Building the Octagon Pt.26 Pier Pressure 2.

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Friday, and another very wet day, on an and off, as I propped up all four posts for the pier. Thanks to their almost upright stance they are remarkably docile against tipping. The floor aperture restrains them against any untoward movement. I must have been out of my tiny mind to think I can just drop a heavy mounting and OTA[s]on top of this arrangement! The only thing keeping it upright is its considerable momentum. The footprint, inside the pipe is far too small to provide any serious stability.  My [airy] theory about the necessary mass and anchorage being provided at the base was as substantial as the air surrounding the pier preventing it from crashing.

Which leaves me looking at triangulation of the column in one form or another. A suitably massive construction of heavy timbers could greatly increase the footprint with heavy bracing ensuring a connection between the tall poles and the ground. The result would be ugly and fill the interior of the lower building with obstructions to normal movement and usage. A medieval windmill in all but name and originality.

The other, most obvious, alternative would be the use of stranded steel guy lines. Their connection would be best achieved at just below obs. floor level. Having cables descending at 45° would be only slightly less cumbersome than timbers as far as restriction of ground floor usage is concerned. Which leaves one with some form of internal, or external, gin pole [or A-frame] for each cable emanating from the column. A gin pole is not laterally stable to the lines of applied forces. At least not naturally in this universe.

Which means providing bifurcated guy lines outboard of the building. [Simple but not ideal] Or providing a triangular [A-frame] gin pole to stop sideways tipping. With a single, adjustable guy line exiting the building through an aperture to be well anchored to the ground. An A-frame could stand close to the inside or outside of the walls so cause little or no disturbance to normal use of the lower room.

Either option would provide extra perching space for the garden birds. Quite what it would look like is quite another matter. The guy lines would also need total isolation from the building's structure. "Wind in the wires" is unlikely to lead to vibration since observation is unlikely in much of a wind.
The major downside is the orientation of the guy lines relative to the obs. building. If three cables were provided at 120° apart then at least one cable has to come forwards. The alternative is to use the shed alongside as one, high level anchor. The other two would then lead of to the SW and NW where only trees and shrubs presently grow.

The amateur astronomer's more usual approach to tall piers is a massive concrete foundation. A cast concrete, brick or block pier is then raised off the block. Usually with some form of internal steel reinforcing bars or mesh. Hollow chimney blocks have a following. I had already denied myself any wet concrete in the construction of my observatory. I wanted, and demanded of myself, that the site be able to be quickly and easily returned to a bare parking space. Perhaps a suitable site for a normal garden shed. Leaving a vast concrete slab and pier behind for the next owner of a home is unfair IMHO. It denies the new owner any freedom concerning their normal use  that space.

The obvious question which must be asked is: How important is pier isolation from the obs. building? With only one user, who isn't planning on any form of dancing during observations.. how much vibration would actually be carried into the pier and thence to the magnified image? Can the pier be a simple column to take the heavy vertical loads. With normal anchorage to the obs. floor to avoid toppling. Could steel cables be used to anchor the pier to the building itself, with only rather limited isolation, to keep the tall pier upright?

Click on any image for an enlargement.

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