41 ms·

Why is that? I've already put all my knowledge on the topic on display so I can't tell if this is a joke about aesthetics or if there is actually something neat

Why is that? I've already put all my knowledge on the topic on display so I can't tell if this is a joke about aesthetics or if there is actually something neat having to do with the gaps

hinkley 3y ago
I know from tales of professional cycling that those metal bridges are vastly disproportionately responsible for broken bones. They're also squirrely when wet, and if you've ever ridden across one on a bike or a motorcycle, they are fucking terrifying because you can see the river below you, and the railings for some reason tend to be very low. And while I understand that many bridges don't really prevent runoff into the water flowing beneath them, metal mesh bridges really can't.
- maxbond 3y ago
  I'm not sure you're talking about the same thing though, I think they meant these thermal expansion joints, not necessarily a bridge made from metal mesh: https://i.pinimg.com/originals/4d/82/91/4d8291e022bd14c87985df11fad1be1e.jpg https://i.pinimg.com/originals/4d/82/91/4d8291e022bd14c87985... I know that picture is a mesh, I thought it had the best detail, but they're on paved bridges too: https://siamagazin.com/wp-content/uploads/2018/03/23h32h-min-1.jpg https://siamagazin.com/wp-content/uploads/2018/03/23h32h-min...
  - hinkley 3y ago
    I think they meant the former, because otherwise having expansion joints that don't expand doesn't really accomplish anything. The concrete still expands, hence the need for the joints.
    - maxbond 3y ago
      My reading is that they were saying this expansionless material isn't likely to be useful for bridges, because thermal expansion is well managed by these traditional techniques. I agree though that both the thermal coefficient of the teeth probably don't impact their performance (the expansion in the teeth relative to the width of the gap is negligible, so we can probably pretend we're using expansionless teeth already), and I've never had an accident on one of these bridges, but I'm sure you're right and that the mesh concentrates the force on your bones like a golf club.
      - pfdietz 3y ago
        And because this material is 35% nickel. Nickel costs $21/kg.
    - jaclaz 3y ago
      The whole idea with reinforced concrete is that concrete has thermal expansion very similar to that of steel, thus both steel rebars and the concrete that encloses them move in the same manner. A rule of the thumb with bridges is that the movements (thermal expansion/contraction + elongation or contraction due to loads + in case of pre-stressed concrete fluage/creep) is in the range of 6-9 mm every 10 m. Expansion joints on (long) continuous beam bridges can thus need to have very large displacement, up to 1,200-1,400 mm are relatively common.
- thaumasiotes 3y ago
  > and the railings for some reason tend to be very low This isn't just a problem on bridges. Guardrails in shopping malls seem to come up to thigh height, well below a normal person's center of gravity. I hate it and I can't understand who thought that would be a good idea. I want guardrails that -- if I should happen to be propelled into them -- will stop me from falling over the side. Not rails that will tip me over headfirst.
  - hinkley 3y ago
    And then on a bike it hits you below the knee.
- burnished 3y ago
  I'm actually just talking about the expansion joints! Like one or two places that are metallic, the rest can be concrete. I havent biked over one but I expect that they are one of the least hazardous of the hazardous terrains - I'm comparing them mentally to pot holes and rail lines (not sure the right for them, street car lines that as it turns out are perfectly shaped to get a bicycle tire stuck in while at speed). Just between you and me, fuuuuuck those metal grates on bridges. I'm grateful that I have almost always had the opportunity to get onto a good sidewalk/path instead of navigating a bridge with an 'interesting' surface.

Why is that? I've already put all my knowledge on the topic on display so I can't tell if this is a joke about aesthetics or if there is actually something neat having to do with the gaps

hinkley 3y ago

I know from tales of professional cycling that those metal bridges are vastly disproportionately responsible for broken bones. They're also squirrely when wet, and if you've ever ridden across one on a bike or a motorcycle, they are fucking terrifying because you can see the river below you, and the railings for some reason tend to be very low. And while I understand that many bridges don't really prevent runoff into the water flowing beneath them, metal mesh bridges really can't.

maxbond 3y ago
I'm not sure you're talking about the same thing though, I think they meant these thermal expansion joints, not necessarily a bridge made from metal mesh: https://i.pinimg.com/originals/4d/82/91/4d8291e022bd14c87985df11fad1be1e.jpg https://i.pinimg.com/originals/4d/82/91/4d8291e022bd14c87985... I know that picture is a mesh, I thought it had the best detail, but they're on paved bridges too: https://siamagazin.com/wp-content/uploads/2018/03/23h32h-min-1.jpg https://siamagazin.com/wp-content/uploads/2018/03/23h32h-min...
- hinkley 3y ago
  I think they meant the former, because otherwise having expansion joints that don't expand doesn't really accomplish anything. The concrete still expands, hence the need for the joints.
  - maxbond 3y ago
    My reading is that they were saying this expansionless material isn't likely to be useful for bridges, because thermal expansion is well managed by these traditional techniques. I agree though that both the thermal coefficient of the teeth probably don't impact their performance (the expansion in the teeth relative to the width of the gap is negligible, so we can probably pretend we're using expansionless teeth already), and I've never had an accident on one of these bridges, but I'm sure you're right and that the mesh concentrates the force on your bones like a golf club.
    - pfdietz 3y ago
      And because this material is 35% nickel. Nickel costs $21/kg.
  - jaclaz 3y ago
    The whole idea with reinforced concrete is that concrete has thermal expansion very similar to that of steel, thus both steel rebars and the concrete that encloses them move in the same manner. A rule of the thumb with bridges is that the movements (thermal expansion/contraction + elongation or contraction due to loads + in case of pre-stressed concrete fluage/creep) is in the range of 6-9 mm every 10 m. Expansion joints on (long) continuous beam bridges can thus need to have very large displacement, up to 1,200-1,400 mm are relatively common.
thaumasiotes 3y ago
> and the railings for some reason tend to be very low This isn't just a problem on bridges. Guardrails in shopping malls seem to come up to thigh height, well below a normal person's center of gravity. I hate it and I can't understand who thought that would be a good idea. I want guardrails that -- if I should happen to be propelled into them -- will stop me from falling over the side. Not rails that will tip me over headfirst.
- hinkley 3y ago
  And then on a bike it hits you below the knee.
burnished 3y ago
I'm actually just talking about the expansion joints! Like one or two places that are metallic, the rest can be concrete. I havent biked over one but I expect that they are one of the least hazardous of the hazardous terrains - I'm comparing them mentally to pot holes and rail lines (not sure the right for them, street car lines that as it turns out are perfectly shaped to get a bicycle tire stuck in while at speed). Just between you and me, fuuuuuck those metal grates on bridges. I'm grateful that I have almost always had the opportunity to get onto a good sidewalk/path instead of navigating a bridge with an 'interesting' surface.