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#1 RGM

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Posted 19 November 2012 - 08:59 PM

I have a couple of questions for those that have built piers from wood. It will be for a G11 mount with around a 40 pound refractor payload. The pier will be about 4 feet max in height above ground level.

I plan to use concrete to secure the pier into the ground.

My questions are. 1. What type of wood, do I need pressure treated because it will be in concrete?

2. What should the cross section of the pier be, 6" x 6", or larger?

3 Is wood a bad idea? It is easier for me to use wood, but would consider steel if necessary.

Thanks

#2 shawnhar

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Posted 19 November 2012 - 11:22 PM

Pressure treated 6x6's, glue and bolt 4 of those together and put it 4 feet in the ground you should be just fine for like 10 years. I have seen the argument that the wood will contract and swell causing your alignment to go out, but I have my scope on a pier made from 4x4's, the alignment will stay solid for 8 months in temps ranging from 20-100 degrees, good enough for me.

#3 stmguy

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Posted 20 November 2012 - 06:01 AM

If you go with wood in concrete make the concrete pier big enough and put in some anchor bolts so you can go to a steel pier later if you want to. I went back and forth in my mind between wood and steel and I ended up with a 6" well casing bolted to concrete, not sorry at all I did it.

Norm

#4 RGM

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Posted 20 November 2012 - 06:36 AM

Norm, I like the idea of putting in bolts - just in case. I am still in the design stage and looking at both options.

shawnhar, I thought pressure treated would be best, it makes sense to use it

#5 roscoe

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Posted 20 November 2012 - 06:55 AM

Bob,
I also suggest pouring your concrete base with bolts in it, and bolting your pier to them. Anything above ground doesn't have to be PT, unless it's going to be out in the weather. An old carpenter trick for putting wood on concrete is to put a layer of tarpaper, like a roofing shingle, between them, or for a post, a piece of lead flashing. Four pieces of lumber bolted together will be more vibration-resistant then one large piece, and will resist warping better. The post will be just as strong hollow as solid. To put ends on it, you could use sturdy lag bolts countersunk into the end plates, plywood would be better for the plates than solid wood. Two pieces of 3/4" glued together is a pretty solid sandwich......
A problem with PT lumber is that it has a very high moisture content, and will certainly shrink and perhaps warp as it dries out, a process which takes months at best.
Even framing lumber (2x4's and the like) is only semi-dry, and will also shrink as it dries. The ideal is the stuff you've had stashed in the rafters of your garage for years, that's thoroughly dried out.....
Russ

#6 Loren Toole

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Posted 20 November 2012 - 07:51 AM

This is an interesting subject, there are many threads on this topic in the Observatory forum. I built a wood pier around 2003 and have used it since, the dimensions are similar to your proposed pier but my scope is much smaller. Deflection maybe an issue with heavier scopes. Here's one discussion thread and a pic:

http://www.cloudynig...d=Observator...,,,,,,,f80,,,,,,,&Words="wood%20pier"&Searchpage=7&Limit=25&Main=2789243&Search=true&where=bodysub&Name=&daterange=1&newerval=5&newertype=y&olderval=&oldertype=&bodyprev=#Post2790906

#7 D_talley

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Posted 20 November 2012 - 09:07 AM

I have a couple of questions for those that have built piers from wood. It will be for a G11 mount with around a 40 pound refractor payload. The pier will be about 4 feet max in height above ground level.

I plan to use concrete to secure the pier into the ground.

My questions are. 1. What type of wood, do I need pressure treated because it will be in concrete?

2. What should the cross section of the pier be, 6" x 6", or larger?

3 Is wood a bad idea? It is easier for me to use wood, but would consider steel if necessary.

Thanks


I just built a wood pier for my G11 last week. It is made up of 4 6x6 pressure treated wood. I placed it in a 3 foot hole and poured concrete to lock it in place.

Here is a link to my post:

Wood Pier for G11

#8 mich_al

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Posted 20 November 2012 - 12:18 PM

Pressure treated 6x6's, glue and bolt 4 of those together and put it 4 feet in the ground you should be just fine for like 10 years. I have seen the argument that the wood will contract and swell causing your alignment to go out, but I have my scope on a pier made from 4x4's, the alignment will stay solid for 8 months in temps ranging from 20-100 degrees, good enough for me.


I had a little different experience. My wood pier is 4 6x6 bolted together and 10 feet above grade and in a very large piece of concrete (about 3x3x56")l. I have a CGEM with a 30lb OTA on top permanently. Sometimes it holds a 'good' alignment for days and sometimes not. This spring I did a test. I placed a marker about 300 ft away (just below horizontal) and pointed the scope at it. Every morning for a couple of weeks I would check to see where the scope was pointing. It often pointed an inch or two left or right and sometimes up to an inch up or down. Anytime the scope was moved or powered it was always returned to the marker. Nothing in the structure was loose permiting movement. Temperatures did not change dramatically during my testing. I believe the wooden pier must have been twisting/flexing to cause the deviation in pointing. Not a showstopper for me but this is what I observed.

Al

#9 Lorence

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Posted 20 November 2012 - 02:54 PM

I have a couple of questions for those that have built piers from wood. It will be for a G11 mount with around a 40 pound refractor payload. The pier will be about 4 feet max in height above ground level.

I plan to use concrete to secure the pier into the ground.


Why do you think the concrete is necessary? I've got a wooden pier, no concrete, no problems. See Dwight's thread above.

If a 30,000 pound excavator couldn't move that pier do you think a telescope will?

#10 mich_al

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Posted 20 November 2012 - 04:58 PM

I think if you drive the pier far enough into hard clay it won't need concrete. If it gets there some other way it will need something substantial to stabilize it.

Al

#11 D_talley

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Posted 20 November 2012 - 07:55 PM

I have a couple of questions for those that have built piers from wood. It will be for a G11 mount with around a 40 pound refractor payload. The pier will be about 4 feet max in height above ground level.

I plan to use concrete to secure the pier into the ground.


Why do you think the concrete is necessary? I've got a wooden pier, no concrete, no problems. See Dwight's thread above.

If a 30,000 pound excavator couldn't move that pier do you think a telescope will?


If your ground conditions will let you work without concrete ok, but in my case I am only 3 feet into the ground that is not clay so I needed to use concrete to lock out any motion.

#12 Raginar

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Posted 20 November 2012 - 07:57 PM

Al,

Are you saying your pier is over 10' high? I could see where a wood pier would not hold a good alignment due to warping/twisting/flexing.

#13 JJK

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Posted 20 November 2012 - 10:04 PM

I used two 4"x8" cedar beams (left over from a pergola project) bolted together to make an 8"x8" square pier. The pier sits about 3.5' in well-packed clay soil (no concrete) and 3' above ground. The entire pier was primed and then painted (three coats of exterior paint). I added a bit of gravel at the bottom of the hole to keep any water that percolated below ground away from the bottom of the pier.

The pier supported an AP900GTO mount and an AP155 f/7 EDF refractor plus some astrophotography kit. There has been very little movement of the pier over several years.

It'd certainly be nicer to have a wider pier (four 6"x6" posts sounds good) and have the pier dug a bit deeper in the ground.

#14 123456

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Posted 20 November 2012 - 11:57 PM

the last pier i built for a friend we use 4 PT 6x6 LAG screwed and glued and sunk it about 4 ft deep in packed gravel ..no concrete at all and its about 5ft out of the ground too , packed in gravel to save the post from moisture . we do all our fence post this way
very little warping after the first season . once you pack it in ...your not going to move it , it will snap first

it holds a 12.5 newt EQ setup

joe

#15 RGM

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Posted 21 November 2012 - 09:02 AM

Thanks for all the comments about not needing concrete. I never really thought about it before. It does make sense that under the right conditions, concrete is not necessary.






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