Haha, the logic is just convenience...

John in NM
The layout is for ease of forming. The building is octagonal, with a central octagonal cupola supported by four posts. So the pattern is to first pour triangular slabs with the sonotube under the post at the peak of the triangle - the forms can lap past the side of the sonotube footing. Then the trapezoidal slab between the two triangles is poured, and only a small bit of formwork is needed where the short top of the trapezoid forms one side of the square under the cupola.

Of course, two of those pictures were taken when the form work became less easy - the entry to the building is on one of the triangular sections, so it had to be left for last.

When I was earlier thinking of pouring a big, single slab, putting in joint lines was going to be a bit hard - now they are almost decorative :D

Also, normally this slab would be sure to settle unevenly what with the way it was poured in pieces and having no pins across the joints. In this case I think I will get away with it because the ledge rock (stable yet fractured rock above true bedrock) is only 10 inches or so below the soil surface.

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