As far as gravity goes, a lot of things are more efficient in lower gravity, so a lot of habs will opt to use significantly less than earth gravity. I think half earth gravity is pretty popular overall.

Leetsepeak wrote:

Something I'm curious about, if you build a Torus Habitat and it has one large ring as the main structure, if you had a smaller ring on the interior of that ring (I guess this photo (http://i.stack.imgur.com/hg9Eq.jpg) demonstrates what I'm talking about, basically?) and it was all being spun at the same rate, would it have the same gravity? Is there a good reason not to have multiple rings in your torus habitat? Failing having one ring in the interior, could you have two rings side by side? Sorta like 2001: A Space Odyssey, but obviously spun properly for gravity. Also, is Earth standard gravity specifically undesirable for habitats? Or do people just not really use earth gravity as a standard anymore?

"Same rate" is slightly unclear. You are most likely referring to revolutions per minute but it is possible you are referring to the linear velocity. Assuming you are referring to RPM then the inner ring will have less gravity than the outer ring. If you are speaking of linear velocity then the inner ring will have more gravity than the outer ring. Think of it this way; if you drive in a 50' circle in 60 seconds you will feel much lower g-force than if you drive a 5000' circle in 60 seconds. On the other hand if you drive a 5000' circle at 100 mph you will experience much lower g-force than if drive a 50' circle at 100 mph. This is why you slow down when you need to make sharp turns. As for a good reason not to have multiple rings of the same size; if you take the material for both rings and you use it to make a single ring it will naturally be larger than the twin rings while still taking roughly the same amount of energy to spin up as the two rings. This is advantageous because on a spinning habitat you have an issue with what are known as tidal forces. As a simple example of tidal forces if the torus has a radius of 20m then the top of a person's head will experience only 90% of the gravity affecting their feet (since the distance from their head to the center is only 18m). On the other hand if the radius is 200m then the top of the person's head will experience 99% of the gravity affecting their feet. On the other hand once a ring is built it is built and you can't really expand it. To grow a ringed habitat you might extend the central hub and then build a second parallel ring (I would then build tubes connecting the two rings at multiple points so you don't have to go through the hub but most multiple ring designs I've seen don't do that). It would also be possible to build inner and outer rings and use them for activities that don't require standard gravity. As an example you could use inner rings for power production since that doesn't tend to require gravity. Materials storage would also work well since lower gravity would make it easier to move the materials. Outer rings might be used for food production. The outer ring would have more space and the plants could be genetically modified to deal with the heavier gravity although constantly transitioning from normal gravity to higher gravity would not necessarily be pleasant for the workers. This might mean heavy bot use or stronger sociological separation as you have a 'farmer caste' who prefers to spend most of their time in the outer ring while most people prefer to spend most of their time in the central ring (the gravity difference probably won't be high enough that people on the central ring can't go to the outer ring unless the outer ring has a significantly larger radius).

One note: there is real benefit in doubling up rings, albeit not in the way the OP mentioned. Having two rings side by side and spun at a slight variance allows for a gyroscopic effect that allows the habitat at large to control its angle without expending reaction mass, which is overall good for efficiency. In particular, one oft-omitted feature of the Stanford Torus is the mirror, which is essential for effective inner system life (the tether gets missed a lot too). Twinned rings allow for that habitat to stay pointed properly at the Sun throughout its whole orbit. It does mean that the two rings would be inaccessible except via the central spoke, of course. This is the reason classical O'Neill cylinders are paired, and why I wonder why most fiction depicts them singly, since a megastructure like any space habitat could use any cost-reducing measure available to it.