22.03.2021 Belt and braces.

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Monday 22nd. 30F, misty with a white frost. 

Today I shall press on with the woodwork. Adding 2x6 joist extensions and sandwiching them between pairs of 2x4 cross braces above and below.

It is vital to tie the new framework into the older building. To add stiffness to the whole structure. Just because something is nominally round does not mean it is inherently stiff. A thin roll of paper certainly isn't. That much was obvious. When the western frames flexed to my pushing and pulling on the top ring of braces. 

The north and south felt very solid as soon as the cross braces were fixed to the old veranda joists. The large gap out to the west had nothing to connect to at the time. Everything in the building must be a belt and braces arrangement. Multiple layers of security. Working in parallel [and series] against flexure and component separation. A circular building does not lend itself readily to normal triangulation by diagonal struts.

I think I might bolt the frames together near the tops instead of relying entirely on lots of skewed, 4" screws. The plywood cladding will provide a very stiff [stressed] skin effect for the frames themselves. Tearing a 12mm plywood clad frame apart would need tremendous forces.

The weak point is in the joints between the [paired 2x4] vertical posts. The 4" [100mm] Torx screws have incredible holding power. Though still not that of large coach bolts and 2" square, roofing washers. 

Banding the entire top ring of the building with a perforated steel roofing strip would also help. However,  it needs shallow grooves to lie flat. Otherwise its thickness will  push the cladding  plywood off the faces of the frames. Which have been deliberately angled on the table saw to accept the plywood on completely flat faces. The small router could groove the posts lightly by hand guiding. The required cut is not very deep.

Or, I could use perforated roofing plates on the insides of the post joints. Though placing them externally would be much stronger against separation in simple, geometric terms. Again this would require the timber be relieved slightly to make room for the depth of the plates and their fixing screws. Only if they were fitted on the outside of the posts. Bolting the posts together would be so much easier.

Eventually, the plywood top ring will be heavily screwed down into the tops of the frames. Thereby adding another layer to the overall rigidity of the structure. The ring will act as a flange. Its horizontal depth having much greater resistance to flexure than its modest vertical depth. Much like a joist uses its greater depth on edge. To achieve its superior, vertical stiffness. Without adding the enormous weight of nominally square timbers. 

Today I competed all the joist extensions using sandwiched, dog's tooth washers to lock them to the original joists. Cross braces were well fixed to tie the extensions into the frames. Then finished the upper ring of cross braces at mid-wall height, observatory level. It all feels very solid now. Still plenty to do. The images above were taken at dusk.

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