4 ms·

How is the issue of a horizontal slip different for a tunnel versus for an above ground structure? Neither piece of infrastructure can be economically designed

by abduhl 11y ago

How is the issue of a horizontal slip different for a tunnel versus for an above ground structure? Neither piece of infrastructure can be economically designed to withstand a seismic movement if the structure crosses the fault where the offset occurs. Why does the possibility of an offset result in a tunnel being an "emphatic no" while still allowing for this pylon idea?

btilly 11y ago
The pylons are built with an internal structure allowing them to move horizontally both ways, and vertically. The tube itself can remain stationary in the air. We do not have a cost effective way to allow the tube to remain stationary if it is attached to the ground and the ground moves. And avoiding buckling and sudden bends kind of matters when you're traveling at mach 1. Search for "earthquake" in http://www.spacex.com/sites/spacex/files/hyperloop_alpha.pdf http://www.spacex.com/sites/spacex/files/hyperloop_alpha.pdf to confirm that this is a design factor the Elon explicitly considered.
- abduhl 11y ago
  I'm sorry but no - this does not answer the question. Expansion joints and "lateral dampers" do not make the problem of a joint offset go away. In order to resist a horizontal or vertical offset of 20+ feet with the tube "remaining stationary" then the pylon must be built to allow an extra 40+ feet to account for this movement. How, exactly, does the pylon not suffer the same design flaw as the tunnel under the use case that you yourself brought up of a horizontal or vertical offset occuring along a splay?
  - btilly 11y ago
    Where do you get your 40+ feet figure from? If each side can move the tunnel 10 feet relative to itself without breaking, you should theoretically be able to handle up to a 20 foot slip between plates. One side bends one away, the other the other way. The pylons move 20 feet relative to each other, and only 10 feet relative to the tube. That said, my information is based on the blueprint that Elon published and I already linked to. It includes graphs showing numerical simulations of how their design is supposed to respond to earthquakes. If you have further questions, you can start with that, then download the designs that they and others have produced and do your own work. I am not an engineer. However I do know that engineers have done exactly what I suggested and come to the conclusion that his design is feasible.
    - abduhl 11y ago
      The 40 foot figure assumes that the direction of movement is unknown and that one pylon remains fixed. I suppose you could just have each pylon capable of handling 20 feet of three dimensional movement but I doubt the economics work out in favor of that option rather than choosing towers rationally. That said, your answer is an appeal to authority. Just because a feasibility study says it is feasible does not actually make it feasible. I skimmed the document you linked and nothing in it pertains to large ground movements that would occur due to fault offset - it is all with relation to ground shaking rather than fault slip. Further, it is common knowledge, in fact, that underground structures generally perform better under ground shaking as compared to above ground structures except when the structure crosses the fault which experiences the offset. That is, tunnels work better than pylons except at the point where the large offset occurs. I understand that you may not be an engineer. I am an engineer, with my area of expertise being in geo-structural interaction, and I am telling you that your statement about a tunnel not being feasible because of earthquakes is NOT correct. There are plenty of other reasons why a tunnel makes no sense for this job, but seismic vulnerability is not a good one. Whether his design is feasible or not is a moot point.
      - btilly 11y ago
        Slip is defined by relative displacement, not absolute displacement. See http://earthquake.usgs.gov/learn/glossary/?termID=174 http://earthquake.usgs.gov/learn/glossary/?termID=174 for verification. So in a 20 foot slip, neither side actually moved 20 feet. Secondly it is very important that the tube remain still while the pylons move relative to it. Don't forget that we have objects moving down that tube at close to the speed of sound. It doesn't take a large local kink to be a fatal problem for people inside. You really don't want that tube to move. Enough so that keeping the tube still that it is an explicit design consideration to keep it still while the pylons expand and contract for thermal reasons. Thirdly I well know that earthquakes are generally less of problems underground than on the surface. But trading an easy problem in most places with an impossible problem at fault boundaries is hardly an improvement. That is why I have only focused on the potentially impossible problem. Fourth, your response was just as much an appeal to authority as mine. But my appeal to authority was an appeal to authorities whose credentials are independently verifiable, whose conclusions have been put up for public comment, and which other authorities have independently questioned, criticized, and mostly verified. Your authority is based on an anonymous claim of expertise made on the internet, in a thread where you made a basic mistake about how earthquakes are measured, with no actual analysis backing it up. Which authority is more believable?
msandford 11y ago
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.
elektromekatron 11y ago
You can include base isolation for pylons crossing known faults. - https://en.wikipedia.org/wiki/Base_isolation https://en.wikipedia.org/wiki/Base_isolation

by abduhl 11y ago

btilly 11y ago

The pylons are built with an internal structure allowing them to move horizontally both ways, and vertically. The tube itself can remain stationary in the air. We do not have a cost effective way to allow the tube to remain stationary if it is attached to the ground and the ground moves. And avoiding buckling and sudden bends kind of matters when you're traveling at mach 1. Search for "earthquake" in http://www.spacex.com/sites/spacex/files/hyperloop_alpha.pdf http://www.spacex.com/sites/spacex/files/hyperloop_alpha.pdf to confirm that this is a design factor the Elon explicitly considered.

abduhl 11y ago
I'm sorry but no - this does not answer the question. Expansion joints and "lateral dampers" do not make the problem of a joint offset go away. In order to resist a horizontal or vertical offset of 20+ feet with the tube "remaining stationary" then the pylon must be built to allow an extra 40+ feet to account for this movement. How, exactly, does the pylon not suffer the same design flaw as the tunnel under the use case that you yourself brought up of a horizontal or vertical offset occuring along a splay?
- btilly 11y ago
  Where do you get your 40+ feet figure from? If each side can move the tunnel 10 feet relative to itself without breaking, you should theoretically be able to handle up to a 20 foot slip between plates. One side bends one away, the other the other way. The pylons move 20 feet relative to each other, and only 10 feet relative to the tube. That said, my information is based on the blueprint that Elon published and I already linked to. It includes graphs showing numerical simulations of how their design is supposed to respond to earthquakes. If you have further questions, you can start with that, then download the designs that they and others have produced and do your own work. I am not an engineer. However I do know that engineers have done exactly what I suggested and come to the conclusion that his design is feasible.
  - abduhl 11y ago
    The 40 foot figure assumes that the direction of movement is unknown and that one pylon remains fixed. I suppose you could just have each pylon capable of handling 20 feet of three dimensional movement but I doubt the economics work out in favor of that option rather than choosing towers rationally. That said, your answer is an appeal to authority. Just because a feasibility study says it is feasible does not actually make it feasible. I skimmed the document you linked and nothing in it pertains to large ground movements that would occur due to fault offset - it is all with relation to ground shaking rather than fault slip. Further, it is common knowledge, in fact, that underground structures generally perform better under ground shaking as compared to above ground structures except when the structure crosses the fault which experiences the offset. That is, tunnels work better than pylons except at the point where the large offset occurs. I understand that you may not be an engineer. I am an engineer, with my area of expertise being in geo-structural interaction, and I am telling you that your statement about a tunnel not being feasible because of earthquakes is NOT correct. There are plenty of other reasons why a tunnel makes no sense for this job, but seismic vulnerability is not a good one. Whether his design is feasible or not is a moot point.
    - btilly 11y ago
      Slip is defined by relative displacement, not absolute displacement. See http://earthquake.usgs.gov/learn/glossary/?termID=174 http://earthquake.usgs.gov/learn/glossary/?termID=174 for verification. So in a 20 foot slip, neither side actually moved 20 feet. Secondly it is very important that the tube remain still while the pylons move relative to it. Don't forget that we have objects moving down that tube at close to the speed of sound. It doesn't take a large local kink to be a fatal problem for people inside. You really don't want that tube to move. Enough so that keeping the tube still that it is an explicit design consideration to keep it still while the pylons expand and contract for thermal reasons. Thirdly I well know that earthquakes are generally less of problems underground than on the surface. But trading an easy problem in most places with an impossible problem at fault boundaries is hardly an improvement. That is why I have only focused on the potentially impossible problem. Fourth, your response was just as much an appeal to authority as mine. But my appeal to authority was an appeal to authorities whose credentials are independently verifiable, whose conclusions have been put up for public comment, and which other authorities have independently questioned, criticized, and mostly verified. Your authority is based on an anonymous claim of expertise made on the internet, in a thread where you made a basic mistake about how earthquakes are measured, with no actual analysis backing it up. Which authority is more believable?

msandford 11y ago

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.

elektromekatron 11y ago

You can include base isolation for pylons crossing known faults. - https://en.wikipedia.org/wiki/Base_isolation https://en.wikipedia.org/wiki/Base_isolation