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Equipment Discussions >> Observatories

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Christopher EricksonModerator
Carpal Tunnel
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Reged: 05/08/06

Loc: Waikoloa Village, Hawaii
Re: Pier diameter? new [Re: JJK]
      #6042380 - 08/23/13 07:52 PM Attachment (12 downloads)

Here is another variation on the X pier that has less mass.

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TheSheriff
sage
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Reged: 07/15/07

Loc: Yellow n Green, Duck Country
Re: Pier diameter? new [Re: JJK]
      #6042627 - 08/23/13 10:37 PM

Quote:

Quote:

Quote:

Hey Dan,

How did you bolt your pier plate to that stack of bricks? Pictures?




Haven't gotten that far yet. I'm gonna try to make more progress this weekend but the plan is to stuff some newspaper in the voids of the blocks then pour concrete in just the upper block then set anchor bolts for the pier plates that the mount will bolt to. We'll see how it goes!




Why not stuff the voids with broken pieces of blocks or clean stones?




I'm an admitted "Overkill" type, but after all that work, why not just fill the entire void with concrete? Or at least, fill all but the top foot with damp sand and crete the remainder.


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roscoe
curmudgeon
*****

Reged: 02/04/09

Loc: NW Mass, inches from VT
Re: Pier diameter? new [Re: TheSheriff]
      #6043159 - 08/24/13 08:37 AM

Chris,
Looking at your latest sketch reminded me of the look of a z-braced radio tower - how well do you think a short section of something like Rohn 25 would work as a pier top?
Russ


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dawziecat
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
Re: Pier diameter? new [Re: tomcody]
      #6043203 - 08/24/13 09:12 AM Attachment (6 downloads)

Quote:

If it were me, I would use a metal pier ( I like ATS piers) as you can easily adjust ( level) if the the foundation shifts or settles.
But a properly sized concrete pier will work fine ( Dan,s piers offer good kits for concrete piers).
Rex




Many thanks for the tip on Dan's Piers, Rex. I had not heard of them.

I have found a contractor to do the concrete work. Furthermore, he says he can get his 4X4 into the site on the very rough track the excavator has made. It just may be necessary the excavator haul the truck back out!

The budget will not allow one of those very fine ATS piers.

So I plan on a 12" concrete pier, topped by a plate from Dan's Piers that will directly accept the Astrophysics 119FSA adapter for the AP1100.


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Midnight Dan
Postmaster
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Reged: 01/23/08

Loc: Hilton, NY, Yellow Zone (Bortl...
Re: Pier diameter? new [Re: dawziecat]
      #6043281 - 08/24/13 10:14 AM

Good plan! I think that should work well for you. Don't forget the rebar in the concrete. 2 pieces should be plenty, but many people use 3.

-Dan


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dawziecat
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
Re: Pier diameter? new [Re: Midnight Dan]
      #6043329 - 08/24/13 10:40 AM

Quote:

Good plan! I think that should work well for you. Don't forget the rebar in the concrete. 2 pieces should be plenty, but many people use 3.

-Dan




Hmmm . . . is rebar really necessary?


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Midnight Dan
Postmaster
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Reged: 01/23/08

Loc: Hilton, NY, Yellow Zone (Bortl...
Re: Pier diameter? new [Re: dawziecat]
      #6043352 - 08/24/13 10:49 AM

Absolutely! Concrete is extremely strong in compression so rebar will not help one bit in supporting the weight of your gear. But, it is very weak in tension or shear, which means it can easily break due to relatively small sideways forces such as movement of the ground in freeze-thaw cycles.

-Dan


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dawziecat
Carpal Tunnel
*****

Reged: 10/20/10

Loc: Rural Nova Scotia
Re: Pier diameter? new [Re: Midnight Dan]
      #6043384 - 08/24/13 11:10 AM

Quote:

Good plan! I think that should work well for you. Don't forget the rebar in the concrete. 2 pieces should be plenty, but many people use 3.

-Dan




Hmmm . . . is rebar really necessary? I hadn't planned on it. I don't want to mess with the stuff unless I really have to!
I read all about it adding strength against shearing forces but they're not really a concern. All I see rebar doing is getting in the way of properly seating the L bolts and allowing voids to form in the concrete?


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dawziecat
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
Re: Pier diameter? new [Re: dawziecat]
      #6043394 - 08/24/13 11:16 AM

Whoops . . . sort of a double posting done inadvertently.

Thanks Dan, I guess a couple lengths of rebar will be necessary. Not happy about it but . . .


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tim57064
Pooh-Bah
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Reged: 08/23/12

Loc: Southeast South Dakota,USA
Re: Pier diameter? new [Re: dawziecat]
      #6043554 - 08/24/13 12:46 PM

If you do not use rebar,it will not be long before the pier breaks apart.It is not that expensive here yet not sure where you are located.I am sure though that it would be more expensive to replace the pier.

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mclewis1
Thread Killer
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Reged: 02/25/06

Loc: New Brunswick, Canada
Re: Pier diameter? new [Re: tim57064]
      #6043602 - 08/24/13 01:14 PM

Terry,

I'd use 3 pieces of rebar with some thin wire between the pieces to hold it in shape. Size the rebar length to go down into the base and up to a point just below the top surface. You shouldn't have any problem keeping it away from the sides or the points where you want your J bars. You'll also be able to use a long pole (broom handle or similar) down the middle of the rebar to help prevent any voids as you're pouring the concrete.

A slightly wetter mix (think runny but lumpy oatmeal) will also make the pour around the rebar and tamping to remove voids easier.


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Midnight Dan
Postmaster
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Reged: 01/23/08

Loc: Hilton, NY, Yellow Zone (Bortl...
Re: Pier diameter? new [Re: dawziecat]
      #6043647 - 08/24/13 01:43 PM

Quote:

... I guess a couple lengths of rebar will be necessary. Not happy about it but . . .




I think when you're done you'll find that the rebar was not a big deal at all compared to everything else. It's cheap, and easy to work with. Just buy some cheap steel wire to wrap around it hold it in the shape you want. It doesn't have to be fancy or pretty - it will be hidden inside the concrete.

For mine, I just used two pieces wired together like the photo below. Seems like most people in the threads here on CN use 3 pieces. Either way, it's probably the least of your worries when building an observatory.

I think the slightly running mix suggested above is a good idea. Mine was pretty stiff and I ended up with some small voids on the side - nothing that's a real problem, but it just didn't look as good as I'd like. Be SURE and agitate the concrete to help it flows everywhere in the form and helps to eliminate voids. I used another length of rebar to run it up and down in the mix and also side to side. I'd stop to agitate about every quarter of the pour.



-Dan


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HunterofPhotons
Pooh-Bah


Reged: 04/26/08

Loc: Rhode Island, USA
Re: Pier diameter? new [Re: tim57064]
      #6043650 - 08/24/13 01:43 PM

Quote:

If you do not use rebar,it will not be long before the pier breaks apart.....




OMG!!!!!!
When I think of all the concrete columns that I've poured without rebar it just makes me sick. I've left so many people with ticking time bombs.
Admittedly none of these columns have broken apart yet (even afters decades of use), but oh the humanity when they do shatter and collapse.

Concrete columns have been around for thousands of years.
I don't know of any that had rebar yet they are still standing.
The tension forces on our astronomical columns are virtually non-existent for all practical purposes. There is no need for rebar from that quarter.
Are there any situations where rebar may be needed?
The only one that comes to mind is if you have built your observatory on steeply sloping land that suffers from severe frost in the winter. In that instance rebar may save your pier from coming apart. It won't save it from being pushed out of plumb, but at least it will be in one piece.
Other than that you don't need rebar.
The ironic thing is that most of the rebar installations that I have seen in observatory forums are incorrect and they are causing problems rather than preventing them. They pound the rebar into the ground and pour on top of that. Rebar needs to be totally encased in concrete. When rebar gets wet, like from goundwater, it rusts and expands.
Guess what happens to the concrete. It splits open.

dan k.


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Christopher EricksonModerator
Carpal Tunnel
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Reged: 05/08/06

Loc: Waikoloa Village, Hawaii
Re: Pier diameter? new [Re: roscoe]
      #6043680 - 08/24/13 01:57 PM

Quote:

Chris,
Looking at your latest sketch reminded me of the look of a z-braced radio tower - how well do you think a short section of something like Rohn 25 would work as a pier top?
Russ




Rohn tower segments are designed to be ultra-light and a bit flexible for survivability in wind. My instincts say that they would be vulnerable to higher-frequency (50-500hz) sympathetic vibrations but that's only a guess.

The self-supporting Rohn tower segments would be a bit better than the guyed-tower segments.


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Mary B
Vendor - Echo Astronomy and Electronics
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Reged: 05/21/10

Loc: Minnesota
Re: Pier diameter? new [Re: Christopher Erickson]
      #6043749 - 08/24/13 02:37 PM

I had 30 feet of rohn 25 bracketed to the house. Talk about resonate and howl in the wind!!!

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rimcrazy
sage


Reged: 03/03/12

Loc: Overgaard, AZ
Re: Pier diameter? new [Re: dawziecat]
      #6044797 - 08/25/13 08:39 AM

I've posted pic's of this before. My pier is 10' high from concrete base to the top. 8' from the level of the floor. It sits on over 12,000 lbs of concrete on a 5x5x3 block with a 2'x2' round pier in the center. The second level up bolts to the concrete pier at the floor level with 8 bolts. There are 12 bolts that anchor the pier to the concrete at the bottom. Bolting the mid level up loads the beams to put tension on them to reduce vibration. The asymmetric design reduces/eliminates harmonic ringing. Using steel allows for rapid cooling and reaching thermal equilibrium.

Having to go 4' down is a bit tough for getting below the frost line. Mine is only 18" down which makes it much easier. Chris's X-Design is quite robust and would do a very good job for you. Obviously, you either need to be able to do the steel work yourself or have access to someone that can do that work for you.

Here is the CAD design:



I have a top plate adapter. This is a 1/2" plate steel waterjet cut from a CAD drawing. The large holes bolt the plate flush down to the top of the pier. The smaller holes are all tapped. Cost of the plate including waterjet was $90



It's designed to fit a Meade LX200 wedge and a AP1600GTO Flat surface adapter. The wedge is gone and I'm waiting for the AP1600 to be delivered. I don't see the need at all for level adjusting. It's not required and IMHO reduces the rigidity of the overall mount. My concrete was poured very level and the pier is quite square. After 10' you can see that the top wedge is only slightly off level.



Again, level is not required. What is required is accurate polar alignment.

Finished pier





Hopefully this gives you some ideas for what you might want to do.


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Midnight Dan
Postmaster
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Reged: 01/23/08

Loc: Hilton, NY, Yellow Zone (Bortl...
Re: Pier diameter? new [Re: HunterofPhotons]
      #6044909 - 08/25/13 10:17 AM

Quote:

OMG!!!!!! When I think of all the concrete columns that I've poured without rebar it just makes me sick ... oh the humanity when they do shatter and collapse ...

The tension forces on our astronomical columns are virtually non-existent for all practical purposes. There is no need for rebar from that quarter.




Sarcasm aside, tension definitely exists in-ground pier. It comes from two sources. The first is any sideways motion of the ground. When any thick object like a pier experiences a lateral force, the side away from the force is in compression while the side nearest the force is in tension.

Think the ground is stationary? Take a look at any farm fence built of wood poles stuck in the ground. If it's been there for more than a few years, the poles lean every which way due to to heavy rains, soil migration and settling (especially on sloping sites), and frost heaves. And that brings up the second tension force - frost heaves.

Frost heaves occur when the ground is saturated with water near the surface and then freezes. Since water expands when it freezes, it grips objects like piers tightly and then lifts them upward, again due to the expansion. This uplift force is one reason why its a good idea for the bottom of the pier, under the frost line, to be larger than the rest - to resist the uplift force and keep the pier in the ground. However, if you have a footer keeping the bottom stationary, and a frost heave trying to lift the top, you have - wait for it - tension!

Since the OP is in Nova Scotia, frost heaves area a very real concern and rebar in the concrete will eliminate that concern. Your experience with columns brings to mind the mind the old saying - just because something has been done and not failed, doesn't make it a good design. Will it fail if rebar is not used? Perhaps not. But putting rebar in is easy ... replacing a broken pier is not. Just my 2 cents, but I would not put in a pier without rebar.

-Dan


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HunterofPhotons
Pooh-Bah


Reged: 04/26/08

Loc: Rhode Island, USA
Re: Pier diameter? new [Re: Midnight Dan]
      #6045043 - 08/25/13 11:44 AM

Just out of curiosity, how many mount piers have you seen pulled apart by frost heaves?

dan k.


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Christopher EricksonModerator
Carpal Tunnel
*****

Reged: 05/08/06

Loc: Waikoloa Village, Hawaii
Re: Pier diameter? new [Re: rimcrazy]
      #6045125 - 08/25/13 12:45 PM

Excellent design!

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TimN
Pooh-Bah


Reged: 04/20/08

Loc: Ontario, Canada
Re: Pier diameter? new [Re: Christopher Erickson]
      #6045303 - 08/25/13 02:29 PM

I can't comment on other climates but the OP has a similar climate as mine. Concrete columns are used a lot for cottage supports etc. in my area and all installations follow 3 rules. 1. Go below frost line 2. Larger at the bottom 3. Use rebar.
The only exception on depth is when you connect to bedrock and even then you use rebar.

Soil and other conditions may mean all these steps aren't necessary but why take a chance with your pier? Rebar is so inexpensive and easy to install.


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