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Using "proper" U-form, shelf brackets to fix the base ring to the dome is looking a bit expensive! For no real reason I have settled on the completed dome weighing 500kg or 1000lbs. The GRP dome itself is supposed to weigh 220kg. Or about 500lbs.
To which I have added the plywood shutter and slit ribs. Plus the base ring. I have cut out the "roof" segment to make the observation slit. Then filled it back in again with the overlapping shutters.
The only, real GRP addition is to cover the open doorway. Partially filled with the shutters. Partially with spare triangles of GRP on either side. I suppose I should work out a rough area of the open doorway. Then calculate the added weight from closing it in as a fraction of the total.
The curved area of a hemisphere of 2.15m radius is 29m². The area of the doorway is about 2m². So 220kg = 29-2 = 27m² = 8kg/m². The door area weighs 16kg extra. Making the GRP 236kg in total.
Plus all that Baltic birch plywood for the ribs at 22kg per sheet. I have lost count of how many sheets I have bought. Let's assume 10 sheets for 220kg. That doubles the original weight to about 500kg. So not that far out as a wild guess. There are brackets and screws and bolts too but not that much more weight.
4.3m Ø x Pi = 13.5m circumference. Call it 12m without the shutter area. Even at a rather mean, half meter spacing that means 24 brackets. [Ignoring the shutter area.] Halve the spacing again and I'm looking at nearly 50 brackets!
Hopefully 50 brackets would spread the upward loads sufficiently. 500kg/50 = 10kg each. Assuming an all up weight of 500kg that is a 500% safety margin. I had better start saving for more shelf brackets! Then I can start saving for more rubber washers to seal the fixing bolts. Two bolts per bracket? Eek!
The three, dome segments are held together by bolts at 20cm spacing. I used 40cm spacing on the shutters. Which looked quite appropriate. 50cm spacing wouldn't look too out of place on the skirt for the base ring bolts.
What about 30 brackets at 40cm spacing? Better safe than sorry? 500kg/30 =16kg per bracket. That's still a 400% safety margin with a claimed load capacity of 65kg each. I wonder what the safety margin has been allowed for by the manufacturers? It would be risky to claim 65kg if that was all they could tolerate as a fail weight! Do I want to risk losing a bracket by giving one a destructive load test?
Using simple, bent metal, angle brackets would be half the price but leave the strength as a complete unknown. I could have used the slotted brackets which I used on the outer shutter ribs. Half the price of shelf brackets but a lot [literally] rides on the base ring. Every kg has to be resisted in an upward direction.
If the ring gives way then the dome drops. Probably to jam fast on top of the building. Even the act of lowering it into place on the rollers could easily put excessive local loads on small sections of the ring. All depending on the skill of the crane operator. Which might still be me at the controls of a Manitou! 😳
A trip to town provided 13 more shelf brackets for a total of 30. Of rubber washers, they were, yet again, lying through their teeth. Only four packs on the rack. Their website claims 30 packs in stock at that branch. The staff member on the checkout told me that they were on order and waiting for more stock. Last time I had a 30 minute telephone call [including waiting time] with customer services who told me the shop should provide what they had. There was no rationing. I was twice refused an order for ten packs on their web shop too. Lying seems to be their stock in trade.
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