4 ms·

This is totally hearsay, but I've heard from multiple older engineers that as computer models have been improved over time the safety factor has been reduced as

This is totally hearsay, but I've heard from multiple older engineers that as computer models have been improved over time the safety factor has been reduced as we have more accurate simulations. If the safety factors from the '50s had been used it's possible this bridge would never have been designed the way it was.

dredmorbius 7y ago
I've noticed from videos of recent bridge failures (FIU and the Taiwain Arch Bridge collapse a few weeks ago) how fast the failures occur. Contrast with, say, the Tacoma Narrows bridge failure in the 1940s. I suspect current designs are far nearer critical tolerances. Even under the ridiculous dynamics of the Tacoma Narrows incident, ultimate failure took hours.
- userbinator 7y ago
  Even under the ridiculous dynamics of the Tacoma Narrows incident, ultimate failure took hours. It took 4 months from construction to failure, during which time it was already flexing far more than normal.
  - dredmorbius 7y ago
    The FIU and Taiwan's Nanfang'ao Bridge collapse (https://en.wikipedia.org/wiki/Nanfang%27ao_Bridge https://en.wikipedia.org/wiki/Nanfang%27ao_Bridge) each precipitated in seconds. Contrast Tacoma Narrows, whose final throes were long enough for crowds to form and traffic to be cleared. Tacoma: https://youtube.com/watch?v=mXTSnZgrfxM https://youtube.com/watch?v=mXTSnZgrfxM FIU: https://youtube.com/watch?v=BSq8295GFk4 https://youtube.com/watch?v=BSq8295GFk4 Nanfang'ao: https://youtube.com/watch?v=mjsZbbG5fag https://youtube.com/watch?v=mjsZbbG5fag Morandi, Genoa, Italy: https://youtube.com/watch?v=_gEMep8xLHk https://youtube.com/watch?v=_gEMep8xLHk I-35W, Minneapolis, MN: https://youtube.com/watch?v=74JNl5n-YdI https://youtube.com/watch?v=74JNl5n-YdI
jaclaz 7y ago
The (approximate) rule of the thumb is that anything in reinforced concrete calculated by "hand" has an "implied" additional safety factor of 5-10%, mainly due to roundings by excess in the calculations, at least this has been my experience in the mid 1990's, when for a couple of bridges existing hand made calculations had been re-made with computers. It is a small (but good) thing, though it would have IMHO not been enough to save this particular bridge, here the issue is evidently a wrong static model, or - less likely - a wrong assessment of loads. Besides the initial (huge) calculation error, what is really preoccupying is that they didn't consider as serious the cracks, which as seen in the video/photos on youtube are the typical (severe) shear cracks, that should have been ringing all alarms. The poor structure, as often happens, did all it could to "signal" the issue, what I find bordering being criminal is ignoring or severely underestimating those clear signals.

dredmorbius 7y ago

I've noticed from videos of recent bridge failures (FIU and the Taiwain Arch Bridge collapse a few weeks ago) how fast the failures occur. Contrast with, say, the Tacoma Narrows bridge failure in the 1940s. I suspect current designs are far nearer critical tolerances. Even under the ridiculous dynamics of the Tacoma Narrows incident, ultimate failure took hours.

userbinator 7y ago
Even under the ridiculous dynamics of the Tacoma Narrows incident, ultimate failure took hours. It took 4 months from construction to failure, during which time it was already flexing far more than normal.
- dredmorbius 7y ago
  The FIU and Taiwan's Nanfang'ao Bridge collapse (https://en.wikipedia.org/wiki/Nanfang%27ao_Bridge https://en.wikipedia.org/wiki/Nanfang%27ao_Bridge) each precipitated in seconds. Contrast Tacoma Narrows, whose final throes were long enough for crowds to form and traffic to be cleared. Tacoma: https://youtube.com/watch?v=mXTSnZgrfxM https://youtube.com/watch?v=mXTSnZgrfxM FIU: https://youtube.com/watch?v=BSq8295GFk4 https://youtube.com/watch?v=BSq8295GFk4 Nanfang'ao: https://youtube.com/watch?v=mjsZbbG5fag https://youtube.com/watch?v=mjsZbbG5fag Morandi, Genoa, Italy: https://youtube.com/watch?v=_gEMep8xLHk https://youtube.com/watch?v=_gEMep8xLHk I-35W, Minneapolis, MN: https://youtube.com/watch?v=74JNl5n-YdI https://youtube.com/watch?v=74JNl5n-YdI

jaclaz 7y ago

The (approximate) rule of the thumb is that anything in reinforced concrete calculated by "hand" has an "implied" additional safety factor of 5-10%, mainly due to roundings by excess in the calculations, at least this has been my experience in the mid 1990's, when for a couple of bridges existing hand made calculations had been re-made with computers. It is a small (but good) thing, though it would have IMHO not been enough to save this particular bridge, here the issue is evidently a wrong static model, or - less likely - a wrong assessment of loads. Besides the initial (huge) calculation error, what is really preoccupying is that they didn't consider as serious the cracks, which as seen in the video/photos on youtube are the typical (severe) shear cracks, that should have been ringing all alarms. The poor structure, as often happens, did all it could to "signal" the issue, what I find bordering being criminal is ignoring or severely underestimating those clear signals.