Dome build: Lifting the base ring safely onto the wheels.

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I have been pondering different ways to ensure the ring will be round rather than oval. If I use cross braces the completed ring must be able to be lifted onto the rollers without obstruction. The double thickness ring will not be light enough to handle easily unaided due to the sheer size making it very awkward. Perhaps I can build a lifting jib using the 5m [16'] ladder and the two builders stepladders? The problem is avoiding the ladders causing an obstruction in themselves.

Perhaps a simple inclined plane using the long ladder and the boat winch [or just a rope] is the answer? The inclined plane itself [the ladder] could slide up over the octagonal top ring under the control of the winch. Bringing the supported ring with it.  The ladder would easily slide between the rollers and could then be removed when the ring is safely in place. A rubbing strip fixed to the octagon ring will protect it.

My fear is that pushing the bare ring up the sloping ladder would break it when it reached the top and flopped downwards onto the pier. If I was at the top and pulling a rope, attached to the middle of the cross-braces, I would be in a position to 'field' the ring and lower it safely down onto the rollers from the stepladders. Though the top end would be high overhead even from the observatory floor.

It gets even easier if I allow another ladder to slide up the big ladder. This allows the bottom of the big ladder to be pulled out to provide a much more gentle slope. The winch cable can run over the top rung of the 5m ladder and connect to the bottom of the shorter, sliding ladder. The shorter ladder has the usual hooks for the rungs. So will be safely locked at regular intervals on the lift should I need to go down and adjust anything. Though the winch does have the usual locking tumbler pawl on a ratchet wheel which most boat/winch owners seem to rely on.

The sliding ladder will be easily stiff enough to support the ring throughout the lift and lower. It is also lighter and rides nicely between the rails of the wider and heavier, big ladder. A single X-brace to support the ring would be tied to the sliding ladder. A rope would be attached to the top of the sliding ladder for greater control of the load. I think I finally have a workable plan.

Tuesday: A miserably wet, cold and windy morning clearing after lunch. So there's a possible window open to cut the new arcs. And there was. I marked the first arc with the beam compass at 1.5 and 1.6m on the plywood. Then checked with an existing arc to see if both were accurate and matched each other. I then used the arc as a template to mark seven new 15mm arcs which were sawn out with the jigsaw. Each freshly sawn arc weighed 1.75kg. 15 arcs to complete the whole ring @ 30mm thickness = 26.25kg = 58lbs! Manageable but difficult to handle due to the sheer size.

I have to decide how to join the new ring onto the old. I cut the ends off square on the last one and made half lap joints. It might be stronger to use skewed butt joints on the second ring lamination and stagger the joints relative to the first. The wheels should roll more easily over a diagonal joint. Though I hope to use the longer arcs cut from the 4'x8' hardboard on the underside of the ring to achieve even smoother joints. The birch plywood is in 5x5' sheets so makes shorter arcs.

A contact warned me against having hills and hollows in the base ring's desired perfect flatness. Which would make the ring drag at intervals as the wheels had to climb 'uphill.' Hence the move to a double thickness ring and hardboard running surface.

Wednesday:  A promising, sunny autumnal day has now been muffled in thick mist and heavy overcast! At least it is dry so I can continue work on the dome's base ring.

I shall arrange and glue the original ring together on blocks laid out on what remains of the parking place lawn. The blocks can be carefully leveled with a long straight edge and 4' level.

The image shows the original base ring has been glued, clamped and screwed at the lap joints. The beam compass has been re-drilled and a tent peg used as a much firmer pivot than the original 4" screw. I use a loop of cord to hold up the business end of the compass to make quick work of a circuit. In fact I went round and round checking roundness and gently adjusting the tent pegs pushed through each joint.

The diagonals on one end of each arc of the second ring lamination will be used to scribe the next joint. I shall use the diagonal cuts as butt joints and rely on the lower ring, with staggered joints for even support.

The horizontal, bottom struts of the dome segments will heavily reinforce the completed 35mm thick base ring. With a second horizontal strut on the inside of each segment, to match the outer one, the ring will not dare to warp.

I had a trial with the sliding ladder and found the rung hooks did not want to lift clear when a rope was used for lifting. So I reversed the sliding ladder to put the hooks outwards. The ladder then rose almost effortlessly and gently tipped over as it reached its balance point. Where it tipped down gently onto the pier. After which it was very easy to carry the far end of the ladder over to the opposite side of the octagon. I will add some blocks to pack up the octagon ring and the pier to ensure the ring does not catch on the projecting wheels once it is safely lifted up top.

It looks as if this method will serve perfectly for lifting the finished base ring safely into place. I am quite tempted to lift the entire dome skeleton up the ladder after construction on the ground. Though I would have to beware of the dome tilting backwards and outwards away from the ladder. The base ring alone has no outward rotational moment like the dome would probably have.

Click on any image for an enlargement.

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