A new tall pier idea.

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I have a massive concrete pipe in the garden looking for serious purpose into its old age.

100cm tall x 70cm diameter x 60mm thick. [40" x 28" x 2.5"]  It is far too heavy to lift and is a real struggle just to tip it over. [Carefully of course!] It was quite handy for rolling and compacting the gravel for the shed pad some years back. I don't remember clearly how I managed to get it upright afterwards. Probably using levers and increasing heights of support blocks.

I'm thinking the pipe could form the base of a tall, tapered wooden pier. With half of the depth of the pipe buried in the observatory site's aggregate material it could hold four, tall, near vertical, 10cm x 10cm [4"x4"] wooden posts.

Perhaps I should utilize four of the same concrete car port anchors at the bottom inside the pipe? The sturdy timbers would rise to join each other at the top of a tall, narrow pyramid. Where they would be bolted firmly together with horizontal, threaded rods [studs] for a really solid connection. A further length of "4x4" could be fixed between the tops of the four posts to increase the overall dimensions where it meets the mounting base. This could provide a pivot point for the mounting in azimuth via a sturdily dimensioned, vertical stud.

The concrete pipe down at the bottom of the pier would then be stuffed with self compacting sand and gravel for further stability. Being completely protected from the weather the posts would not be subject to much change in moisture levels other than atmospheric variation.

Let's see now:

Pi x D = circumference x thickness = V [volume] of concrete in the pipe's own structure.

V = 100H x 70 x D x th [thickness.] 1cu.m of concrete weighs 2400kg.

V = 100 x 220 x 6cm = 132,000 cc = 132 liters or 2400/0.132 cu.m = 320kg = pipe's own dry weight

Plus the weight of four concrete anchors and the back-fill of sand and gravel of course.  

Internal volume of pipe = Pi x r x r x height. 

3.142 x 35 x 35 x 100 = 380, 000cc or 380 liters or 0.38cu.m. 

Sand and gravel = 1922kg/cu.m. x .38 = an additional 730kg plus or minus the four concrete anchor posts.

320 + 730 <1000kg p="">

If I filled the pipe with concrete: 1cu.m = 2400kg/m^3 x [volume] 0.38 = 900kg.

The difference in weight between concrete and gravel fill is probably not worth the effort of mixing the concrete. Moreover the timbers would shrink over time leaving a gap between the posts and the concrete. The aggregate should not suffer this fate and should remain in intimate contact with the wooden posts. The self-compacting sand should provide heavy damping to avoid vibration excited by the mounting drives or touching the telescope for focusing.

The stiffness of the tapering wooden structure should be an order of magnitude better than a parallel sided steel pipe of the same diameter as the top of the pier. While no lightweight structure, the timber posts should be lighter than  a similar structure made of concrete-filled blocks. The danger with a top heavy structure is its becoming a compound pendulum. With a very low, poorly damped vibration period. Once touched or excited at the top it would go on swaying for long periods. 

To all intents and purposes the pier would act as a solid, tapered wooden pier, very firmly anchored to the ground at the bottom. Its total height would be four meters but one meter would be lost inside the heavy pipe. A further meter would reach above the raised platform but be completely isolated from the platform itself. This should completely avoid footfall vibration of the telescope mounting. This is  thanks to the long path between the platform and the massive concrete pipe. Rather than solid concrete short-circuiting the path, between the pier base and the platform, the sand should help to isolate any vibration.

The hope of such a design is to avoid the likely flexure of a single wooden pole of normally available dimensions. Variations in moisture content might cause warping and affect the polar pointing accuracy of the mounting experienced by a single post. Multiple posts in a tapered design should be far stiffer and each should help to counteract warping in any of the individual posts.

The option exists to sink the concrete pipe to full depth. The top half buried in the sand and gravel and the lower half buried in the site soil. This would provide a [theoretically] better anchorage against rocking but would increase the pier post length [and mounting leverage at the top] by 50cm or about 19". The sand in the pipe could always be removed for dismantling if the pier proves too prone to vibration. Concrete filling would make this utterly impossible. Swings or roundabouts?

The advantage of a fully sunken pipe is that the posts need only be sawn off for the pipe to become a flat surface at ground level. Adding a little extra gravel would make it completely invisible. Without the chore of breaking up, or moving a huge and massive concrete eyesore should the platform site ever need to be used for parking in future.

Digging a hole in the ground soil, to sink the lower half of the pipe, would not be too onerous. The hole would probably aid setting the pipe upright if made large enough to allow the pipe to be tipped, once safely rolled into place.

Even more importantly, the four posts rising cleanly from the ground would not form such a huge obstacle to the use of the space under the platform. This relatively large, 5x4m area, could become a a storage shed or "warm room" for imaging. A large concrete pipe rising 50cm or 19" from the floor would be a considerable obstruction to normal, everyday use. The pier design has very low thermal mass so will not affect the local "seeing." The under-platform construction should also be of wood or boards to avoid solar gain. 

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