Mechanics in the news

MichelleLOyen's picture

Since I am an alum of the University of Minnesota, when I was a PhD student I lived only a few blocks from the site of yesterday's catastrophic bridge collapse in Minneapolis.  The statics analysis of a truss is almost the first thing learned by every undergraduate engineering student, and appears to be relevant here.  It is interesting to see words like "fatigue crack" and "vibrations" in the news .  In light of such events, never has there been a better time to step forward and emphasize the importance of mechanics in daily life!  Each time we drive across a bridge we are relying on the engineers of years past to have done their job correctly.  We can argue on this forum about the meaning of elastic modulus for a carbon nanotube , or what happens to dislocations in micropillar compression , but all around us macroscopic human-scale mechanics is in our lives, and unfortunately now also in the news.  Perhaps in this case the engineers of the 1960s were a bit lax: the interest and faith in steel as a wonder material may have caused a lack of design redundancy that has now apparently taken many lives.  I recommend the book "Why Buildings Fall Down " as a fascinating read and study of engineering/mechanics principles suitable for a lay audience... and I truly hope that in tragedy we are reminded of the fundamental importance of our great field of mechanics. There is an important educational lesson in this as well.  Getting the basics "right" is not an option--human lives are on the line if we don't put significant efforts into training the next generation of bridge designers and inspectors.  Mechanics is at the core of our modern existence.


Aaron Goh's picture

terrible

This is terrible.  When I was in academia, I had the chance to cross the Atlantic twice to Minneapolis to do some consultancy work.  I have not heard from my local contact but hope she is unaffected.  Let's hope management and mechanics sort something out so that this will not happen again.  Also while we are going nano and bio let's not forget the age old problems of water and flooding that still make us anxious.

 


Ajit R. Jadhav's picture

Re: Mechanics in the news...

Dear Michelle,

Good that you bring this up, in a timely manner... Things like these have a greater impact if brought out when memory (or the pain) is still very much fresh in mind.

I quote off-hand, and I have forgotten exactly where I read it, but I guess it was in T L Anderson's book, in the very first, introductory chapter of that book (second edition, early 1990s). He was quoting a past study which had concluded that the annual loss in dollar terms due to fracture was of the order of a billion dollars (*), and further, that most of this loss could be prevented by applying the technology that already existed back then.

(*) It would be great if someone could please correct me with the exact figure... All that I now remember is that it was a huge figure.

It would also help to put this exact figure in perspective--say, comparing it with the recent NSF budget, for instance. Or, the world-wide total annual market for CAE-related software. Etc.


zhan-sheng guo's picture

very bad news. everyone

very bad news.

everyone should do something


Henry Tan's picture

English style versus American style on mechanics

I realized that while American mechanicians are so loving nano- and bio- stuffs, many English mechanicians are still pursuing down-to-earth mechanics, which I believe is more crucial for a healthy mechanics, and many more important and real issues still unsolved there.

Fancy is needed and challenging, but everyday mechanics, with many unsolved problems, still needs our attention.

 


Re: Bridge collapse

Henry, 

Though my knowledge of the reasons for the collapse is limited, I don't think that the problem is just a design flaw.  Bridges are built with huge factors of safety which makes detailed fracture and fatigue based design somewhat irrelevant. 

I think that a more serious problem is that the money spent on the inspection and maintenance of old bridges is limited.  Because of the cost of labor in the US, each inspection and maintenance cycle is performed by a tiny number of people over a long period of time (sometimes a year or more per cycle).  Considering the complexity of the structures and the heavy flow of traffic over them, I wonder how easily inspectors can detect flaws that develop rapidly due to corrosion fatigue after a snowy winter (with lots of salt on the roads).  I believe that the mechanics is well understood but there is still some way to go to convert that understanding into useful engineering fixes.

The nano/bio push by the US government is driving the behavior of the mechanics community - which is what the intention of the policy is.   The US wants to stay ahead in the innovation race just as it did for the semiconductor revolution.  The rest of the world is following the US's lead and trying to get in on the action early.   And if nano/bio determine the future of the world that's certainly the way to go.  

There will always be some macroscale engineering problems that need to be solved.  However, those problems are unlikely to lead to revolutionions in the way we do things.  On the other hand, nano/bio innovations have the potential of changing what those macroscale engineering problems are.   I have my doubts about how much classical continuum mechanics can contribute to bringing about those innovations.

I  disagree that everyone in the US is working on nano/bio.  Though such a twist in a proposal certainly helps, the end goal of most research on mechanics continues to be to predict the response of the macroscopic world.  Finding funding is harder that it was 20 years ago - but that's probably because there are more players on the field now.

Given all that, I still wonder whether we are seeing the beginning of the demise of theoretical and applied continuum mechanics as a subject worth doing research on.

 


Ajit R. Jadhav's picture

Re: Re: Bridge collapse

1. Biswajit, as to (lack of) inspections, I think you are right on target. Specifically, inspections with a fracture mechanics and NDT approach are needed. The airline industry has demonstrated the effectiveness of frequent inspections.

In the context of US, one of the major factors driving the cost of labor up is, of course, the ridiculous sort of immigration laws you have out there! From my years of experience related to NDT in India, I can vouch for the availability of the skilled to the very highly skilled inspectors relatively inexpensively. Furthermore, it is no longer the "unreliable" Indian certificates--these days, the American certification examinations--ASNT Level I, II, III, are regularly held in India. US could avail of their specialized skills at a lower cost, but only if your politicians were not to keep beating up the populist (and unrealistic) drums.

2. On another sub-thread, my mind reels over the money the US has spent in the past for "leadership" in areas like: artificial intelligence, robotics, 100 MPa-sqrt-meter tough ceramics... The list would go on and on... Why, if all the posturings were to be believed, we would have got a "cure" for cancer already thanks to the "pursuit" of "leadership" by and in American science...

Closer to the issue here, and closer to the truth, it would be (and would have been) far wiser to spend money on manually conducted inspections, the inspectors having been equipped with the suitable and, if necessary, advanced instruments and systems, rather than pursue so many half-promising avenues like automatic condition monitoring. The American love for automation can be understood and appreciated, but wisdom still is necessary in deciding the areas of application where to pursue automation. Acoustic emission remains half-promising after 25+ years of research--and is of suspect utility for any composite, e.g., cement-concrete. Still, Americans pursue AE as if it were the silver bullet. (As to the golden bullet: There is the emerging area of "smart" sensors that would have readjusted loading in that collapsed bridge... Dream on...)

What you need is the right technology for the right job. Not necessarily AE (acoustic emission) coupled with RFID coupled with powerful communication servers uploading data via high-bandwidth satellite links to powerful supercomputers that still cannot subtract noise from the signal and make a head or tail of when or if the structure is going to fall--something a well trained man could determine when equipped with a portable instrument, if allowed to immigrate. That is the point.

(Since I spoke *against* *Americans*, a clarification is obviously in order: I have nothing against AE research as such.)


MichelleLOyen's picture

re: Bridge collapse engineering

Actually, all indications are that this failure was indeed due to a design flaw in the bridge.  The thing was simply insufficiently redundant, and so the failure DID come down to basic fracture mechanics associated with fatigue crack growth in high strength steel, the ability of inspectors to locate problematic sub-critical cracks, and it was quite probably a chain reaction failure due to buckling of a single steel part, not unlike previous failures in the Silver Bridge and the Mianus bridge.  It was simply not a bridge sufficiently designed for redundancy, a topic expounded on in great detail in "Why Buildings Fall Down."  No system of human inspections could statistically catch every single small crack that was about to go critical when the bridge itself is not sufficiently redundant.  It's like building a suspension bridge with a single cable instead of bundles of cables; we would not build this bridge today, and we need to look hard at the bridges in service that might have gone up in this era (in this case, just prior to the Silver bridge failure).

I also can't be the only person who has noticed that high profile civil engineering departments in the US have been increasingly hiring people working in nano/bio, at times causing me great puzzlement.  There will hopefully be some sort of recognition based on this incident that there is still a lot of important work to be done in civil engineering and we can't let the traditional aspects of this subject die in the midst of funding constraints and sexy research.

 


Ajit R. Jadhav's picture

Re: Design flaw in the bridge

Apparently, conformance to the design standard specifications followed in America won't necessarily get you a bridge design that was good enough.

But, of course, this assumes that the fault really was with design. ... That may not ultimately turn out to be the case. Failure analysis takes time. We all should perhaps wait...

And, is it alwys possible to build redundancy in designs? I am not so sure. Not just from cost angle, but also from weight, fabrication and maintenance angles. Redundancy is easier to build into electronic products. For civil or mechanical structures, probably, rechecking the design from angles other than strength--such as structure stiffness (buckling), vibrational modes, resonance is more important than redundancy as such. I am not sure they always checked these matters back in 1960s. Neither do I know the status as of today in practical civil engineering.


De Xie's picture

Research is like a fashion show

Currently, not only in the United States, research in science and technology is becoming a fashion show rather than a pursuit of the truth (or driven by the human curiosity).

Great majority of those so-called achievements are judged simply by  accounting how many junk papers and useless approaches are made.  As to mechanics, everyone talks about bio, nano, atom, MEMS .… These just look like the elegant and expressive cloths for fashion shows not for daily dressing.   As a result, we didn’t see any breakthrough like FEM, HRR or J-integral for many years.

The only good thing for such circumstance is to generate some young "fashion show models" who win the fame and the money.  Nothing more than this.

But who cares? 

This is the reality, not right or wrong.


Vladimir Vinogradov's picture

I agree that the nano/bio

I agree that the nano/bio research is important, very interesting, financially encouraging, modern and fashionable. All this is true. But we still should remember that we don't drive nano, don't fly nano and don't rely on nano in our everyday life. But we are going to fly new Boeing 787 made of composites, we need buildings and bridges that do not collapse, and there are plenty of unresolved solid mechanics problems that require our attention and better understanding. In my opinion, disproportional attention to one specific branch of mechanics and its exclusive financial support does not help to any of this. 


Nano/Bio could have tremendous impact

Nano/Bio could have a tremendous impact, even in bridge building. For example, I think that by dispersing nano-materials in the steel matrix one could make a material which is much stronger/fatigue resistant than the normal materials. Just because we live in a macro world, there is no need to neglect research at the nano scales. After all, macro is made up of micro (nano) building blocks. A better understanding will lead to better materials. Please correct me if I said something wrong: my background is not in nanotechnology.

 

 -Amit Ranade 


Zhigang Suo's picture

Imagine continuum mechanics as a revolutionary idea

A while back I wrote a few lines with a title, Flip test: imagine continuum mechanics as a revolutionary idea.  In learning about hydrogels in recent months, I have been once more reminded of the remarkable versatility of continuum mechanics, as well as its great intellectrual depth.

An old example illustrates another point.  When Griffith (~1920) and Irwin (~1950) formulated fracture mechanics, theory of elasticity was well established.  But these individuals combined the well established theory of elasticity and a few other ideas to produce fracture mechanics.  Once the basic ideas became clear, a wonderful period in mechanics followed.  It is hard to predict innovation, especially in a field like mechanics with such a long and rich history. 


Aaron Goh's picture

Nanomania

In my opinion, we have always played around with our materials except we call it materials science or engineering or chemistry. 

For example, if we take the first paragraph in Virginia's text for the Journal Club Theme this month and remove the word nano, the remaining text is still ok and there is absolutely no loss of information.  The science remains the same and only the size is different. 

One may argue that the second sentence may then sound a little funny, but it is still correct, because even at macroscale, there are materials that 'may be highly compliant, heterogeneous, or possess unique morphological characteristics'.  One such materials is something we put into our mouths everyday to give us pleasure and fill our stomachs.