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Tuesday 11th 50F, heavy overcast and raining steadily. I have the shutters draped over the slit to keep the rain out. The large ratchet strap is maintaining security against gusts or gravity.
My knee is hurting again after yesterday's crouching to fix the support posts and remove the slotted, angle iron. Fortunately the rain gives me an excuse to exercise my keyboard instead of doing anything constructive. At least it gives me a chance to think through my next steps. My usual, random task approach is not always the most efficient use of my time.
It is logical to fit the slit ribs first before working further on the shutters. The slit ribs will provide support for the shutters as well as the dome. The ribs can be supported on blocks on the ground and secured by the new posts. This will save me having to make the base ring immediately. Which would otherwise close off the doorway to normal foot traffic. I have the dome at the lowest possible level above the ground at the uphill point. The slope would raise the hurdle a few inches at the slit/doorway. I am excused all hurdles at my age!
I could use the dome as a workshop in the rain. However it is very dark in there when overcast and the shutters closed. Getting the router trammel and supports in there would need the shutters open or removed. There would be masses of sawdust falling on the bare, trodden earth ground. The noise of the the router would be amplified horrendously by the dome! So, all in all, best avoided.
I bought a transparent, net-reinforced tarpaulin. It could be stretched over the slit to let the light in, as intended. Though it would be a wet nuisance to get in and out of the dome. Guy lines would need to be loosened to obtain access and tightened again afterwards. When there is no longer any access to them from inside the dome. Or the tarpaulin would sag. Or flap wildly in the wind. The raw [sawn] edges of the GRP would not be kind to tensioned plastic. Too much hassle unless I have a flash of insight.
The base ring has been exercising my thoughts at odd moments. I have a raised, horizontal, moulded, GRP, reinforcing rib just above the dome skirt. Which could be a complete nuisance or used to my advantage. I always prefer the latter.
If the base ring [made of 2x12mm] laminated ply is arranged just under the moulded rib, then it cannot rise. Which spreads the loads evenly into the dome without the need for mechanical fixings.
A second ring placed just above the moulded, GRP rib means that this plywood ring cannot sink any lower. So that becomes the retention ring for the hold down disks.
Now join the two rings with narrow, horizontal strips of plywood, laid over the moulded GRP rib. The twin rings and shallow spacer become a very stiff H-beam. [RSJ style.] The channel could even become a smooth running surface for the steering rollers. Instead of their running along the edge of a cut plywood ring.
The inability of the steering rollers to escape from their channel would provide a further level of security against the dome lifting in a storm. With my familiar security disks, borrowed from the plywood dome, on top of the [skateboard wheel] steering rollers.
Both rollers and disks would be attached to very stiff, pressed, stainless steel, shelf brackets. Which are, in turn [sic] attached to the main support roller, pressed steel forks.
I get a compact, completely reliable, rotation and hold-down system regardless of dome orientation. No need to set, and usually forget, storm/security latches. It is all there in the rough drawing but not to scale.
I went over and double checked the moulded GRP rib on the dome. It is not a precision device. Nor is it placed with great precision. A closed cell, foam strip is applied to the inner surface of the dome and then glassed over. The geometry of the rib's completed depth increases the stiffness of the skirt.
I checked with a vernier caliper today rather than a tape measure. Though there is a 40mm projection there are radii where the moulding meets the dome. These make the height of the rib closer to 50mm.
The height from the skirt is also slightly variable. There is a minimum of 50mm above the trimmed edge. So, the sandwich, base ring idea still works but needs care. Some packing may be required in places to ensure the active [roller] base ring is both smooth and horizontal. This is easily checked with the laser level.
My hope is that the central plywood strips, or kerf sawn batten[?] being precisely cut on the table saw, will force exact spacing on the rings above and below. The rings cannot be any thinner than 2x12 thick because of the need for staggered joints. Laminating these large rings [4.3m Ø]would require a very large, flat surface which I do not have. Nor would the completed rings be remotely manageable if so fabricated. So they must be be built in-situ and clamped together. Supporting blocks could be used to ensure the rings are level. Again checked with the 360° laser level.
One other worry is the use of using only two plywood laminates. A serious no-no with plywood. The grain in the plies will cause warping [curvature] unless odd numbers of sheets of plywood are used. Or, a central "filler" material is used between the plywood rings. The extra "filler" providing the necessary stability. It does not need to be the same material or thickness as the sandwiching plywood. Provided it can accept wood glue.
The ring arcs are all cut from across the sheet of plywood for greater stiffness. Which means that normal grain orientation does not apply. Which is not quite as bad as having two sheets of ply simply glued together. Which would be guaranteed to cause severe curvature. Flatness is highly desirable in a base ring. Variation will cause the dome to rise and fall. Placing variable loads on the drive mechanism. As the rollers climb and fall on the undulations. The share of the overall loads on each roller will also vary with dome rotation.
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