13.05.2021 Dome base ring re-design.

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Thursday 13th 51F, heavy overcast with rain or showers promised.

All this messing about between showers, under a depressingly, leaden sky, is finally paying off. Instead of rushing into things I am spending more time thinking than doing.

A full depth, lower, dome base ring doesn't really need to exist below the rolling track. The moulded GRP rib offers potential location but does not need a formal ring below it. Though the GRP rib is probably too insubstantial to accept clamping bolts. It is certainly not strong enough for wood screws. While a suitably narrow, plywood ring could act as a clamping surface to make the GRP rib fully structural, encased within plywood. 

A narrower, lower, plywood ring, with a firm packing ring over the GRP rib, could form a very stiff sandwich. 

The GRP rib's position and resistance to deformity, would be greatly amplified. To the advantage of the upper "rolling" ring. Which needs to resist the loads applied by the heavy dome to the inverted, rotation rollers. Loads which tend to press the ring upwards.

The dome's own rib, clamping bolts and lower plywood ring would locate the rolling ring immovably. While further stiffening the dome skirt against "out of round" distortion. The ring of clamping bolts needs to be close to the fibreglass rib to avoid cantilevering the plywood rings. Which also helps to keep the bolts well away from the rolling track.

The idea is to avoid, or at least minimize, the number of mechanical fixings penetrating the dome skirt. Though it might be useful to have a horizontal ring of bolts. Clamping the vertical plywood [packing] ring more tightly to the GRP rib and dome. The loads on the GRP rib should be more evenly spread by the plywood "casing." Fortunately, the lower dome is more thickly glassed over. This is to provide a sturdier base for when it is [normally] resting on the ground. [As an animal shelter.]

I did some more measuring, double checking and mocking-up potential layouts with a rotation roller.

The dome is 4.3m Ø or 2.15m radius. The building is 4.2m Ø or 2.1m radius. Both measured across the points of the 14 sides of the 50x150mm [2"x 6"] top ring. That is a difference of 10cm in diameter or 5cm in radius. If I bolt the roller, support forks across the "points" of the top ring then there is only a 5cm gap to the inner dome wall.

The rolling track can be at 10cm from the dome wall. With the roller ring 12cm deep. The lower, base ring need not be more than 60cm deep. To allow for the clamping bolts and load spreading washers. The vertical strip between the rollers can be 50 to 55mm high to ensure clearance over the GRP rib.

All that said, I have some misgivings about clearances. The dome needs to be perfectly round to ensure clearance. A base ring only 12cm wide will exert little pressure on the dome to confirm to its own radius. The vertical "steering" rollers have only limited control over dome concentricity. If these are fitted to tight clearances they bind. Rather than forcing the dome to conform. 

The steering rollers must be absent when the dome is lifted into place. Otherwise the skirt could easily fall upon them. Causing damage or even denying the dome the ability to come to rest safely on its rotation rollers. Levering won't help at that point. The dome becomes locked in limbo. 

The very small clearance of 5cm in radius is a bit of a worry. It places very tight tolerances on the dome's lowering into place. I could saw off the top ring points to achieve a slightly smaller radius and more room for manoeuvre. This is not the sort of thing which can be easily managed once the dome is in place. 

I curved the last top ring using a plywood arc as a template. Making the top ring smaller means I'll need a slightly deeper [i.e. broader] rolling ring to ensure the rolling radius is still manageable. The roller ring can be made as deep [vertically] as desired. So flexure can better resisted simply by adding more rings on top. 

Curving the building's top ring proved to be pointless. The "points" [at the joints] hardly overlapped the supporting timber. Only about 10mm could be gained [in radius] by rounding the points back to the timber.

 

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