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Tuesday 22nd
As I was lying there, half awake, waiting for a suitable time to get up, I had some new insights into my dome project.
Most of these ideas relate to the considerable weight involved. About 500lbs or 220kg for the entire dome. Each segment weighs 70kg or 160lbs. This weight must be fully respected and planned for. I presently have no idea if each segment can be safely manhandled while working alone. So I need to pre-plan how assembly of the basic [3 segment] dome will be accomplished.
If I cut away an observation slit this will seriously weaken the central segment. So I thought of threaded rods [studs/all-threads]. These can temporarily bridge the large gap of the slit in the central segment. To replace the short bolts which usually hold the segments together. Nuts and washers will still clamp the individual segments together. While the distance between the segments will be fixed by the rods. Hopefully, regardless of the observation slit's absence from the previously, self-supporting structure.
The spacing between the flanges at certain bolts holes should be measured, marked and noted for the studs before the slit is cut. The studs could be fitted at intervals during the actual, slit cutting process. To avoid hindrance of the jig saw but closely following it to secure the structure against distortion.
If the dome can be safely lifted by a ring on top. Then it can be safely lifted by a jack inside the dome. It just needs a stable, pyramidal, timber "rise and fall pier" in the centre. So that the jack's considerable lifting force can be safely applied. 4x4 timbers make sense for building the pier.
A 4x4 timber "piston" can be restrained upright by crossbars in the pier's structure. So that the jack can remain down low where it can be most comfortably applied. It might need nothing more than a car, scissors jack for the lift. These have both the capacity and considerable linear movement.
Once lifted the entire dome can be rotated for work to be carried out on the lower edge. Once the observation slit is cut out there will be far less strength at the top of the dome until reinforcement has taken place. So timing of each building stage is important. Lifting becomes more difficult after the slit is cut out. It would need long strops or ropes reaching down to the base flange.
The dome base ring will be of laminated plywood bolted around the existing, moulded flange. The dome rotation rollers will support the entire dome by this base ring. So it needs to be both flat and strong. It follows that the dome ought to be assembled on top of the pre-levelled ring. Which will ensure the segments are level when everything is bolted together. Epoxy resin might be better than wood glue for "structural woodwork."
The laser level will be ideal to set up adequate supports for the dome base ring before lowering the dome on top.
The base ring can be initially cut oversized in outer diameter. Then trimmed back afterwards using a flush router bit. A suitable depth of material [plywood] needs to be built up at the edge of the fibreglass. To allow a weather skirt to be fixed to the dome's lower edge. The skirt will be vulnerable during the big lift of the dome onto the building. So is best applied after the "main event."
The donor igloo's, wide doorway needs to be strongly bridged by the base ring. To avoid it spreading wider after cutting out the observation slit. This need must be kept firmly in mind as the base ring is laminated together from limited lengths of plywood arcs. The [metric] 4x8 sheets set the maximum limit on arc length. So the actual position of the butted ends of these arcs ought to be considered.
The observation slit needs serious reinforcement to avoid weakness at the edges. I thought of making solid timber flexible by multiple saw kerfs. Though multiple laminations of thinner material might be stronger.
Birch plywood strips could be built up on the inside edges of the slit. Being held by spring clamps would be easy after the slit is cut out. This reinforcing strip will then be screwed at short intervals to the edge of the slit to stiffen the central segment after its major surgery.
Only then can both ribs be applied for the bi-parting shutters to open and close against. The laminated plywood rib arcs will further help to reinforce the edge of the slit. These ribs will carry on down from the zenith to the base ring at the doorway. Further reinforcing the base ring at its most vulnerable point.
A thicker board will space the slit ribs on the base ring and beyond them to the dome itself. On either side of the doorway cut-out. The curved triangles, outside the slit ribs, can then be filled in. I thought I could laminate suitable filler panel moulds on the dome'¨s external surface. Though these area will be lost behind the open shutters during use they will be highly visible at all other times.
Fibreglass matt and resin could be applied inside the dome to tie in the slit ribs more securely. I am not sure how securely GRP will adhere to the inside of the dome. It might have protective coatings applied during manufacture. Perhaps it just needs a wipe with acetone where resin is to be applied? I ought to ask the manufacturer about this.
I keep wondering if glass matt or cloth could be laminated between layers of plywood for increased strength and stiffness. Though this would need suitable temperatures to harden properly. So it will have to be put off until [probably] late spring. With global warming it is possible for it to be warm much earlier than usual.
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