2nd August 2021 Shutter rib brackets.

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Monday2nd 58-68F, bright. Expected to be breezy with sunny periods. 

Today's jobs: In no particular order:

Bolt brackets between the slit ribs and the dome.

Bolt brackets between the shutter ribs and the fibreglass, shutter covers.

Progress: I used a simple jig to mark and drill the shutters 10mm Ø at 40cm intervals for the brackets. This is twice the spacing of the dome's 20cm, segment, clamping bolts.

I carefully checked the gap between the shutters and ribs before screwing the brackets into place. Then  I was able to pull the shutters down  onto the ribs with the 10mm nut on the inside. Starting at the bottom and working upwards ensured there were no bulges. Which would cause gaps.

It is quite a stretch to reach the top of the dome from any of the four ladders. They are angled optimally but they really aren't long enough. I have to lean out over the dome with my knees pressing on the top rung.

I almost finished fixing the brackets for the central ribs before lunch. The sun has been shining and the air temperature has reached 68F. So it is very warm in the dome.

Marking the outer rib brackets will need far more care. There is quite an overhang on the outside edges. So I may have to drill small pilot holes from the inside.

It wasn't quite as difficult as that. I measured the overhang and then added the width of the jig. Two layers of 12mm ply and the distance to the centre of the hole in each bracket = 4.5cm. Only one bracket left to do on one outer rib. Then I start on the other. Do I call them left and right shutters? Is this looking outwards? Or looking inwards from the outside?

The job is very time consuming. Drill bracket to enlarge 8mm hole in short leg to 10mm in the drill press using a machine vice. Mark hole spacing on the outside of the dome. Measure overhang and mark in from the edge. Drill 10mm hole working from a ladder on the outside of the dome and insert bolt. 

Climb back down and go back inside the dome. Fit the bracket over the bolt and retain loosely with the nut. Change 10mm drill for a Torx driver. Drive five Torx 5x25mm wood screws into the plywood rib. Finally tighten the nut to snug down the cover onto the rib.

It doesn't sound much but every bracket and every hole has to be exactly right. Too close to the rib and the bracket won't fit over the bolt. The height of the bracket and the angle of its bend has to be exactly right. Or it leaves a gap between the rib and the underside of the fibreglass cover. Too far from the rib and the bracket stands away from the plywood rib. There are nine holes to a row/rib. 26 brackets fixed so far. Another ten to go to complete the shutters. 

Then I start on bracketing the dome/slit ribs. There is small, lateral gap between the slit ribs and the dome. The brackets may/will need longer [short] legs. I chose the shutter rib brackets carefully to keep the bolts a safe distance from the edge of the fibreglass shutters. I feared break out under severe stress. Storm force wind loads, suction and snow loads.

The short leg of these brackets are simply too little for the slit ribs. Lateral tension or vertical forces may tear out the bolts from the edge of the slit cut-out. Which would be a disaster!  The shutters have no lateral forces. It's more about clamping the fibreglass down onto the ribs. 

The slit ribs fit inside the slit cut-out. Which is a severe weakening of the dome. Which it was never designed to suffer. The slit ribs carry the considerable weight of the shutters. So there are heavy lateral and vertical stresses on the brackets and bolts. They had better not be too close to the cut edges of the dome. The more material I can have between the bolts holes and the cut edge the better.

Earlier, I discussed the possibility of supporting arcs of plywood. These would follow the slit ribs but be narrower. They would rest on the cut edge of the dome to help spread the loads. More recently, I have considered applying fibreglass over the gap between the ribs and the dome cut-out. In equal widths up the side of the ribs and out over the dome. 

This would have the unique advantage of waterproofing the gap. Which the add-on plywood arcs could not do reliably. Not even if I used a lot of sealing mastic or silicone in the joint. The fibre-glassing ought to be duplicated inside the dome. Down over the interior of the slit rubs and out across the inside of the dome. Then it would then not be necessary to use any brackets. 

The fibre-glassed ribs would provide enormous mechanical stiffness at the weakest point of the dome. he glass-fibre would supply its own adhesion and location of the ribs within the slit cut-out. The combined load spreading would completely dwarf the use of brackets. 

Glassing in plywood formers and bulkheads is common practice in boat building, sports cars and aircraft. So has a very long history of severe but safe load carrying. Such bulkheads add enormously to the stiffness and strength of the combined structure. 

The downside is severely cosmetic. There is no easy way to disguise the resin wetted, glass matt on the shiny outside surface of the dome. Perhaps a combination of CSM glass inside? With a gap covering plywood rib on the outside with plenty of silicone sealant? I shall think about this overnight.

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