in
> ANY carefully and solidly mounted scope (whatever the materials used) is not
> likely to be the source of much vibration. We have next to zero with our 16" LX200.

What I mean by "cosmetic" is this. I have often run into vibration problems where, when you bang on the system, it rings audibly. These vibrations might be annoying to the ear, but they seldom actually contribute to "vibration problems." The reason is that these are audible, high frequency, vibrations and there is almost no significant motion of the system (amplitude). This is especially true of telescopes. The steel pipe of a pier might "ring" when you hammer on it. But the "real" problems are with the motions of the pier in the range from a fraction of a Hertz to about 20 Hertz or so. You do not hear these, but they do the damage.

For example, the LX scopes, smaller ones, have fork resonances in the 6 to 20 Hz range. These are largely due to the mass of the OTA and the springiness in the bearings. These oscillations, which we cannot hear, are the ones we need to control. Similarly in a pier, we want to get at the sub 1 Hz resonances. The only way to do this is to make the pier stiffer. It is almost impossible to dampen these very low frequency oscillations because damping is dependent on the first time derivative of the motion, that is the velocity component. At low frequencies the velocity, and for that matter, the acceleration components of the force equation are very small. The only one you can work on is the first term (amplitude times stiffness). That means that if you want to make the amplitude small, you have to make the stiffness large.

At frequencies below resonance everything depends on stiffness. At frequencies above resonance mass controls the motion. At resonance, of course damping becomes important. The problem is that we are dealing with a parallel resonant system. That means you have to get one end of the damping element connected to ground (the earth). It is almost impossible to find a mechanical way to do this. Sand in the pier does not do it.

Damping sonic vibrations is satisfying, and very easy, but it does not reduce the very low frequency oscillations which are the killers of the image.

So I preach, stiffness and more stiffness. It works.

But as we have seen many times in Mapug threads, The piers are generally MUCH better that the mounts. This is especially true of fork mounts.

rule

Subject: LeSeuer Pier Recommendation & Other Pier Vendors  Top Button

From: Gerald Miller <gmillera_toknet1.net>

We use the 8" LeSeuer Astropier <http://www.astropier.com/>. Highly recommended, very well made, plus the Polaris mount is better than any other in my opinion; not only is it just as sturdy as any other mount, but the off center mounting allows imaging north of the zenith to a much greater extent, and the elevation and RA adjustments couldn't be any finer. It is kind of pricey, though. But it is certainly very well made and finished.

Editor's note: also checkout these commercial units:
Pier-Tech fixed & variable height piers: <http://pier-tech.com/>
Clear Skys Pier: <http://www.clearskysinc.com/pier.htm>
Galaxy Pier: <http://www.galaxypier.com>
Advanced Telescope Systems: <http://www.advancedtelescopes.com>

rule

Subject: Enlarging/Extended a Permanent Pier  Top Button

From: Mike Dodd <mdodda_tmindspring.com>

Last spring my wife and I built a roll-off roof observatory with an 8" concrete pier. Alas, the pier was too short and less stable than we had hoped, so this month we enlarged it to 12" diameter and added about 14" to the height -- all with the observatory in place around it.

I've posted the story and some photos on my Web site, if anyone's interested in this. There's also some information about using Corian for a scope mounting platform. Point your browser to <www.astronomy.mdodd.com/>

rule

Subject: Initial Orienting of a Permanent Pier   Top Button

From: Shawn Kelly <Kelly_Shawna_tsolarturbines.com>

One easy thing you can do is to use the LX to tell you. Set it up and align AltAz (even if its just on a table-top) and have it point due north by setting circle and level the tube (or lower). Put a stake out as far as you can or up to 50 ft. or so centered in the EP. A second person makes this much easier. Do the same facing south. Run a string between them and you will have a very accurate N/S line that should run right under the scope. You can then offset from that line as needed for each scope on each pier or perhaps even install a permanent marker on the ground for each pier. It'll get you plenty close enough to set up the wedge.

 

rule

Subject: Concrete to Permanent Pier Bolts   Top Button

From: Adam Stuart <amstuarta_tsprintmail.com>

Go to Le Sueur AstroPier website <http://www.astropier.com/> and look at the honeycomb templates used with the J-bolts. I installed my pier two months ago in concrete, isolated from my observatory foundation, and it is solid.

----- Original Message -----
From: Bill Arnett
> Paul wrote:
> > I am in the process of having a steel pier made, and am considering the
> > options available for bolting it down onto a concrete footing (I wish to
> > have the option of relocating the pier at a later stage). Are there any
> > pitfalls / disadvantages with simply embedding threaded rod into concrete -
> > upon which the pier will later be bolted ? I have a vision of them
> > becoming loose over time. Could someone please clarify this for me -
> > and/or provide a better solution ?
> Just make sure you either use J-bolts or put some big washers and nuts
> on the ends to be embedded. And cover the exposed threads somehow when
> you pour the concrete to keep them clean; otherwise you'll have a boring
> job with a wire brush...

rule

Subject: Pier Question Involving Concrete   Top Button

From: Roger Hamlett <ttelmaha_tntlworld.com>

There are two seperate processes involved. One is 'drying', and the other is 'setting'. The latter is actually triggered by the water, but needs _very_ little. In fact concrete that is mixed very dry, will end up stronger than a wetter mix, but because it 'flows' less well, is harder to compact without voids. This is why on 'professional' concrete work, the mix is often very dry by 'handmix' standards, but then vibration, and special compacting tools are used to give a really strong result. Now for the sort of 'mass' concrete needed on a pier, there is no cause to get involved in this sort of complexity, and a mix that flows well, will be much easier to work with.

If the mix 'dries' completely, before it 'sets', then it will be weaker than a mix where the water has remained in place long enough for the chemical reaction to complete. You can cover the concrete to prevent direct sunlight from drying the top, and if plastic is used to do this, the 'damp' is kept in the mix.

I agree wholeheartedly, with the recommendation to hire/borrow a mixer, but it is also worth looking at easier solutions (provided you have a couple of people and wheelbarrows available to move the mix quickly). If (for instance) there is a building job going on locally, then it is well worth talking to the contractors supplying the pre-mixed concrete (especially for things like foundations, where the mix will not have any 'special' additives). The lorries doing this, typically deliver six to eight cubic metres of 'mix'. Now if they are being asked to do a job needing (say) 4m^3, you can do a deal where the 'spare' load is delivered to you, for a lot less than it would normally cost, provided you are prepared to take it at a time that suits them... I did this for a pier base, and had a tarpaulin ready to take the mix. My base needed 2.5m^3 (one cubic yard is not a lot, double check your calculations here, since you do not want to be 'wrong' when having a load like this delivered...). Then with three people, one shovelling into the barrows, and two wheeling the mix to the rear, and pouring it into the hole, the entire mix was moved in less than an hour. You will find it better (as the replier sayes), to actually support your pipe properly, and get it held firmly in place, before starting to pour. Basically you want to have as little as possible to do except move concrete!. I 'hand cast' a 6" base, at the bottom of my hole, then using stones, positioned the tube where I wanted it. Then built a timber frame to hold the top of the tube, so it was straight. Only once I was certain the tube was straight, and would stay that way if I 'leant' on it really hard, did I start with the main concrete. You need a support like this, since otherwise the tube _will_ move, even if you insert it into the concrete after pouring.

rule

Subject: Pier Mounting Plates: One vs. Two --part 1 of 5  Top Button

From: Jerry Stephenson <stephejla_tsongs.sce.com> Date: Mar 2002

For a steel pier, two plates at the top provides for nice adjustability but adds weight. I would instead try to use just one plate at the top to lessen the weight. The more weight you have on top of your pier, the more the rigidity is challenged. Your scope, guidescope, camera, wedge, etc. will be heavy enough. I would first try to make sure the top plate is welded very squarely to the pipe. Then, I would use the bottom attaching bolts to level to the top plate. As I've posted before, it is very important not to try to pull your plate down against the top of your concrete footing/anchor. Instead, use jamb nuts on top and bottom of the bottom plate to float it above the concrete. This eliminates the possibility of a "soft foot" associated with a bump in the middle of the concrete keeping the corners from contacting. This also, conveniently provides a leveling adjustment. If you want your adjustment axes to be NS and EW, put the CORNERS of your bottom plate at the compass points, rather than the flat sides. This lets you adjust across the diagonals with only two bolts where the other configuration requires a more tedious adjustment.

In a concrete pier, the same could be done by floating your single plate above the top and using the 4 bolts to adjust it. Not that two plates is bad, I just think that if you can make one plate work, you eliminate a little weight. I thought the Corian Cutting boards was a novel idea. Modern materials are getting stronger and cheaper all the time.

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Subject: Pier Mounting Plates: One vs Two --part 2   Top Button

From: Mike Dodd <mdodda_tmindspring.com>

Jerry Stephenson wrote:
> Not that two plates is bad, I just think that if you can
> make one plate work, you eliminate a little weight.

The trick to "making one plate work" is to build a pier large enough to space the plate-mounting bolts far enough so they clear the scope base or wedge that you plan to mount. One plate doesn't work if the mounting bolts interfere with the stuff on top of the plate.

I'm slowly moving to the opinion that a 16" or 18" pier is worth the effort. That should be large enough for the bolts to clear the scope/wedge, and it sure improves the stability. We started with an 8" pier, then increased it to 12". If we had just gone to 18", we could have used a single plate. But we were concerned about removing it, should we ever lose interest or sell the house. Not a good compromise, IMHO.

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Subject: Pier Mounting Plates: One vs Two --part 3   Top Button

From: Dale Hooper <Dale.Hoopera_tsdl.usu.edu>

Unless I've missed something, if you only used one plate - you would either need a BIG pier, a "small" wedge, or jam nuts. In other words, the bolts on your pier are at most a bit less far apart than the diameter of the pier. I.e. if you have a 12" diameter pier your bolts are less then 12" apart. The bottom line is that with one plate unless the pier is really big your wedge would have to rest on top of the bolts coming from the pier. However, with jam nuts I suppose you could make it so that the bolts didn't protrude above the plate. But ---

Personally, I used two plates because of the aforementioned alignment, level, and concrete issues. The bottom plate is steel and the top one is a really nice aluminum one that Ken Milburn made for me. It's just as beautiful as the wedges he builds.

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Subject: Pier Mounting Plates: One vs Two --part 4

From: Mike Fuller <mfuller1a_tsatx.rr.com>

I also used the 2 plate configuration and have been very happy with it.  <http://fullerastropix.web.com/>

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Subject: Pier Mounting Plates: One vs Two --part 5 of 5   Top Button

From: Frank Loch

I favor and have used the 2 plate approach. My plates are 3/4" Aluminum. The bottom plate attaches to 6 - 3/4" bolts which set 18" into the concrete pier (14" diameter). You can see pics of this at: <www.lochsjewelers.com/observatory.htm>

Editor's note: to see the construction details on a 2-plate mounting system, go to the SuperWedge Topic.

rule

Subject: Grounding the Pier

From: Doc G

----- Original Message -----
> I am in the last stages of construction of my observatory and have
> installed 110v outlet up the concrete pier and am considering
> anchoring the outlet to the metal pier I am having constructed on top
> of it. Basically the metal outlet box will be fastened to the metal
> pier via sheet metal screws.
>
> QUESTION: Are there any concerns in doing this, as I am sure that
> this will also ground the base of the LX200?

You should ground everything in sight. But, remember that you must not ground the negative side of the LX200 power supply. This point in the circuit is actually "floating" below ground by a few tenths of a volt. Especially when using batteries to power the scope, you must not use the batteries to power any other equipment.

This is a well know problem with the LX200 classic. It is the result of measuring the current, the red bar indicator, with a "looking" resistor of value 0.1 ohms. This is a basic design flaw in the LX200 classic electronics.

It is perfectly fine to plug the Meade power supply into the grounded outlet. In fact that is the best way to power it. You will then plug the power from the supply into the power input to the telescope. That too is correct. The body of the telescope is connected to the true electronic ground of the scope. This is an internal ground reference point. The telescope base (body) is normally grounded through the wedge and pier in any case.

The problem that many have had is when an external battery is used for the scope and also for other equipment like a CCD camera. If you power every piece of equipment from the power supply designed for that equipment you will be fine. I also recommend a ground connection to all other metal parts of the observatory that are reachable. The power line to the observatory should also have a ground lead. For additional safety a GFI circuit breaker for the power to the observatory is a good idea.

I also recommend use of high capacity outlet strips with surge protectors. A good surge protector will save electronic equipment in some cases. They are not perfect, but better than nothing. It should also be pointed out that surge protectors are not forever good. They wear out if they are called to action very often. If you use one, advised to do so, get one with a large capacity. It might cost $60 but it is worth the cost to protect your expensive electronics.

Editor's note: also see the Grounding topic.

rule

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