Building the Octagon Pt.2 X-rated.

*

It has been helpfully suggested that I hire some scaffolding for the project. Not only would this contain the posts until they can be safely braced but it would help later stages. I'll have to see what is possible. The steeply banked, narrow edges to the site and the shape of the building does not really lend itself well to scaffolding. Not unless the scaffolding was erected inside the framework. Tower scaffolding units could work for the erection of the outer framework. I'll have to give this some serious thought.

I am inclined to mass produce cross braces out of 2"x6" which would firmly fix the distances apart  of the 4m tall posts. The brackets on the footings do have some leeway thanks to oval holes instead of round drilled holes. Such sturdy, pre-drilled [?] cross braces would not be too heavy to work with at only 1.2m long. They could easily double as floor joists down at the ground level and up at observatory floor level. The advantage would be that the orientation of the posts would also be firmly fixed. A third brace at the top of each twin pole frame should provide the desired stability for lifting into place. The 1"x4" timber props will be fixed to the tops of the frames prior to their being lifted to vertical.

I have also obtained enough 2"x4" timber for diagonal bracing of each lower frame. These would fit in an X-form cross between the lower cross braces and the underside of the obs. floor braces/joists. Unlike the much simpler cross braces, these diagonal stiffeners would need a skewed miter cut on each end. A diagonal notch at mid-point would be needed where they cross. Though this notch would not be skewed since the X-frames would be perfectly square to each other. Only the ends are skewed to fit between the 22.5° faces of the main posts.

I am not absolutely sure that a full X-bracing is really needed. Each lower frame could have a single diagonal strut arranged as zig-zag diagonals working around the building to provide resistance to each individual frame "lozenging." A single diagonal strut would become push-pull. While each alternate strut would compliment the strut in the next frames.

The accepted plan is to cover the octagonal framework with commercially grooved 4'x8' cladding plywood. This would provide a very effective stressed skin if it were well fixed to all sides [and diagonal crossbar] of each frame with screws.

At this moment I am leaning towards building and erecting a four initial frames arrangement. To be joined later by pre-cut 2"x6" cross braces once these four frames are safely erected and diagonally propped across the diameter with 1"x4"s and horizontally braced with 2" x4"s. [Or 2"x6" to become joists though the span is quite small. ]

The proof of the idea is to build my first frame to see if it is [remotely] manageable by one [very determined] but elderly, optimist. The frame can be pushed upright at [breathless] intervals with an overlong 2"x4" ground prop [or props] attached to one of the upper cross braces. I think I may leave out the upper cross braces to reduce the weight to be lifted. If you hear no more from me tomorrow then I have failed. 😇 

Click on any image for an enlargement.

*