27.03.2021 Fixing it all together.

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Saturday 27th 41F, heavy overcast, wet and windy. Not a great day for outdoor DIY! I might enjoy a rest for my aching hands, arms, knees and back. I ought to exercise my brain. Just for a change.

The exact details for fixing the shutter ribs and dome ribs is still under [internal] discussion. Small, galvanized, steel brackets are a simple solution for fixing plywood arcs to GRP. Though with serious, potential rain leakage issues. A colour coordinated, silicone bead? The brackets would need insets cut into the plywood. Or they would push the shutter ribs apart by their own thickness.

Rows of fixing screws [nuts and bolts] are not the most decorative detail. Stainless steel lasts well but will always remain bright and "eye catching." So called "Climate" finish is rather golden and eventually dulls. Not available in many screw heads. Countersunk [CSK] head could be much lower impact than domed. Which usually end up looking like rivets on an iron, railway bridge. Exterior nuts just look amateurish.[IMO]

Hot dipped galvanized is an option and soon much duller against uniform GRP green. The highly rust prone, zinc flashed hardware is too awful to even consider. It practically rusts in the hideously expensive, bubble pack. Bolt heads can be sealed against the rain with some kind of flexible washer under the heads. Or a blob of silicone around the hole before bolt insertion and tightening. O-rings?

Using resin and glass to join the plywood to the GRP is another option but requires dry, summer warmth. It also needs some form of temporary but rigid fixing while the resin sets. The roughness of the cured CSM [chopped strand mat] is very unattractive when visible on a finer surface like gel coat. Particularly at a highly visible seam. Is anything "highly visible" on top of a 4m high building? Pride over function? You only do it once. So why compromise on the details? Get it right first time. 

Late morning and the weather brightened up to 48F in sunny periods. So I measured the temperature of the dome facing the sun. 106F outside and 96F inside. This thing is going to soar in temperature in high summer!

The next step was confirming the centre top of the dome. The cloth tape read 3.4m up to the top of the central ridge from the skirt. The same measurement was taken from both sides at the nominal equator. I then used the fibreglass tape again to measure 3.4m from the skirt, opposite the doorway, to the top centre of the same ridge. This allowed me to mark the very top of the outside of the dome. With some confidence of reasonable accuracy. 

With the weather holding I drilled up through the holes in the arched tube over the doorway. I then fitted 10mm bolts instead of G-cramps/clamps. Packing out the bolts with large washers will help to avoid possible head injury from projecting bolts. As I duck under the arch to access the dome while it is still on the ground. [I caught my shoulder instead. So I inverted the dome headed screws for safety] The sturdy, steel tube will support the front of the dome as I cut out the observation slit.  

The final exercise, as it darkened for rain again, was a fuller mock-up of the observation slit ribs. These would be partially sunk into the dome surface. So ignore the radii and bulk of the plywood arcs in the image. The arcs are also being pushed out by the rainwater lip on the open doorway. The almost matching, shutter ribs close against the dome ribs when the shutters are open or closed.  

I have been searching online for heavy duty 100kg drawer slides in 500mm full travel for the shutters. I'd need two pairs. Top and bottom. Not cheap at about £45 per pair. It is difficult to judge the actual loading but it is probably safer to be pessimistic. They will not get much exercise but need to be at least rust resistant. Stainless steel are even more expensive.

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