How to build this?

> For all you engineers. If we want a spaceship with a rotating torus
> section how do we mate it with a non-rotating spindle?

Actually, it's not very hard. Along the central axis you have a transfer
tunnel.

You just have a bearing to hold it centralised and multiple layers of
sliding seals around the periphery.

Between each layer of seals use vacuum pumps to minimise any leakage to
space. Vacuum pumps are capable of reducing the remaining leakage down
to countable molecules if you really want that.

> WHat would the structural problems be? How would you insure pressure
> worthiness?

There aren't any. You build it; then pressurise it up to a few times
atmospheric to test it. If it doesn't explode and you haven't got any
big leaks, it's going to be fine. Atmospheric pressure is 10 tonnes
(force) per square meter- which sounds a lot; but aluminum suitable to
keep it in is surprisingly thin- for example the pressure hatch on the
Apollo lunar lander- you could poke your finger through it (if you were
feeling suicidal; ok in practice, they ran the lunar lander at 1/3 bar
which made it 1/3 thickness, but still.)

Incidentally, a hole has to be surprisingly big to give significant
leakage. Aircraft are designed to lose a whole window, and still make it
down from 35,000 ft to 10,000 ft before all the air gets sucked out and
everyone passes out (minutes); Concorde had small windows for that
reason (it flew higher at 55000 ft so took longer to get down.) Big
holes lose pressure disproportionately faster than thin gaps around
things due to viscosity effects. So, if the pumps fail, you aren't going
to die, although you gradually lose valuable atmosphere; and so it might
be a good idea to close hatches either side.