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>>Will you build a bridge that’s inexpensive, but will have to be replaced in 25 years, or will you spend twice the money for more concrete and more steel to ma

>>Will you build a bridge that’s inexpensive, but will have to be replaced in 25 years, or will you spend twice the money for more concrete and more steel to make it last for 50? > Of course the second (which is what is actually usually done) the issue being that in real world after 50/60/70/80 years the bridge won't be replaced/repaired . Why does that time extension hold for the 50-year bridge but not the 25-year bridge?

bumby 4y ago
It does, but to a different probability of failure. A 25-year bridge extended to 35 years will have a higher probability of failing during that 10 year extension than a 50 year bridge extending life to 60 years. The most common survivability analysis is a Weibull model which typically has a longer right tail to the failure distribution.
- knorker 4y ago
  The fair comparison is 25 extended to 30 vs 50 extended to 60, though. In terms of cost per year of having a bridge.
  - bumby 4y ago
    You’re right, I misinterpreted your point being about absolute age extension, not relative
jaclaz 4y ago
The point I was trying to make[0] is independent from the actual planned duration of the structure[1]. Of course it would apply also to the less lasting one. In a perfect world, besides ordinary maintenance (which rarely is performed anyway in a timely manner) when a bridge is near to its planned end of life (let's stay on the 50 year duration) it should be inspected and - if it is the case - its life could be extended by - say - 10% or 5 years, BUT there should already be present: 1) the projects and all authorizations for the new bridge 2) a reserve of money ready to be spent on the new bridge Then (and this is actually an increased cost/waste) these "granted" extensions of life could be repeated a number of times BUT should be "hard-capped" anyway to - say - 30% of the planned duration, in name of more certain safety, it will happen that you will demolish and rebuild a perfectly good bridge, but you will also in many cases prevent a disaster. What happens in the real world is that once the 50 years have passed, an inspection is made, and it is determined that everything is good enough, let's make another inspection in 5 years time or let's make this minor repair, and let's see what happens in 5 years time, rinse and repeat until the structure has double the planned lifetime and finally something happens (be it a structural failure, an accident, whatever) that forces to close the bridge (or to reduce load on it or in the worst case the bridge collapses) and at this point starts the race to make the project, get the authorizations, gather the money, etc, which adds two or three years to the time needed to have the new bridge built and usable. If you prefer, the issue I was trying to point out is that usually we need an "unexpected" event to trigger an "emergency response" whilst the bridge nearing its end of life is something known and expected. [0] which is that once something is built, for some reasons it is often considered (wrongly) to be eternal [1] which anyway is only a very rough estimate, not an exact number, duration for reinforced concrete or steel bridges depends on so many factors that is impossible to determine accurately a possible "structural" life.
- bluGill 4y ago
  The vast majority of bridges do not fail. Sure a few get in the news because they fail early, and some are sabotaged (earthquake, war, or some other event that cause an early failure), but the vast majority of bridges last until they are intentionally torn down.
  - jaclaz 4y ago
    Sure they don't, but they also do not last for eternity (without maintenance or repairs). The theme is more about whether to build "massive" (and expensive) more lasting structures or build more economical but less lasting structures and how to properly evaluate the expected lifetime of these structures. We know empirically that many (reinforced concrete) structures built 70/80 years ago (some even less than that) start showing a number of issues (that often require "heavy" repairs) besides the few that made it to the news. We don't have that many examples of earlier constructions of this kind (pre-stressed concrete is relatively recent as a technology), but we do know that in many countries a large number of bridges were built in the '60's and '70's so they are nearing the time they will reach the age we already found to be critical.
    - bluGill 4y ago
      In short the system of inspections is working.
      - jaclaz 4y ago
        >In short the system of inspections is working. Actually no, it isn't working because it cannot possibly work, at least not with decades old structures (that however do not collapse because they were often over-engineered, because after all they weren't built so shoddily and for a thousand other possible reasons). A bridge inspection more often than not amounts to a visual inspection, to detect cracks, check bearings, but while some elements of the structure are visible (as said bearings and concrete surface) the state of internal cables is hard or impossible to check, though there are some new techniques that look promising (though they are not yet AFAIK properly documented/standardized), as an example, JFYI: https://www.youtube.com/watch?v=pKWxpHsLBRk https://www.youtube.com/watch?v=pKWxpHsLBRk And there is another key point, norms have obviously changed a lot in the several decades since the construction, original calculations and drawings may have been lost, and there is the need to re-do the calculations with the new standards of loads and check whether the current loads/frequency of traffic is compatible with those the original project took into account.

bumby 4y ago

It does, but to a different probability of failure. A 25-year bridge extended to 35 years will have a higher probability of failing during that 10 year extension than a 50 year bridge extending life to 60 years. The most common survivability analysis is a Weibull model which typically has a longer right tail to the failure distribution.

knorker 4y ago
The fair comparison is 25 extended to 30 vs 50 extended to 60, though. In terms of cost per year of having a bridge.
- bumby 4y ago
  You’re right, I misinterpreted your point being about absolute age extension, not relative

jaclaz 4y ago

The point I was trying to make[0] is independent from the actual planned duration of the structure[1]. Of course it would apply also to the less lasting one. In a perfect world, besides ordinary maintenance (which rarely is performed anyway in a timely manner) when a bridge is near to its planned end of life (let's stay on the 50 year duration) it should be inspected and - if it is the case - its life could be extended by - say - 10% or 5 years, BUT there should already be present: 1) the projects and all authorizations for the new bridge 2) a reserve of money ready to be spent on the new bridge Then (and this is actually an increased cost/waste) these "granted" extensions of life could be repeated a number of times BUT should be "hard-capped" anyway to - say - 30% of the planned duration, in name of more certain safety, it will happen that you will demolish and rebuild a perfectly good bridge, but you will also in many cases prevent a disaster. What happens in the real world is that once the 50 years have passed, an inspection is made, and it is determined that everything is good enough, let's make another inspection in 5 years time or let's make this minor repair, and let's see what happens in 5 years time, rinse and repeat until the structure has double the planned lifetime and finally something happens (be it a structural failure, an accident, whatever) that forces to close the bridge (or to reduce load on it or in the worst case the bridge collapses) and at this point starts the race to make the project, get the authorizations, gather the money, etc, which adds two or three years to the time needed to have the new bridge built and usable. If you prefer, the issue I was trying to point out is that usually we need an "unexpected" event to trigger an "emergency response" whilst the bridge nearing its end of life is something known and expected. [0] which is that once something is built, for some reasons it is often considered (wrongly) to be eternal [1] which anyway is only a very rough estimate, not an exact number, duration for reinforced concrete or steel bridges depends on so many factors that is impossible to determine accurately a possible "structural" life.

bluGill 4y ago
The vast majority of bridges do not fail. Sure a few get in the news because they fail early, and some are sabotaged (earthquake, war, or some other event that cause an early failure), but the vast majority of bridges last until they are intentionally torn down.
- jaclaz 4y ago
  Sure they don't, but they also do not last for eternity (without maintenance or repairs). The theme is more about whether to build "massive" (and expensive) more lasting structures or build more economical but less lasting structures and how to properly evaluate the expected lifetime of these structures. We know empirically that many (reinforced concrete) structures built 70/80 years ago (some even less than that) start showing a number of issues (that often require "heavy" repairs) besides the few that made it to the news. We don't have that many examples of earlier constructions of this kind (pre-stressed concrete is relatively recent as a technology), but we do know that in many countries a large number of bridges were built in the '60's and '70's so they are nearing the time they will reach the age we already found to be critical.
  - bluGill 4y ago
    In short the system of inspections is working.
    - jaclaz 4y ago
      >In short the system of inspections is working. Actually no, it isn't working because it cannot possibly work, at least not with decades old structures (that however do not collapse because they were often over-engineered, because after all they weren't built so shoddily and for a thousand other possible reasons). A bridge inspection more often than not amounts to a visual inspection, to detect cracks, check bearings, but while some elements of the structure are visible (as said bearings and concrete surface) the state of internal cables is hard or impossible to check, though there are some new techniques that look promising (though they are not yet AFAIK properly documented/standardized), as an example, JFYI: https://www.youtube.com/watch?v=pKWxpHsLBRk https://www.youtube.com/watch?v=pKWxpHsLBRk And there is another key point, norms have obviously changed a lot in the several decades since the construction, original calculations and drawings may have been lost, and there is the need to re-do the calculations with the new standards of loads and check whether the current loads/frequency of traffic is compatible with those the original project took into account.