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Monday 10th 60-68F and heading for the warmest day this year. Rather cloudy and cooler than yesterday.
That said, there must be enough material to resist applied loads to the dome and shutter areas. The shutters are supported only at the top and bottom by the heavy duty drawer slides. Which means the base ring and zenith board area need the most strength [stiffness] to avoid distortion. So local loads on the observation slit arcs is limited.
Snow loads are unlikely to matter much in the warming climate. The dome's slippery surface and predominately steep curvature will not encourage snow build up. Wind loading is an unknown but domes are inherently stiff. So I just need to replace the missing stiffness from cutting out the large, observation slit.
I see one 12mm birch ply arc at full depth. With two minor [half depth] arcs. One inside and one outside, sandwiching the dome. This has twice the value in that it provides an improved wind and weather seal. While simultaneously hiding the cut edge from view.
Applying the heavy "sandwich" plus main arc as one complete unit is very unlikely to be easy. So it will need a stepped approach. The steel brackets can be fitted to support the inner arc against the underside of the dome. The main arc can follow with the outer arc already attached. This will provide the opportunity to apply clamps. Simple, spring clamps can easily manage the limited thickness of the arcs. With the brackets providing solid support throughout.
The outer arc will help to carry the loads of the heavier full depth rib as it is lifted into place. Followed by clamping. Glue and screws will be used to hold the sandwich construction together. End slotted braces, fitted across the open slit, can hold the main arc quickly and firmly in place. Allowing time for clamping. Vertical F-clamps can then be used to ensure a close fit between inner and outer "sandwich" arcs on the GRP shell.
All this must be achieved while working alone from a stepladder. I already know that a double thickness rib is heavy, flexible [laterally] and very ungainly. So I need to use the materials themselves as self-supporting components. If only to avoid the need for building complex, temporary supports. Which, by their very nature, are usually obstructive.
A vertical clamping post could be helpful to support the rib's weight during the actual construction. Though the main arc could be horizontally slotted to fit over and under the dome at the zenith. This would provide automatic location and support by the dome material itself. Once location is established, the bottom of the arc can be safely lifted onto place on the base ring.
Which [logically] must itself, already be in place to support the arcs. Yet, I'd much rather not have the base ring impeding the fitting of the arcs. The base ring being well off the ground would act as a serious hurdle to dome internal access. The temporary, angle iron, dome support is seriously lowering the already limited headroom.
I cannot raise the cross bar without interfering with the rib location. The dome must be supported without distortion until the observation slit ribs are in place. I can't see any simple way to avoid the vital crossbar. It is stopping the dome, on either side of the open slit, from pushing inwards.
Solid posts, sunk into the ground, could support and locate the bottoms of the slit ribs. These supports being removed later. When the base ring is constructed. This would allow unimpeded foot access between the posts. The ribs would then take over support duties and the angle iron, cross bar removed. A temporary one being fitted above head height to allow easy dome access between the ribs.
I wish I had thought of support posts sunk into the ground earlier. If they were tall and stiff enough, they could resist the contraction of the dome across the open slit. As well as supporting the dome and slit ribs, during construction.
I used some roofing batten for the dome support posts. 45x70mm nominal. I put a neat point on one end of each post with the bandsaw. Once inserted into the prepared ground they proved nicely stiff. Without the bulk of anything larger. The same steel brackets as yesterday but reversed left for right. I could hardly hammer the posts in through the dome itself. So they went just outside the doorway and low enough to avoid any conflicts with ribs or shutters.
The dome sagged 2cm at each side of the arched doorway without support. So I jacked up each side in turn before screwing the brackets to the posts. The sag was neutral as far as slit width was concerned. It remained unchanged.
I have ordered some Gorilla wood glue. That should fix the plywood laminations together. It seems very popular in the US but is not generally available over here. Only available online.
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