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Thursday 6th 43F, overcast with late morning rain. I have been carefully examining the details of the shutters before I start to do anything more serious. If only to avoid wasting materials.
After searching online I found some sturdy [roofing] angle brackets to connect the plywood ribs to the fibreglass shutters. I looked at the few brackets I happened to have lying around and then decided on an asymmetric bracket. It was lucky I have looked around online. The local builder's merchant chain wanted twice the price I eventually paid at an online plumbing outlet.
An important aspect of the brackets was the lack of a reinforcing rib. The shutters are fully, three dimensional. i.e. Spherical. While the radius is over 2m and therefore hardly curved, it is not flat. So I may need to "dress" each bracket to its exact location. The moulded facets on the shutters also add extra complexity. So attaching the plywood ribs to the GRP shell needs a single point for simplicity. Hence the short legged bracket. The smaller foot, than a more typical, symmetrical bracket, will hopefully aid location. Each "foot" will be likely to need a specific angle where it meets the GRP shutter.
I still have plenty of spare, 10mm galvanized, dome-head screws, nuts and washers. I wanted to avoid having to drill the brackets. So chose one with a suitable hole in the short leg. This will be bolted through the fibreglass. I intend to use about 15 brackets per shutter rib. Or 30 in all per shutter. Complete overkill, perhaps, but I don't want to lose a shutter in a storm. Particularly when access will be so poor. I may modify this decision on the quantity of brackets. The dome has to have rib brackets as well! I could use half the number and see how firm it feels. Perhaps order more brackets later.
The long side of the brackets will be bolted, with several smaller screws, to the plywood ribs. Probably needing slightly counterbored holes to avoid projecting metalwork hitting the matching shutter rib. Which will be pressing against one another when the shutter is fully open or closed. The ribs will laminated from 2x12mm birch ply. For about an inch in total thickness. I still need to decide which outdoor glue to use. While the plywood may be protected to some extent it would be foolish to use "indoor" wood glue.
I was tempted to use tap washers to seal the heads of the screws. Though I have only had one slight sign of a water droplet beside the very top screw on only one side of the dome after heavy rain. Suggesting that the large 20mm heads are already providing a very good seal against the GRP gel coat. The shutters are mostly sloping. So any leakage around the screws will run straight down. If the top screws should begin to leak, over time, I can access them though the open slit. From a ladder, inside the dome, resting on the observatory floor. Just as I do with the present dome.
In a perfect world I could have used resin and glass fibre to secure the ribs to the GRP shutters. Temperatures have been so low this spring that this has been impossible to consider. Perhaps I can glass in the base ring? Though brackets and bolts are a much surer way to be able to dismantle the whole lot. Handy when I start on a bigger dome. That was a joke aimed at The Project Manager. 😉 Fortunately, for all of us, The Head Gardener doesn't read my blogs. 😳
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