Dome build: New struts, please!

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As Friday remained dry, I built a single gore leaning against the shed. A batten was screwed to the shed wall at 1.5m [5'] to provide the dome's inside height measurement. The outside of the bottom strut was placed at 1.6m from the shed wall. I clamped two spare arcs to meet at the batten on the shed wall. Then re-fixed the original, horizontal struts at higher positions to bring the 60cm level to 55cm in width. This was the average of measuring all 16 gores on the previously competed, strut-free dome.

Trial gore/segment with struts at 50cm [20"] centers: The aluminium speed square allowed me to measure the angles and new strut length to be placed at 50cm center height from the ground.

I then trimmed a new, "middle" strut until it fitted snugly without distorting the ribs. I shall now make a longer, upper strut to fit above the middle one. Followed by a new gore using these new strut lengths to ensure they fit together side by side. 

Saturday: Constant fine drizzle but I decided to continue with building a standard [pattern] gore. The horizontal struts were reset at 50cm from the ground and then at 50cm centers above that.  Note that this spacing is measured in straight lines rather than circumferential because the plywood cladding panels will be flat. The image text gives the general idea.

Both ribs of the gore/segment rest nicely flat against the shed side as a rough check for lateral curvature. The loose arcs were clamped as usual to extend the gore to the batten ledge on the shed at 1.5m [5'] high. Because of the large observation slit cutout, [60cm or 2' wide] the top rib extensions will be much shorter in practice.

Although the rib extensions look correct to the eye there is considerable leeway before a full and true arc of a circle is achieved.

As the dome's circumference across the flats is 10m then the full rib arc should be C/4. Or 2.5m measured across the flats from the ground up to the zenith. The rib's top section beyond the top horizontal strut must also be another 50cm to reach the pole. This is vitally important to avoid the gores drooping when fixed together side by side. So the lower ribs must be tilted back until the final 50cm dimension is reached. This is achieved by sliding the upper rib extensions outwards [telescopically] while still resting on the 1.5m high support ledge.

The total lengths of the ribs must be correct. Otherwise the ribs won't come together to meet at a single point exactly at the pole. Then all the struts will be of the wrong length. This is easier to visualize if the gores are imagined to be rolled out flat on the ground as a long thin, isosceles triangle drawn on paper.

The total rib length [or gore height] must match the dome's polygonal circumference/4. While the struts must fit snugly between the two straight sides drawn from the base to the apex at the correct distance apart. [50cm or about 20"]

Having typed this I panicked and had to go out and check the new gore up against the shed again. The "top end" looked much too short on the image above. Fortunately the real thing measured only an inch too long. This was measured along the ribs themselves. When, in fact, it should be measured on a perpendicular from the horizontal base strut. Which in this case makes the gore/segment the correct length if I had measured it properly. Phew!

Better the gores are too long, rather than too short. As this ensures the upper arc of the dome is not slightly "pointed" at the top. [Lanceolate.] With the danger of the dewshield clearing nicely at the zenith but striking the dome ribs at lower pointing altitudes. Clearance is further complicated by the large lateral offset of a German equatorial mounting on either side, east and west of the pier. I'm hoping it all comes right in the end. Or I shall have a lot of firewood to get us through the winter!

Click on any image for an enlargement.

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