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Imagine, if you will, that you have to raise eight, tall, timber posts. Each is heavy, 4"x4" in girth and all are of four meters [12'] in height. All of them must be accurately arranged in an octagonal layout. The posts do not even enjoy the stability of being inserted into holes in the ground. In fact their bases are resting on a floppy, height-adjustable bracket, screwed into a concrete block buried in sand/gravel.
How do you go from a stack of sawn timber to eight vertical poles? Needles to say, in my usual haphazard way, I have been ignoring the problem in the fervent hope of inspiration wheedling its way into my consciousness while I wasn't actively thinking about it. Just writing about a problem usually triggers an insight, usually leading to a range of possible solutions. Which is often the reason for my endless scribbling on my blog in the first place. The more solutions I come up with the worse the decision making process to follow. Then I talk it to death on virtual piece of paper and come to a tentative conclusion.
Quite obviously I can't run around actively propping the tall posts up "freehand" while I fix cross pieces to join them all together. I may be a bit of a clown but I am not a circus juggling act. Nor am I ten feet tall!
The most obvious solution to the problem of erection is to build frames of pairs of posts. These will need timber braces to arrange enough stiffness to avoid "lozenging." Nor must the braces impede further frames from being erected alongside those already upright. Adjacent frames should aid the stiffness of the structure without catastrophic collapse of the whole arrangement. Ideally, the builder should survive the task to completion as a minimum reward for success! 😎
The most obvious solution to the problem of erection is to build frames of pairs of posts. These will need timber braces to arrange enough stiffness to avoid "lozenging." Nor must the braces impede further frames from being erected alongside those already upright. Adjacent frames should aid the stiffness of the structure without catastrophic collapse of the whole arrangement. Ideally, the builder should survive the task to completion as a minimum reward for success! 😎
Fortunately the frames are quite narrow at about 1.2m [4'] despite their considerable height. The main option lies between temporary bracing with scrap lumber. Or to use finished, internal diagonal and cross bracing. Since the poles are 4"x4" then 2"x4" seems the most obvious choice for the diagonal or X-braces.
The weight of a completed frame does become quite an issue. Particularly if cross bracing is added to the mix. Two adjacent frames can't both have upright posts. Which means no direct connection is possible until the next-but-one frame is safely upright. Quite obviously the first frame must be held securely in place without toppling. Attachment to the shed, at eaves level, makes good sense for the first frame. I can drop rectangular 4" x 8" nailing plates down from the underside of the exposed rafters of the overhang. Then add [say] 2"x4" shaped props to safely fix the first frame securely. The distance is only about a hand width at the shed's fascia board level.
The weight of a completed frame does become quite an issue. Particularly if cross bracing is added to the mix. Two adjacent frames can't both have upright posts. Which means no direct connection is possible until the next-but-one frame is safely upright. Quite obviously the first frame must be held securely in place without toppling. Attachment to the shed, at eaves level, makes good sense for the first frame. I can drop rectangular 4" x 8" nailing plates down from the underside of the exposed rafters of the overhang. Then add [say] 2"x4" shaped props to safely fix the first frame securely. The distance is only about a hand width at the shed's fascia board level.
Then what? I have a fixing at ground level but no visible means of support for the next frame 10' away. Just to add to the fun there are shallow 22.5° angles between the eight sides. So there is no serious reinforcement between frames as they join each other, towering there in [virtual] in mid air... remember, this is only a thought experiment...
The image alongside shows one 12' post, propped and standing against the shed eaves for support. The observatory floor is at eaves level 2.6m high + floor thickness. The dome ring will be fixed at the top at ~1.5m higher still. The idea is to carry all the loads right up to the dome ring in one go. Or, rather, down to the ground via the eight solid posts and ground anchors.
Straight braces projecting out from the first frame can allow the opposite frame to be erected. So that's two frames upright and safely in place. Now the two "side" frames of the eight can be added using the original props as supports for more temporary cross restraints. That just leaves pre-cut braces to be added between the four standing frames to complete the octagonal structure.
I shall use plenty of ground props to allow me to work on the loose cross braces from step ladders. Well, it's a plan! One I didn't have when I started typing an hour ago.
Click on any image for an enlargement.
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