Hmm, I'd not thought about that - which makes sense.
However, given a horizontal roadbed and a near horizontal stay cable (the case when you have a very long bridge without very tall towers), surely any element near the centre will have almost no vertical moment to act against its weight. I assume that's port of the reason why even today, the very longest bridge spans are suspension ones.
]]>I have been across the Tsing Ma Bridge on a train - main span 1,377 metres - so it's possible if you take it gently. However, that was the airport link train in HK, so it not be as much of a challenge as a heavy freight train or a high speed express.
]]>There is no fundamental reason why the deck couldn't be under as much tension in at the furthest-out sections as it would be in compression at the nearest sections if there were no tension applied, or anything in between. That tension is hard to achieve while the bridge is only half finished, but perfectly doable when the last pieces of the deck are joined. Like putting the keystone in a masonry arch, but for tension instead of compression, the whole thing is a lot more stable when everything is joined up than while it's being constructed.
I don't know of anyone who's built a bridge under those assumptions. I think... If one started at the center, with a center section and two cables, one up to each tower, and then worked towards the towers from there, that might get the geometry right for the loads during assembly.
So . . . bringing this back full circle, how would this work for large rotating cylindrical colonies in space? I'm guessing that for the truly large habitats, say, 10 km in radius, you're pretty much going to have to use some sort of tension support structure. And this would certainly be a case where both the center and near parts would be experiencing the same amount of compression/tension.
]]>(Actually, it's a complex of bridges - including spans of 940, 990 and 1,118 metres as suspension bridges, and two shorter spans of 420 metres each as cable stay bridges.)
To compare, the longest span not part of a suspension bridge would appear to be the Sutong Bridge in China, with a 1,088 metre span.
]]>I'm guessing that for the truly large habitats, say, 10 km in radius, you're pretty much going to have to use some sort of tension support structure.
I think I saw someone treat the cylinder as 'a suspension bridge with no ends and no towers'. Think bicycle wheel, basically, but on a huge scale.
]]>I've wondered ever since if things had gone differently and the LTTE had managed to hold out against the government, if the new nation would have wanted to build some project like a bridge to connect the mainland and the island as a demonstration of the unity of the Tamil people, and what effect that would have had in the long term on the politics of the region. And I have speculated about a resurgence of the Tamil insurrection a generation or two from now, and how it might be affected by proposals for building such a bridge. That speculation is part of the background of one of the characters in an SF novel that I've been working on in fits and starts for the last couple of years. It may even be finished someday. The point is, anyway, that large-scale engineering projects often have long-term, and sometimes unexpected, consequences to the political and cultural life of the regions where they're built.
]]>However, given a horizontal roadbed and a near horizontal stay cable (the case when you have a very long bridge without very tall towers), surely any element near the centre will have almost no vertical moment to act against its weight. I assume that's port of the reason why even today, the very longest bridge spans are suspension ones.
Isn't this just simple trig to a first approximation? The tension in the cable divided by the vertical component is tan theta, or vertical force divided by tan theta gives the tension in the cable (again, roughly.) So for an angle that is, say 10 degrees off the horizontal, tension has to be roughly six times the vertical component, and since tan theta is approximately theta for small angles this is an approximately inverse-linear relationship; halving the angle to five degrees doubles the tension to twelve times the vertical component.
So which is cheaper, thicker cables or taller support towers :-)
]]>Yes, there are relatively few long (non-light) rail suspension bridges, but as I pointed out, they do exist, with some among the longest 25 bridges in the world.
It may be interesting to examine the third longest span in the world is on the Great Belt Bridge in Denmark. This is a combined road/rail route - but while the cars go over the box girder bridge and the following suspension bridge, trains go over the box girder bridge and then dip into tunnels to follow the route of the suspension bridge.
So it looks like a case where planners have a different balance between tunnels and bridges for cars and trains -- it's not that you can't put trains over bridges, it's that the tipping point between a bridge and a tunnel for a train tends to come out different from the same case with cars/trucks.
One factor in this, I suspect, is that bridges are preferred to tunnels for road traffic because of the horrific problems you can get when the inevitable crashes do occur in tunnels. With an electric train, even if you do have a crash (much less likely), you're much less prone to getting a fuel fireball pushing through the bore (carrying burning trucks into the tunnel on your train is plain stupid). On a bridge, your fire is much more localised.
(PS I think your spell checker is playing up: it's 'catenary', not 'cantenary'.)
]]>Indeed so.
I'm always amused by how little people in general, when considering engineered artifacts, tend not to appreciate the problems of construction, maintenance and deconstruction. They look at the bridge and see something that works - they don't consider that every intermediate step in its building also has to work. For a pyramid, that's not too difficult - a half-built one won't fall down, and temporary ramps up it are not too hard. But that great stone arch? It was probably built on a wooden frame that filled most of the aperture.
]]>Still, it's obscured about 85% of the sun's disc, which isn't bad as far as cooling is concerned. It's also much simpler as far as orientation is concerned - you can basically build something with a dirty great sunshield, and have the cold and dark parts one side, and the Earth-pointing telemetry on the other. In most other places, you have to worry about differing and varying directions for the Sun and Earth. You don't get the Earth shining its light at your scope, and the Moon is almost not a problem either, being much closer in, and you on the dark side.
]]>Why would their first desire be to settle a gravity trap? Especially one with an atmosphere that will complicate any attempts to land and to reach orbit?
They're going to go for the first system that's rich in asteroids (and that's probably not binaries like Alpha Centauri) and build more pylons. It's far easier for them to build another habitat with perfect living parameters than to terraform a planet to match the parameters we have become finetuned for through evolution. At a pinch they might consider mining airless moons since it would be relatively easy to launch stuff into space using rails when you don't have to care about atmospheric friction. When you've figured out how to live indefinitely in space you're free, liberated, no need to lock yourself in the gravity prison again.
]]>