3 ms·

Because steel is very ductile and tough, where as concrete is very strong, but brittle. Make a tunnel in the ground with concrete and if the ground moves, your

Because steel is very ductile and tough, where as concrete is very strong, but brittle. Make a tunnel in the ground with concrete and if the ground moves, your tunnel moves too, along with it. Unless your tunnel is stronger than the ground. Which it isn't. Or if it is, it's a 10ft diameter tunnel with 40 foot thick walls. Which is SEVERELY uneconomic. Make it out of steel and put it in the air, and things get a lot different. The air doesn't make the tunnel move so instead of being surrounded on all sides by things which can exert force (in the ground) it's instead only acted upon by the pylons. The pylons are supposed to be around every 100 feet. That means if the earth moves 20 feet sideways between two pylons the steel has a chance to bend and move rather than be sheared. And you can design the pylons to be strong enough to hold the tube up, but not so strong as to break the tube. So if things move a long distance, the closest pylons will break free from the tube and the tube will be supported by pylons which are further apart. That reduces the angles necessary to keep the rest of the tube on the remaining pylons which support it, and that means you're asking even less of the steel. Sure the tube might get some bends in it that it wasn't designed for, and it might even sag down. But you can give the cars braking mechanisms that'll automatically be triggered in an earthquake so that the forces experienced when encountering these new features isn't so unpleasant. The alternative with the concrete tube in the ground is that the tube shears into two parts which are no longer connected. That means that the tube suddenly stops and you're crashing into the dirt at 500+ mph.

abduhl 11y ago
If the pylons are every 100 feet and the joint offset is 20 feet between pylons then that implies a shear strain of .20. This is LARGE for a shear strain. Assuming a shear modulus of 11500 ksi then a shear strain of .2 results in a shear stress of 2300 ksi which is well beyond the ultimate strength of any steel that I've ever worked with. Now your argument that the tube will just break free of the pylons seems like it has merit; however, the question of "why do we have all these pylons every 100 ft if we only need them every XXX ft?" should immediately spring to mind. The answer obviously lies with the rigidity of the tube between support points. In order to get into the realm of the possibility for the steel being able to take a 20 ft offset you're talking about putting supports once a mile, at most. Again, neither the pylon idea nor the tunnel idea have a feasible/economic engineering solution for displacement along a fault if it occurs along alignment.
- msandford 11y ago
  > why do we have all these pylons every 100 ft if we only need them every XXX ft? Because most of the time people want to have their design include a safety factor. Further there's a huge difference between "what this system is designed to do all day every day" and "what this system will do in the 5 minutes between an earthquake beginning and the time that the system is fully halted until repairs can be made" Have you ever seen the remains of a power pole hanging from a power line? That would seem to indicate that there exists excess strength in the system, or that the line support structures are substantially overbuilt. http://i2.cdn.turner.com/cnnnext/dam/assets/130601193956-15-midwest-weather-0531-single-use-image-horizontal-large-gallery.jpg http://i2.cdn.turner.com/cnnnext/dam/assets/130601193956-15-... I suspect that in a hyperloop you could lose 5-10 pylons in a row and what would happen is that the tube would sag well beyond normal operational parameters between the remaining pylons, quite possibly even touching the ground. The reason that so much extra strength will probably exist in the system is that there's a limit to how much deflection you can tolerate without jostling passengers too much for them to buy tickets on a regular basis. And that small deflection means that there's a lot of extra strength in the system, even though it's not strictly needed just to hold things up. This is why machine tools have giant castings well in excess of the strength needed for the machine to stay together. It's because the Young's modulus matters far more than the yield strength or the ultimate tensile strength when you're trying to build a machine that can hold a sub 0.001" tolerance. So if you happened to be traveling in that overbuilt tube at 500mph just after it's fallen to the ground, you'd quite likely bump (or smash) your head on the sides and ceiling of your vehicle. But what you wouldn't do is slam into the dirt that's now at the end of the tube like you would if your buried concrete tube was sheared 20ft by a fault line.

abduhl 11y ago

If the pylons are every 100 feet and the joint offset is 20 feet between pylons then that implies a shear strain of .20. This is LARGE for a shear strain. Assuming a shear modulus of 11500 ksi then a shear strain of .2 results in a shear stress of 2300 ksi which is well beyond the ultimate strength of any steel that I've ever worked with. Now your argument that the tube will just break free of the pylons seems like it has merit; however, the question of "why do we have all these pylons every 100 ft if we only need them every XXX ft?" should immediately spring to mind. The answer obviously lies with the rigidity of the tube between support points. In order to get into the realm of the possibility for the steel being able to take a 20 ft offset you're talking about putting supports once a mile, at most. Again, neither the pylon idea nor the tunnel idea have a feasible/economic engineering solution for displacement along a fault if it occurs along alignment.

msandford 11y ago
> why do we have all these pylons every 100 ft if we only need them every XXX ft? Because most of the time people want to have their design include a safety factor. Further there's a huge difference between "what this system is designed to do all day every day" and "what this system will do in the 5 minutes between an earthquake beginning and the time that the system is fully halted until repairs can be made" Have you ever seen the remains of a power pole hanging from a power line? That would seem to indicate that there exists excess strength in the system, or that the line support structures are substantially overbuilt. http://i2.cdn.turner.com/cnnnext/dam/assets/130601193956-15-midwest-weather-0531-single-use-image-horizontal-large-gallery.jpg http://i2.cdn.turner.com/cnnnext/dam/assets/130601193956-15-... I suspect that in a hyperloop you could lose 5-10 pylons in a row and what would happen is that the tube would sag well beyond normal operational parameters between the remaining pylons, quite possibly even touching the ground. The reason that so much extra strength will probably exist in the system is that there's a limit to how much deflection you can tolerate without jostling passengers too much for them to buy tickets on a regular basis. And that small deflection means that there's a lot of extra strength in the system, even though it's not strictly needed just to hold things up. This is why machine tools have giant castings well in excess of the strength needed for the machine to stay together. It's because the Young's modulus matters far more than the yield strength or the ultimate tensile strength when you're trying to build a machine that can hold a sub 0.001" tolerance. So if you happened to be traveling in that overbuilt tube at 500mph just after it's fallen to the ground, you'd quite likely bump (or smash) your head on the sides and ceiling of your vehicle. But what you wouldn't do is slam into the dirt that's now at the end of the tube like you would if your buried concrete tube was sheared 20ft by a fault line.