Alternative very tall piers?
Posted 13 January 2013 - 06:04 PM
I am seeking feedback to my alternative ideas for a very tall telescope pier. Visual observations would take place on an 8' high wooden platform.
As I use refractors this would require a mounting some 7' higher than the platform. So the pier would need to be at least 15' high.
A "normal" 15' tall, poured concrete pier on a poured concrete foundation is completely out of the question for any number of reasons which I needn't elaborate.
Naturally any pier should be isolated from the platform. Otherwise I could just hoist up my massive, welded steel (normally ground based) 3-legged pier and use that resting on the platform. Though vibration might then be a problem.
I could build three separate timber piers to locate the feet of the existing 3-legged, steel pier through holes in the platform. The piers could be tied together with more timber.
I thought of a pyramid of three heavy timber posts joining a single 8' tall pier post just below floor level. Say 6" x 6" pressure-treated lumber. A tripod is very stiff and could be supported from cast concrete, ground anchors normally used for fixing the main support posts of car ports. Unfortunately, joining the three legs rigidly to the single pier post on top gets rather complicated.
Then I thought I could use an existing (storm felled) telegraph pole for the entire pier height. Resting it on a suitable slab (to stop sinking) and fixing it firmly in place with my heavy timber tripod just below floor level.
Being isolated from the platform it should be sturdy enough for visual use. Steel plates would tie the structure together in a similar form to a heavy duty, video tripod.
Are there any constructive thoughts so far?
Posted 13 January 2013 - 07:08 PM
How about building the pier with concrete blocks mortared together? You could use pairs of regular 8x8x16 blocks, or you could use chimney blocks, which are about 18" square. As you build them up, you could fill the cores with concrete, this system allows you to build 8" at a time......
If you want to use PT, perhaps you might consider making a 4-leg tripod, perhaps still tied into a single 6x6 for the last part. (Use 1/2" threaded rod to make mega-bolts....)
If you're chain-saw handy, you could angle-cut the tops of the legs to match the top section.
Posted 13 January 2013 - 10:29 PM
The idea of building with masonry blocks is good, and others have success in the same way with wood - by laminating dimensional lumber together with glue and bolts to create the size they desire. I have also heard of steel pipe filled with sand to dampen vibration used to create tall piers. An interesting idea is to build a pier that is "non-uniform" in its shape to lessen harmonic issues.
If you build tall and thin - you will likely have issues with sway and vibration no matter what you do - but it's not a killer problem. Do your best to stop the sway no matter what. Vibration happens but do your best to dampen it quickly and you'll still be happy.
Your idea of three piers coming up through your platform to support the three legs of your exiting tripod - with the three piers tied together - seems like a method that could offer good rigidity.
Sorbothane makes a rubber-like vibration reducing product that is effective and affordable. It comes in a variety of shapes and sizes and can be used in creative ways. It works in compression.
Good luck - post pics .
Posted 13 January 2013 - 10:30 PM
Got me interested here.....what is it about a fifteen foot tall pier of concrete that makes it unacceptable when a fifteen foot tall telephone pole (or other arrangement you mention) stuck into a good solid concrete base four feet into the ground is just fine? \
Posted 14 January 2013 - 04:37 AM
With the greatest respect you haven't quite grasped the purpose of this thread.
Imagine if you will, an alternative reality where the site is inaccessible to vehicles. Where the builder works alone using materials which are easily manageable. Wet concrete and blocks have yet to be invented in this parallel universe.
Steel is also best avoided in this scenario. It is heavy and resonant. I have built piers from sand and/or concrete filled, heavy wall 6" pipe and they trembled in the wind!
So I want to explore alternatives to traditional materials. Using timber where possible in manageable units of size and weight.
Triangulation and simple geometric forms, based on triangles, can have immense natural stiffness. Though only where the sides of the figure are stiff or (ideally) in tension.
Imagine that the concrete mixer and block delivery drivers are on strike for the foreseeable future. What then if you desperately need a solid telescope pier 15' high?
Posted 14 January 2013 - 08:50 AM
Btw - it looks like you have a lovely setting.
Posted 14 January 2013 - 01:02 PM
You may be right about the three pillars. A local DIY chain sells ground anchors. These are pyramidal blocks of concrete about 2' high with galvanised steel plates sticking out of the top for bolting timber to. Once buried they don't want to move.
I had a new idea based on triangulation. In Denmark there are lots of old gitter antenna masts. Welded steel constructions for supporting TV aerials, street lights etc.
The downside is I'd need a concrete support block and a crane! Their main advantages are that they are massively heavy, stiff and very cheap secondhand. Probably better than the old telegraph pole.
My garden is in previously unspoilt, very dark countryside. Now several young family neighbours have moved in and leave their security lights on day and night.
It has all become a bit "Close Encounters" recently. So I need to move my former observing situation away from the lights. Or, more probably, move somewhere darker!
Posted 17 January 2013 - 12:00 PM
Posted 17 January 2013 - 05:26 PM