*
The present appearance will be tidied by closing boards at the tops of
the shutters. This will weather protect the drawer slides at the zenith.
These same boards will be well attached to the shutter ribs to increase
their local stiffness.
I have no plans to fibreglass anything onto the inside of the shutters. So the outer shutter ribs cannot be made any lower than 100mm [or 4"] in plywood. [Angle bracket height]
The heavy duty, drawer slides are 55mm tall when placed on edge. The 4" ribs make it almost impossible to make room for the slides under the faceted, shutter porfile. Unless I can find a clever way to have the drawer slides internal to the dome. This is a deal breaker for shutter ribs less than about 120mm deep.
On the previous [plywood] dome I had to notch the [2x16mm x150mm] plywood, shutter ribs to slide over the projecting zenith board. The zenith board has to project, above the dome, to support the drawer slides. I keep saying boards because plywood is the obvious and most easily worked material. Plywood can be stiffened by glue lamination to become as thick as desired.
Stronger and stiffer materials do exist. I have scrap aluminium strip in 10mm x 150mm. This can be sawn to shape and bolted as easily as plywood. Or even made into a plywood-metal-plywood sandwich.
The moulded, dome reinforcing ribs are a very long way from the zenith. Over a meter, in fact. Which will provide nothing in the way of strength at the huge, observation slit cut-out. Where the GRP thickness is really quite modest. It never needed to be strong until I got my hands on it.
A long, reinforcing plate, made up of thin, plywood laminations could be built up as a load spreader. Fitted both inside and outside the dome. By clamping it in ever increasing layers the profile of the top of the dome could be closely matched. Through bolts would help to spread the hoist loads over a large area.
I can use the slit cut out material as a former for clamping up the plywood. So no need to use the top of the dome. I still have plenty of the thin, dome covering plywood. The clamping might straighten out the fibreglass. I'd have to check the profile carefully in case it changes. At only 4mm thick it lacks stiffness.
I had better glue up the plywood plate against much stiffer formers. A long, internal plate could work alone if the fixing bolts had suitably large, spreader washers. If the plate were made long enough it could be shaped and joined to the dome's moulded ribs. Helping to put back the missing strength from cutting out the meter wide slit from the hemispherical form. The plywood plate could even be bolted up against a bed of still wet, fibreglass and resin. For adhesion into the overall structure.
I was going to use the scrap, 10x150mm aluminium as a top reinforcing plate but would need a hydraulic press. Or a professional brake to be able to bend this material. Matching the exact profile might still be a struggle. I'd need to take an accurate, profile template to the owner of the bending brake or press. Aluminium has the advantage of relative thinness in comparison with plywood of the same stiffness and strength. It is also totally weatherproof.
The slit ribs and zenith board can be glue laminated. To provide extra supporting strips inside and outside the dome. This will help to spread the applied loads into the fibreglass. The angle brackets will be bolted though the fibreglass. Which is more about securing the location than adding strength. Perhaps I should use silicone or epoxy resin to help fix these reinforcing ribs to the fibreglass? The silicone would help to disguise the joint and provide a much better seal against the rain. It would not be a huge problem. To lay strips of glass fibre over the insides of the joints between the slit ribs and the dome. Provided we ever get any warm weather this year.
*
No comments:
Post a Comment