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Pier diameter?

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#51 mclewis1

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Posted 24 August 2013 - 12:14 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.

#52 Midnight Dan

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Posted 24 August 2013 - 12:43 PM

... 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.

Posted Image

-Dan

#53 HunterofPhotons

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Posted 24 August 2013 - 12:43 PM

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.

#54 Christopher Erickson

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Posted 24 August 2013 - 12:57 PM

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.

#55 Mary B

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Posted 24 August 2013 - 01:37 PM

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

#56 rimcrazy

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Posted 25 August 2013 - 07:39 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:

Posted Image

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

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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.

Posted Image

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

Finished pier

Posted Image

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

#57 Midnight Dan

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Posted 25 August 2013 - 09:17 AM

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

#58 HunterofPhotons

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Posted 25 August 2013 - 10:44 AM

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

dan k.

#59 Christopher Erickson

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Posted 25 August 2013 - 11:45 AM

Excellent design!

#60 TimN

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Posted 25 August 2013 - 01:29 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.

#61 Midnight Dan

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Posted 25 August 2013 - 02:06 PM

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

dan k.


None. The few piers I've seen have all had rebar in them and have not been pulled apart by frost heaves or any other reason.

-Dan

#62 roscoe

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Posted 25 August 2013 - 10:01 PM

[quote name="Midnight Dan"][quote]Just out of curiosity, how many mount piers have you seen pulled apart by frost heaves?

dan k. [/quote]


I'm a small-town contractor/carpenter, and I've seen quite a few that have pulled apart from frost action - mostly foundation piers under porches and decks. They normally seem to break about 6" below the ground surface, or near the bottom of a j-bolt or short piece of rebar put in to attach a post anchor to.

On the other hand, an 8" pier with a single 1/2" re-bar in it can be cracked, but cannot be broken fully or pulled apart, even if slammed with the bucket of a backhoe in an attempt to break it into pieces for removal. With 2 re-bars, they're hard to even crack, and must be in both cases dug up whole.

While they're not quite as rust-resistant or strong, I often use lengths of threaded rod for re-enforcement, because they serve both strength and anchor purposes at once.
Russ

#63 dawziecat

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Posted 26 August 2013 - 07:35 AM

I have ordered the top plate from Dan's Piers but I can not wait for delivery of the stainless steel L bolts. They are not available locally either. Dan suggested SS threaded rod with a couple of nuts on the end instead. My contractor suggested bending them but how can I bend 3/4" rod?

Another possibility are galvanized L bolts.

So, options are:

1/ SS threaded rod with nuts.
2/ SS rod "home bent" into an L shape (if possible!)
3/ Galvanized L bolts

Opinions?

#64 Midnight Dan

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Posted 26 August 2013 - 07:56 AM

Hi Terry:

You can get the stainless L-bolts at McMaster-Carr here:
http://www.mcmaster....l-bolts/=o8ciz1
BUT ... they are more expensive than the kit from Dan's. You might get away cheaper by asking Dan to overnight his kit to you. He includes the thin-profile jam nuts, stainless washers, and fiber washers that work well for attaching a mount.

Keep in mind that a normal L-bolt is designed to hold something, like a house foundation, tightly to the concrete. The force of the nut tightened against the foundation attempts to pull the bolt out of the concrete, so the "L" part is necessary to resist that pull.

However, your mount is a totally different situation. Most people suspend the mount above the concrete between two nuts on the threads. There is ZERO force trying to pull the bolt out of the concrete. In fact, just the opposite is true - the weight of the mount and optical gear are trying to push the bolts INTO the concrete. An "L" at the end can help a little by spreading out that weight, but it isn't really needed. Concrete is so strong in compression that a simple piece of threaded rod should work fine.

If you want to bend the end yourself, it should be easy to do. Just clamp the rod in a vice and beat on the end with a hammer, or use a pipe wrench to grab and bend the end. It doesn't have to be a 90° bend and it doesn't have to be pretty.

Personally, I'd stick with stainless instead of galvanized. The thread will be smoother which makes it easier to adjust the mount to level. Not sure about other brands, like the McMaster-Carr ones, but in the kit I got from Dan's Pier Plates, the bolts were amazing well made - more like fine machining than the normal crude bolts I'm used to seeing for foundation work.

-Dan

#65 Midnight Dan

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Posted 26 August 2013 - 08:05 AM

Just a reference regarding frost heaves and rebar in piers:

http://www.structure...-deck-footings/

The whole thing is pretty short and can be read quickly. But to get to the pertinent information, scroll down to the bottom and read the two paragraphs between the image of the sonotube with the concrete bell-bottom, and the drawing of the pier with rebar in it.

-Dan

#66 rimcrazy

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Posted 26 August 2013 - 08:14 AM

Just a quick note. If you use SS bolts, be sure you don't use SS nuts, or if you do remember to put anti-seize on the threads.

You can get locking threaded inserts that you simply put into a drilled hole in concrete. This is what I did on my pier. I had lots of bolts to put in (20) and there is no way you can locate that many bolts accurately to holes in the steel. We simply put the pier down, drilled the holes with an industrial drill, put in the threaded inserts and screwed bolts into the inserts. In our case, to be sure, we put a special concrete epoxy into the holes to be sure nothing came out. Probably unnecessary but there is no way the bolts are coming out.

#67 dawziecat

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Posted 26 August 2013 - 08:59 AM

Please don't forget that I am in a third world country! Sometimes that's how I feel about Canada! Getting things shipped here from the US is slow and prohibitively costly. And that's when the merchants agree to ship to Canada at all, and they often don't!

The industrial supplier here informs me I have virtually no chance of obtaining SS L bolts here! And I can't wait for all the delays of having them shipped to Canada from the US. If I could, I'd just order 'em from Dan's.

Now, what's this about not using SS bolts on SS rod lest they seize? Please tell me more about that. I've heard of Loctite, but never an "anti-seize" compound. Teflon tape? Plumber's joint compound?

PS: I have ordered the SS L bolts from McMaster-Carr. We'll see what happens. :question:
Thanks for that link, Dan.

#68 rimcrazy

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Posted 26 August 2013 - 10:40 AM

If you put a SS bolt with a SS nut and you do not use an anti-seize compound you get what acts like welding of the nut to the bolt. It's not really a weld per se but the metal is soft enough that the heat generated by screwing a nut on to a bolt will cause spalling at the surface which, for all intent and purpose, is a weld.

Posted Image

I neglected to do this or rather did not realize the nut I was using (came from the proverbial collection of random nuts in a jar in the shop) was SS and I did not use anti-seize. You can see how far the nut got on to the bolt before it totally seized up. I needed an 18" breaker bar with a pipe to get it off... or rather snap the bolt in this case. I keep this on the mount to remind me of my stupidity.

The simplest way around this is just use anodized or zinc coated nuts if you use SS bolts. The same goes for vice-versa. If you have SS nuts just be sure to use anodized or zinc coated bolts and you won't have a problem. This also happens with aluminum to aluminum bolts and nuts, etc.

#69 dawziecat

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Posted 26 August 2013 - 11:57 AM

Thanks for that Rimcrazy. I'll be sure it doesn't happen.

McMaster-Carr cancelled my order to Canada. :mad:
What I don't understand is why they offer drop-down menus to select other countries in the first place when they won't accept orders from anything but the US?

I think I'll move to Botswana!

#70 roscoe

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Posted 26 August 2013 - 12:03 PM

Just a reference regarding frost heaves and rebar in piers:

http://www.structure...-deck-footings/

The whole thing is pretty short and can be read quickly. But to get to the pertinent information, scroll down to the bottom and read the two paragraphs between the image of the sonotube with the concrete bell-bottom, and the drawing of the pier with rebar in it.

-Dan


An excellent description of the situation!

And....frozen ground will lift EVERYTHING! .....like my Buddy's large bulldozer parked on a concrete slab for the winter, for instance.....

This is something to consider for those who build observatories on foundation blocks on the ground, and piers below frost line.... the building will move while the pier stays in place.

Russ

#71 tomcody

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Posted 26 August 2013 - 01:10 PM

Terry.
ATS uses 3/4" SS all thread rod for their J bolts, they use a straight length of rod and weld a section of the same rod about 2" long at the bottom to form an L shape. (if you have a welder available? may be faster?)
Rex

#72 Midnight Dan

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Posted 26 August 2013 - 01:11 PM

McMaster-Carr cancelled my order to Canada.


Yeesh! You can't catch a break.

Here's a good article on the potential of galling when using stainless on stainless:
http://www.estainles...fstainless.html

I use "Never-Sieze" brand anti-sieze lubricant in situations like this, but according to the article, there are lots of good greases that will work just as well.

-Dan

#73 dawziecat

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Posted 26 August 2013 - 01:45 PM

"Never-Sieze" is probably something else I can't get in Canada! :)

I'll use non SS bolts.

Now I need a welder! :confused: :confused:

Good grief. This whole pier thing is turning into a nightmare! Between international border complications and metallurgical spalling catastrophes, not to mention proper rebar technique, it's enough to make me "rend my garments!"
:bawling: :bawling:

#74 Christopher Erickson

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Posted 26 August 2013 - 01:54 PM

Skyshed is in Canada and has lots of the things you might need.

http://www.skyshed.com/

#75 tomcody

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Posted 26 August 2013 - 03:31 PM

Never seize or a simular anti- seize compound is sold at all hardware and
auto parts stores. its used for preventing bolts on car engines from seizing.
Rex






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