[fhars]

No, they are not. Or, rather, they are only if they follow a(n inverted) chain line (i.e. exp(x)+exp(-x)), but the lower parts of the half spheres favored in architecture are steeper than that, so there is massive tensile stress there. This is, by the way, the genius in Wren's consruction of St. Pauls in London: he hid a chain line dome that carries most of the load between the visible outer and inner half spheres.

[Luc]

> This is, by the way, the genius in Wren's consruction of St. Pauls in London: he hid a chain line dome that carries most of the load between the visible outer and inner half spheres.

I think that's the wrong way around. The inner and outer dome follow catenary curves (they only need to support their own weight), and between them is a cone shape, supporting the heavy ornament on the roof. A 'chain line dome' is not a good way of supporting a heavy load on the roof (just imagine how the shape of the chain changes as you hang a weight in the middle of it).

http://en.wikipedia.org/wiki/St_Paul%27s_Cathedral#Structura... , third paragraph.