FEA in Bridge Failure 4

[unclesyd]

This is not my area of expertise but just saw an update of 35W bridge collapse by the Chairman of the NTSB. He harped on the fact that they a marvelous computer program call FEA, checks notes to see what FEA means.

He stated that it will allow them to reconstruct the bridge and then take it apart one piece at a time to see where the bridge falls and thus the point of failure. It was my understanding that in this type truss that all members were critical.

If you can catch this news conference it will be enlightening to the extreme, on the minus side.

 

[prost]

That's a good one; "checks his notes to see what FEA means." Hey if you can model it with a computer program, it has to be right, right?

It would seem odd that you build airplanes (where weight is the overriding design criterion) with redundant load paths, but you build bridges where weight is relatively unimportant with no redundant load paths. "When it doubt, make it stout doesn't always work," does it?

Though if I were a betting man, I'd bet on corrosion...that is, the bridge performed as designed, but the design and subsequent maintenance (they may have found corrosion, but had few options for figuring how important the corrosion was to struct. integrity) didn't take into account corrosion and what it does to structural integrity. I doubt they'd admit corrosion was the problem, though.

 

[Qshake]

prost - let's not downgrade the profession of engineering with your ignorance of bridge engineering.

Bridge engineering like any facet of engineering, including aerospace, has today different constraints on it than when this bridge was designed 40 years ago. In the mid 60s and for some period prior and after, weight was a significant factor and so bridges were built with that in mind adn many bridge cross-section or member cross-sections varied nearly every 5 feet to thinner plates when possible to save weight. Today, labor is far more expensive and so we don't see this weight optimization.

Moreover, myself being the industry of heavy infrastructure structural engineering bridges, locks, towers etc, I can say that the bridge code, AASHTO, is the most conservative with respect to static and dynamic loads. Hence a non-redundant bridge that has been around for 40 years has seen much more fatigue than an airplane and it has performed as expected. This collapse is a tragedy but there are checks in place to make sure these tragadies don't happen everyday or even every decdade.

Many, many engineers and technicains know about corrosion. Anyone who has been in the industry or has an industry trade magazine can tell you this from all the advertising that corrosion protection system vendors do.

Often times we see members that have flanges or plates that are now 5 times the size they were due to the volumeric expansion of rusted laminated steel. The industry standard is to remove the coating, sand blast to a specific finish as required by the Structural Steel Protective Coating council (SSPC) and wherein necessary replace the deteriorated plates/members.

that this wasn't done here is certainly possible for a number of reasons:

1. The bridge was slated to replaced soon,
2. The bridge was encapsulated rather than paint removal,
3. The work wasn't yet set up or funded,
4. The work was part of a program and other critical areas were being addressed.

All the while, these types of bridges are inspected yearly compared to their redundant counterparts.

It's not that we don't know...but rather a matter of planning and funding and minimizing risk.


Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[unclesyd]

Here is where the news reports probably originated.

Deferring to the FEA experts, if the bridge was modelled to the extent that a recommendation could be made to defer the replacement to 2020 how can a condition exist where it is now considered detrimental to integrity of the structure?

I'm referring to the gusset plates mention in the news release.

http://www.ntsb.gov/Pressrel/2007/080807.htm

 

[prost]

"Design to code" that is now inadequate doesn't relieve maintainers of the need to properly inspect and account for continuing damage effects--of course we all realize that you can't criticize designs to code that were done 40 years ago, and we realize inspections are taking place, however I would argue in this case that the inspections themselves were inadequate--you can't adequately inspect any corroding structure with cyclic loads with just your eyeballs. Why is it considered acceptable to inspect some structure in January, say, and then it fails in August (happened in St. Louis with a cantilevered walkway hanging over a freeway)?

Here's what may be an interesting yahoo search on this very bridge, sent to me by a bridge expert.

http://216.109.125.130/search/cache?ei=UTF-8&p=rust+jacking+bridge+steel&fr=slv8-&u=

http://www.startribune.com/462/stor...bridges+steel&d=aom6q-ljPRYH&icp=1&.intl=usad

 

[EddyC]

The ongoing investigations into the collapse of this bridge will probably reveal that something unusual was the cause. I have a hard time believing that the numerous inspections that were made were inadequate. The knowledge gained from the investigation will most likely lead to positive actions with regard to our national infrastructure, actions that will probably save more lives.

 

[GregLocock]

The inspections revealed that the bridge was cracking in various ways.

As Feynman pointed out in the shuttle investigation, the time to act is not when the system breaks, but when it is showing signs of behaving in a manner that it was not designed to.

I doubt the original designer assumed that cracks and settlement in the piles would be left unfixed for years.



Cheers

Greg Locock

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