12.12.2020 As you were!

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After all this thought, discussion, measurement, photography, examination on the ground, drawing and planning I have decided to enlarge what I have. The building will be expanded directly towards the WSW. The ring of 4x4 uprights will fall on the rim of the veranda in the WSW but will overlap elsewhere. The veranda floor boards can be removed individually to allow for this. While providing support until all these new posts can be safely tied into the structure. 

Moving the centre point of the building bodily towards the WSW will minimise the need for new gravel for concrete anchor footings. A few car trailer loads will provide all that I need. While safely maintaining the same overall distance of the building from the nearby shed. I was hoping to move it further away but can't see how.

My local gravel supplier only has a large, tipper lorry. Which cannot reach the building site. I have absolutely no desire to trundle back and forth with laden wheelbarrows. Like we did last time. 

That took several days after a local groundwork business failed to provide the promised shuttle service between the gate and the groundworks. Moving 20+ tons of gravel a little over 30 meters, entirely manually, is not the sort of thing most septuagenarians do!

The circumference of the new 4.4m dome is nearly 14 meters. So logically I shall have 14 concrete anchors supporting 14 upright posts. This will allow 14x half [8x4] sheets of grooved plywood for external cladding. The actual measurement between posts will be slightly less. Because I need the dome to rain safely outside the supporting building. So it needs to be smaller. Even on the angular points.  The angle between 14 posts is about 25°. I shall use one of the original uprights, nearest the shed, as my geometric, anchor point. Everything else will hinge on that particular fixed point.

The trick is to get all the new posts upright and on a perfect circle at ground level. The steelwork bracket on the tapered concrete anchors is not very user-friendly for lateral movement. Though they do provide height adjustment. Getting the anchors in the right place in their holes in the ground is another problem. The hole has to be made too big and nicely flat on the bottom. So the heavy anchor can be levered about for position. Not easy in two feet deep holes crumbly, self-compacting gravel.

I shall need to make curved, [external] plywood templates for the post circle. Then get them the correct distance from the new centre of the building at the correct spacing between them. They can be temporarily braced with screwed or clamped planks. Before the 2x6s are mitred to length and angle to fit and then screwed between the posts. 

Following an accurate circle at the top too. For the plywood top ring on the building. Which is where the support and dome rotation rollers will sit. It obviously helps if the ring is truly round. So that the dome doesn't rub on it during rotation. A short skirt will probably hang down from the dome to keep wind blown rain out. I used a plastic upstand on the present dome and it rubs on the rubber skirt at times during rotation.

All that I have discussed will allow me to continue to use the present observatory until the dome can finally be replaced by the larger one. Which can only happen after warm weather in the spring. Which will allow GRP reinforcing work to be done along the observation slit, plywood ribs on the new dome. That gives me plenty of time to consider how best to incorporate the new building with the old. Not least how and where to place the big warehouse ladder. If the building is wider the ladder need only be moved to the new, outer edge.

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