There Actually Are some Bridge Engineers Who Haven't Seen

[Jccarlton]

The Gertie Movie:
http://www.youtube.com/watch?v=WEQrt_w7gN4

For the uninitiated the Gertie movie was film taken in the 1930's of the Tacoma Narrows Bridge(Galloping Gertie) oscillation due to winds and subsequent failure due to natural frequency effects. If my experience is any guide engineering students here in the US see that film frequently before they graduate as an example of how even the best engineers get it wrong. The Gertie Movie:
http://www.youtube.com/watch?v=j-zczJXSxnw&NR=1

[Josh Cryer]

Most likely the engineers involved in the project were told to cut corners for profit, and so they did so to the best of their knowledge. The resonance is clearly no where near as bad as Gertie. Could last quite a while longer, even, and may actually be fixed at the taxpayer dime (probably in the end costing more than if they had built it properly).



Fantastic bridge, 7 km long.

[MSimon]

Most likely the engineers involved in the project were told to cut corners for profit,

All engineering is an exercise in cutting corners for profit.

See my sig.

Occasionally you cut a corner too far. That lets others in the field know what the limits are.

The alternative is to overbuild everything. But that compounds the problem. Because you need more structure to support the added structure. Better to live with the occasional failure.

[Josh Cryer]

Better to live with the occasional failure.

That sums up the for profit mentality, absolutely. Sea Dragon was an engineered rocket with margins so high that it would have worked regardless. Ahh, I miss the days when over-engineering things was considered the norm.

[DeltaV]

The Gertie Movie:
http://www.youtube.com/watch?v=WEQrt_w7gN4

The I-beam web in the outside view looks like it's stretching way too much to be physically realistic. And the guy walking along the side doesn't seem to notice the waves. I think someone has been playing with their morphing software.

[Jccarlton]

The Gertie Movie:
http://www.youtube.com/watch?v=WEQrt_w7gN4

The I-beam web in the outside view looks like it's stretching way too much to be physically realistic. And the guy walking along the side doesn't seem to notice the waves. I think someone has been playing with their morphing software.

Here's the Tacoma Narrows Bridge wiki:
http://en.wikipedia.org/wiki/Tacoma_Nar ... %281940%29
That movie is real. I saw it many times before morphing software was even invented or for that matter computers powerful enough to run morphing software even existed. There have also been multiple TV programs starring Gertie, simply because it was such a dramatic engineering failure.

[Jccarlton]

Most likely the engineers involved in the project were told to cut corners for profit, and so they did so to the best of their knowledge. The resonance is clearly no where near as bad as Gertie. Could last quite a while longer, even, and may actually be fixed at the taxpayer dime (probably in the end costing more than if they had built it properly).



Fantastic bridge, 7 km long.

Unfortunately they managed to put the support columns right on the natural frequency wave points. Even 10 meters either way and the oscillations would have been damped out of existence. Don't they teach vibrations in Russia? I can't believe that nobody bothered to calculate the natural frequency wavelength, but there it is on video.

[MSimon]

Better to live with the occasional failure.

That sums up the for profit mentality, absolutely.

Dude. If bridges and other structures were designed so that failure was impossible we could afford a lot fewer of them. That too has its costs. Which is unaccounted for in your engineering theory.

And what does the profit motive get for you? A constant drive to reduce costs and increase capability (do more with less) i.e. it is ecologically sound.

I'd be really interested in how you intend to retain that drive without the profit motive.

[MSimon]

Unfortunately they managed to put the support columns right on the natural frequency wave points. Even 10 meters either way and the oscillations would have been damped out of existence. Don't they teach vibrations in Russia? I can't believe that nobody bothered to calculate the natural frequency wavelength, but there it is on video.

The "natural frequency wave points" are called nodes. That is where you put the supports where you want minimally damped waves. In a musical instrument for instance.

[Jccarlton]

Unfortunately they managed to put the support columns right on the natural frequency wave points. Even 10 meters either way and the oscillations would have been damped out of existence. Don't they teach vibrations in Russia? I can't believe that nobody bothered to calculate the natural frequency wavelength, but there it is on video.

The "natural frequency wave points" are called nodes. That is where you put the supports where you want minimally damped waves. In a musical instrument for instance.

And there were the columns, right on the nodes, just like a violin:
http://www.youtube.com/watch?v=rVzYDIlBQ0w&feature=fvw
with a perfect standing wave effect.

[KitemanSA]

Unfortunately they managed to put the support columns right on the natural frequency wave points. Even 10 meters either way and the oscillations would have been damped out of existence. Don't they teach vibrations in Russia? I can't believe that nobody bothered to calculate the natural frequency wavelength, but there it is on video.

JC,
HUGE amounts of time and energy and money are expended trying to develop and prove methods to calculate normal mode responses in composite structures (metal bridge with jointed concrete decking is one) with varying degrees of success. This may have been the case of doing the best one could and just getting it wrong.

It is VERY difficult to make people make ugly, safe things. This bridge would have been a lot safer if the pilons had been significantly unevenly spaced. But it would have been ugly. People LIKE regular patterns. Unfortunately, so does the destructive side of nature.

[kunkmiester]

What about a natural progression--stretch them out a bit from one side to the other? Plenty of things are uneven but look good, it's a matter of finding the right pattern.

As well, on something that big, a few feet either way isn't going to be very obvious, maybe on a scale model/drawing. How big is the node?

[KitemanSA]

What about a natural progression--stretch them out a bit from one side to the other? Plenty of things are uneven but look good, it's a matter of finding the right pattern.

As well, on something that big, a few feet either way isn't going to be very obvious, maybe on a scale model/drawing. How big is the node?

How fast was the wind? If they make the pylons shorter or longer spaced (unless they are significantly variable, hence ugly) the tuning simply moves a bit up or down the wind speed chart.

Probably should have installed some tuned mass dampers.

[DeltaV]

The Gertie Movie:
http://www.youtube.com/watch?v=WEQrt_w7gN4

The I-beam web in the outside view looks like it's stretching way too much to be physically realistic. And the guy walking along the side doesn't seem to notice the waves. I think someone has been playing with their morphing software.

Here's the Tacoma Narrows Bridge wiki:
http://en.wikipedia.org/wiki/Tacoma_Nar ... %281940%29
That movie is real. I saw it many times before morphing software was even invented or for that matter computers powerful enough to run morphing software even existed. There have also been multiple TV programs starring Gertie, simply because it was such a dramatic engineering failure.

I know the Tacoma Narrows movie is real. I had to watch it in structural engineering class many years ago. I wasn't saying that structural resonance is not a reality. Not having seen the Russian bridge reports before, the I-beam's web-flexing behavior in your first video link looked suspicious, near 17 and 19 sec, like it was stretching-without-tearing more than it should. Chalk it up to crummy YouTube resolution. The Russian calmly walking down the bridge into the high amplitude oscillations is probably just a thrill seeker.

[KitemanSA]

I know the Tacoma Narrows movie is real. I had to watch it in structural engineering class many years ago. I wasn't saying that structural resonance is not a reality. Not having seen the Russian bridge reports before, the I-beam's web-flexing behavior in your first video link looked suspicious, near 17 and 19 sec, like it was stretching-without-tearing more than it should. Chalk it up to crummy YouTube resolution.

First, the Russians tend to use bulb flange beams rather than I beams so they tend to be easier to twist. They trip (i.e., "buckle sideways") fairly easily. They are GREAT in the tensile direction, not so good in compression. Second, I don't think you can see ANY stretching in the beam at that time, it is all twisting of the beam below the grated decking and flapping of the decking itself. At least, I think that is what is happening given the low res images.

Code: Select all

    
\________                _
    |                  _/
    |               _/\
    O             _/    \
                /         O

These are the two extremes as I see them. First you see a broad, light colored "beam" and then it kind of seems to be crushed to no depth. I think they are the first and second graphics above. First the "beam" is down and the deck is mostly horizontal with the tip flapping up, then the whole thing rotates about 45 degrees and the beam gets hidden by the deck.

Or not!