MN Bridge Collpase Part III 1

[WpgKarl]

A guy in my office (who isn't an engineer by the way) had a theory. I believe the bridge spanned over the river without intermediate supports....one would have to assume that there would be some pretty significant expansion joints at each end of the bridge, due to the very long span.

Now let's just say that the work that was being done on deck repairs happened to cause the one or both of the expansion joints to fill with debris and that it was no longer capable of performing per the original design intent.

It was a pretty hot day the day the bridge collapsed....perhaps it tried to expand, but was restrained causing a large build-up of force. To relieve that force, perhaps one of the weakest members in the system buckled, starting a chain reaction (due to a lack of redundancy) which led to the bridge failure? Sounds plausible to me...anybody else's thoughts?

Would this bridge have had more than 2 expansion joints?

 

[Qshake]

It's plausible that the expansion joints could have filled with debris but not likely. Most DOT's have their own inspectors, full time, on site all the time watching stuff like this. Plus with traffic on the bridge only part of the expansion joint would have filled. Of course, the other portion could have filled from previous phase of work...but...

It's more likely that the bridge's expansion bearings didn't work any more any way. Forty years is a long time and it only takes a little bit of debris to immobilize a bearing.

Lastly, I don't recall a chain reaction or a progressive collapse. The main span collapsed in whole and an event prior to that initiated the main span. That's pretty quick.

Regards,
Qshake

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

 

[shin25]

Also, the accumulated dirts will rarely be compacted enough to cause the expansion dams to completely sieze the movement.

 

[hokie66]

The bearings were not working. That has been acknowledged in the various reports produced before the collapse.

 

[prost]

Is it common practice to let a bridge stay open when the bearings don't work? What percentage of bridges have bearings that are known to be non-working and the bridge stays open?

 

[bridgebuster]

I doubt that anyone knows what percentage of bearings are not working. It's usually high rockers and rollers that give the most trouble, which is one reason why they're not used on new bridges.

As far as keeping a bridge open with non-working bearings, see this thread:

http://www.eng-tips.com/viewthread.cfm?qid=130405

Sometimes owners wait too long, even when they've known about a problem for years.

 

[Qshake]

Prost - Of the bridges that have older bearings prone to lock-up from debris, I'd venture a guess to say somewhere in the 50-75% are locked and open to traffic.

Of that group, the majority are bridges on little known roads, as most of the interstate bridges have been replaced in recent years. However, this does include large bridges where bearing maintenance is a huge headache.

Most of these bridges will never see issues as the piers the expansion bearings sit on where detailed similar to the fixed piers for economy in formwork and construction. And since the movement is taken up by the elastic deformation of the piers having the piers designed to handle the lateral load is good. So there is a bit of a safety net for locked up expansion devices.

Regards,
Qshake

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

 

[hokie66]

Seems to me the Albany bridge failure that bridgebuster referenced is another telling argument against the current bridge rating system. The bearings were rated 2, the entire bridge had a weighted rating of 5. So there was no problem because "you must look at the whole system". Bunkum!

 

[bridgebuster]

There was a lot of human error in the Albany situation. The problem fell through the cracks and the bridge nearly fell off the pier.

The problem isn't so much the weighted system it's the people using it. Before inspectors carried lap tops with them there was no weighted system. You had to put some thought into the rating. Now, you enter your numbers, the computer spits out a number, done. You could override the rating but then might have to defend your decision. So you leave the rating as is, issue a flag to cover your ass and walk away. It's up to next person on the food-chain to address the flag.

Another problem with inspection is finding capable people. In the NY metropolitan area good inspectors are a valuable commodity. When agenciess award inspection contracts, they select firms that have experienced people the agencies trust. I've seen too many inspectors who have eyes but don't see.

Usually team leaders are younger engineers with PE's but I've also come across a lot of young engineers who want nothing to do with bridge inspection; that p----s me off (same goes for construction inspection). They don't realize what they can learn. They think they'll be missing something if they leave the office for a year.

Granted, you've seen one rusty beam, you've seen them all, but, spend a year looking at bridges and you'll learn a lot about how they are put together and how things are supposed to work. And when you find a problem, you learn how to diagnose it.

My $0.02 worth.

 

[prost]

Is the problem inspector quality or the process quality?

http://www.suburbanchicagonews.com/couriernews/news/505269,3_1_EL10_A4BRIDGES_S1.article

I think the subheading "Skills spotty at best" is goring the wrong ox, that the problem isn't experience of the inspector, it's the inadequacy of the process--recently we saw a picture in the paper of an inspector with binoculars, looking at a bridge structure 90 feet away! Now how is that inspector, no matter how good he or she is, supposed to perform an adequate inspection with just his eyes and a pair of binoculars, so far away? Look at the variation in the inspectors ratings! Is that inexperienced inspectors making errors, or a bad process for inspecting?

What would I recommend? I don't know, you guys are the experts. If I had to hazard a guess, I would guess either you need to improve the process to reach less variation in the rating system, or start retire (that is, demolish) bridges when the design life is reached.

 

[prost]

In case anybody is interested articles written by the FHA on this very subject:

http://www.tfhrc.gov/pubrds/marapr01/bridge.htm

 

[prost]

and the actual report

http://www.tfhrc.gov/hnr20/nde/01020.htm

 

[Qshake]

Prost - I certainly have no argument with your noting that improvements can be made. If fact, that can be said of a lot of processes that were originated decades ago.

I will note that taken out of context, an inspector using binoculars looks rather suspect, but perhaps there is another facet you're not considering. As as a technical professional you feel it looks bad or inadequate what about lay people. Hence my real concern. You have logical skills most lay people don't.

The inspector could have been engaged in a reconaissance survey of the works, identifying areas that he will note require further inspection. Also, inspectors are trained to identify hotspots in the design plans before hand. As an example, cover plates welded to girders usually gave increased capacity right at the maximum response (in this case moment) and were very efficent and economical, that is until we found out that the weld details were stress risers and under fatigue were prone to cracks, cracks that could propagate into the flange and web.

Minus the cover plates, most slab and girder bridges perform excellently...well the girders anyway. Decks in areas subject to ice and snow always do poorly after about 15 years. So as the inspector looks at the girder bridge many times there is nothing really look for.

Another point to make is that typical structures...common highway overpasses are not fracture critical. In fact, most standard type bridges have a lot of reserve capactiy that is inherent in a simplistic design approach. Bridges of that nature are not designed using 3D plate or shell elements that yield stresses that are more realistic. In fact, the analysis is simplistic but conservative and results in a very good level of confidence.

Regards,
Qshake

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

 

[mdstveng]

I think it is also worth noting that the Dunn Memorial bridge, as noted by bridgebuster, had a partial collapse due to a combination of issues. The poor condition of the rocker bearings in conjunction with the flexibility of the pier, which deflected in the longitudinal direction(i.e. along travel lane), caused a pretty scary scenario.

Regards,
mdstveng

 

[archeng59]

http://www.startribune.com/462/story/1386423.html

CSI News reports that Spanish bridge designer Santiago Calatrava is being supported by a Minnesota real estate consultant to design the replacement bridge.

 

[HgTX]

How much influence does a "Minnesota real estate consultant" have on this process? That bridge needs to be on a very fast track and a not-entirely-insane budget and they're going to have to take a very no-nonsense approach. Such a philosophy does not include bringing in fancy international architects.

This quote from the article really annoys me: "If you have to hire a bridge designer anyway, why not hire a good one?" As if no one but these two or three international stars is a "good" designer? There are some really nice and even award-winning bridges in this country that have come entirely from in-house DOT designers, and plenty others done by "ordinary" design consultants.

Hg

Eng-Tips policies: faq731-376

 

[Qshake]

Hg - I agree wholeheartedly. I was really disappointed about the article.

The other issue I have is bridge designer versus bridge engineer. This is very confusing for the public. They are using the 'designer' in context with architecture and we use the term in the context of engineering. I wish there was a media booklet on this that would set them straight so they didn't confuse the rest of the world.

Regards,
Qshake

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

 

[archeng59]

The comment, "If you have to hire a bridge designer anyway, why not hire a good one?", really irked me. It smacks of contempt for U.S. engineers. In my opinion, the Minnesota engineering associations need to do some PR to inform the general public, and some architects apparently, that the U.S. engineers are just as capable as Calatrava and Menn in the design and engineering of bridges.

 

[bridgebuster]

We went after a bridge job last year; Calatrava was on our team. We made the shortlist but didn't win; his concept wasn't anything spectacular.

He's designing a commuter rail station in the World Trade Center site. The main feature is the retractable roof, which will open only once a year. A huge waste of money.

 

[Robertmet]

Here's another one you can talk about . A new bridge collapsed in Pakistan !

http://www.foxnews.com/story/0,2933,295484,00.html