Mr. Fournier,

A complete torus station is a bit far-fetched at this stage I suppose due
to construction costs (R&D, a multitude of launches, diameter of torus tub
e sections), and having a single habitat connected with a cable (or series
of cables for redundancy) with a counterweight would be cheaper than two co
nnected with a traversable tunnel and hub, I agree. I wanted to suggest a
scalable/modular design that could be augmented proportionately with more a
nd more segments until it became a full torus.
As far as I know (I'm not a mechanical engineer by any means), the curren
t hull structure of the ISS is considerably stronger than it needs to be so
we have somewhat suitable materials already. The stresses you mentioned,
of course, were stresses experienced by spinning. While a tube connecting
two habitats with a large enough diameter to serve as a passageway would be
more expensive, the hull of said tube could contain several cables as well
as life-support systems, storage and gradients of micro-gravity to serve a
s research for partial-gravity experiments. I would advocate a second habi
tat to act as counterweight for safety reasons as well as a more financiall
y justifiable payload. Spent fuel tanks would be impractical because fuel
tanks are rarely empty and even if they are, it is temporary thus necessita
ting constant rearranging. A second habitat of identical mass would be dif
ficult to engineer (say four astronauts piled into one habitat and nobody w
as in the opposite one - would that throw out the orbit?) but cantilevers o
r gyroscopic controls may hold an answer.
The rudimentary sketch I drew was tube-shaped to anticipate the addition
of more modules eventually making a stronger, full torus. Politically and
financially, the initial construction would have to be somewhat habitable t
o justify the years (perhaps decades) of planning and likely billions and b
illions of dollars. Without proper artificial gravity designs to test near
Earth, interplanetary travel carries too great a cost mentally/physiologic
ally to participants in Mars missions.