6.04.2021 Thoughts on lifting a deliberately weakened dome.

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Tuesday 6th, 7.00am. 32F, cloudy but bright with increasing wind forecast. There is a white frost on the dome. 7.40 34F and it is already snowing. Sunshine and wintry showers all morning.

I managed to photograph the dome profile at the top of the slit this morning. The profile is much more exaggerated that it looks on the outer surface. I shall have to scribe to the line in readiness for making a very strong zenith board. 

The option exists for reinforcing the top of the slit with aluminium. I still have a reserve of the 150mm x 10mm aluminium which I used to build the mounting. This could be sandwiched vertically between birch ply cross ribs.

It would be very difficult to match a flat plate to the extreme contours. That would need a serious hydraulic press and an accurate 3D template! There is also the matter of bridging the twin reinforcing ribs near the edges of each segment at the joints. 

I'm going to have to think hard about this. I need the strength at the top of the dome to support the weight when it is lifted into place. The lifting handles which were supplied would locally load the critical area at the large, observing slit cut-out. You can't just remove a metre wide strip out of a segment and expect the original strength to remain. Though spreader bars could join opposing lifting points on either side of the slit. To stop the slit being closed up by the external forces.

Multiple layers of birch ply contoured to the profile as a zenith, cross rib/board? Perhaps carried well beyond the reinforcing ribs to tie those into the structure. The slit ribs would need be seriously joined in structurally. To spread the cross and longitudinal forces. I badly need to spread the lifting loads more uniformly into the entire dome. To reduce the very real risk of the top area being quite literally, ripped out during the lift. 

Multiple attachment points are possible. Numerous, large, eye bolts fitted through the original, segment clamping holes. Shackles attached to these to take the lifting slings. The lifting slings hanging from strong spreader bars/pipes. A sling between two attachment points would automatically balance the loads between them. It all sounds very reasonable until it all starts sliding! Lifting eye-bolts in 12mm have an excess of load capacity.

The alternative is a combination of attachment points. Lifting by the dome skirt as well as across the top of the dome would be safer. To spread the loads much more uniformly. The skirt slings would need to be laterally restrained. To stop them sliding together and directly causing tipping. Or the skirt area could make use of eye-bolts and shackles at the existing joints. These could not move out of place. They would also call on the ribs for load spreading

Balancing the lifting loads between all the attachment points becomes the problem. Though one which could be solved while still on the ground. Perhaps with a timber supporting frame for trying out different lifting arrangements. 

The second image shows the inside of the dome. With a flexible strip of ply following the inner curve. It lies over the twin ribs which have a valley depth of 20mm. I have now copied the zenith profile onto a piece of ply to make a template.

Ideally most of the lifting force is still applied somewhere near the top of the dome. To avoid the C of G of the load rising above the attachment points. Leading to a potential, topsy-turvy situation.

All the lifting gear can be removed once the dome is safely resting on the rotation rollers. Though I still need the strength at the top of the dome for a hoist. Which would be vital for lifting heavy mountings and instruments. Their weight becomes more of a problem with my increasing age. Despite my retaining my ability to lift quite heavy weights while well into my mid-70s. Big, long focus refractors are heavy, awkward AND fragile!

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