Seeing Single

Why doesn’t it fall down? Why don’t the two halves fall off?

I’m sure there’s a sound reason — something to do with loads — but I’ve grown so used to seeing cable-stayed bridges with cables on both sides of the driving surface (here and here and here and here [in hybrid form with a suspension construction]) and, more recently, here . . .

. . . that seeing a bridge with cables just in the middle was disconcerting somehow.

And yet when I go back and actually look at other bridge photographs, I find that some of those bridges also used just a middle set of cables. Here, for instance. So much for my visual memory.

Anyway, this bridge did not fall down nor did it show any sign of distress. So much for my engineering intuition.


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11 Responses to Seeing Single

  1. Tom Watson says:

    Beats the heck out of me. Anything beyond a teeter-totter and I’m lost.

    I’m calling an engineer friend as we speak.


    • Isabel Gibson says:

      Tom – Good luck with that! I’ve tried a few times to get it clear in my head and have about decided that I just don’t think the required way.

  2. Jim Taylor says:

    Does it ever bother you that the people who design these structures only needed to get things 50% right to pass? It scares the hell out of me sometimes!

    Jim T

    • Isabel Gibson says:

      Jim T – I expect we’ll hear from an engineer who graduated way above that mark, but I think part of the answer in this country (and the West in general, I assume) is supervision by additionally qualified engineers plus design factors that codify a safety factor of (if I remember correctly) 100% – so you design things for double their expected load.

  3. Then there’s the pedestrian/bike bridge behind South Keys. If not a cable-stayed bridge, it is a variation thereof.

    First attempt – bad concrete > down it came (by the builder).
    Second attempt – bad cable attachment design > delayed over a year (?) and redesigned and finished.

    I think about those two follies every time I go over it.

    • Isabel Gibson says:

      Jim R – Yes – and to Jim T’s point, both flaws showed up in inspections (I’m assuming). By contrast, the presumed contracting failures seem to have no oversight! For those unfamiliar with the bridge or the saga, here’s a link: Ottawa Citizen.

    • Isabel Gibson says:

      Jim R – Yes – and to that point, both flaws showed up in inspections I’m assuming. By contrast, the presumed contracting failures seem to have no oversight!

  4. Yikes! makes me think of the Tacoma Narrows Bridge in Washington State that came down November 7, 1940 due to wind stress. I remember my engineer Dad telling me about it when I was very young. My nervousness around high bridges no doubt stems from that particular engineering failure.

    • Isabel Gibson says:

      Laurna – Yes, it’s easy to remember notable events rather than a representative sample. And like every other discipline, engineering does fail from time to time.

  5. John Whitman says:

    Isabel – as a retired P.Eng. (Civil) I could go into a long explanation (if I remembered it all) about cantilever designs affected by counter-balanced loads, live loading from traffic, dead loading from the bridge itself and the loads imposed by high winds which can also impose harmonic loading. Since I’m doing well just to remember most of the things that affect the design for a single cable-stayed bridge, I’ll quit while I am sort of ahead.
    WRT to Jim’s comment about only requiring 50% to pass, I can’t let that stand. In my time, engineering was a five (5) year program which amounted to about 60 credit courses in total in which an engineering student was not allowed any “D’s”, i.e. a 50 to 60% numeric score. If you got a “D” you repeated the course and maybe the year, simple as that. Maybe that helps explain why I am the way I am as I never had to repeat an engineering course.

    • Isabel Gibson says:

      John – Well, that’s good to know. In some things (medicine being another that comes to mind), the minimum standard should be more than 50 – 60%.

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