Bridge Failure Comments 3

[Robertmet]

Here are some interesting comments about bridges as a result of the Minnesota bridge failure.Interesting that you can design but can't inspect bridges !

 

[youngstructural]

Although funny, I can see some sense to not allowing bridge designers to inspect unless trained to do so...

Before anyone jumps on me, please let me be clear:

I worked as a bridge maintenance engineer for Public Works and Government Services Canada which owns and operates all bridges accross provincial borders and on federal land (parks, reserves, etc). We inspected the bridges on a regular cycle including monthly visual, annual detailed, and bi-annual comprehensive detailed inspections.

I am a Structural Engineer by education and experience. I have designed many structures, including bridges, and do not feel that my experience as a designer would have qualified me for the work I did as a bridge maintenance engineer. You would certainly be better equipped than a lay person, but you would not be properly ready for the what, where, how and when of bridge inspection.

I do feel very strongly that all bridge inspectors should be Structural Engineers and disagree with the american practice of employing people who's only training is in bridge inspection. You need a great deal of understanding about the materials and underlying systems to do the job well, but you also need some hands-on, practical instruction in the field. That experience also really must come from a senior bridge maintenance engineer, who was in turn trained by their predecessors. In the case of my old employer at Public Works, our group (under various names and structures) had been taking take of most of those bridges for over one hundred years, most of them since the day they were built. There is an in-house body of knowledge, including intimate knowledge of some of the bridges in question, passed down through generations of engineers...

All of that is now being rapidly lost. Our comprehensive inspections were already being farmed out to independant consultants (qualified, but not familiar with, the work/bridges) and although no one explicitly stated that the department was being rolled up, senior engineers were retiring without being replaced, junior engineers were being hired on short-term contracts only (often less than one year... Exactly the amount of time it takes in day to day inspection works to become able to inspect bridges properly), and support staff who had invaluable local knowledge and abilities were being transfered into other areas of work.

I have no problem with changing how something works, but when you modify a system that functions so very well, with your only motivation being cost, you really must accept the possible consequences.

A real shame.

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[civilperson]

Design checking vs. bridge inspection: Structural analysis needed for the first and field training/specialized equipment needed for the second.

 

[JLNJ]

Could the MN bridge have been saved by "inspection" or was there a fatal design error which no manner of inspection could have found?

 

[msucog]

having no inspection automatically means that the problem won't be found until it fails. as with anything whether it's soil, steel, concrete, etc, at least some amount of inspection (before and after) will likely catch a good portion of potential problems (big or small). as you ramp up the types and frequency of inspections, the likelihood of catching problems before it becomes a "major" issue increases but never completely removes the possibility of problems occurring. maybe having a complete design coupled with construction testing and followup inspections will provide a safe and functional structure and be in the best interest of the public.

 

[concretemasonry]

Unfortunately, the reporting system used for bridge inspection is not fully understood and is easily warped by the media for short term publicity. After that, the politicians and budgets have the controls. Too low dedicated gas taxes and deresaed revenues from higher mileage cars compound the problem.

About a week ago, a bridge on I35E (the eastern companion that goes through St. Paul and not Minneapolis) had a 1200# pound piece of concrete that fell off and damaged a car. After road closing and an inspection, it was determined that the bridge was still structurally sound and should be watched until the scheduled replacement date (2012, I think). It appeared that the section of concrete that came off was really not strucrual, but was protecting rebar from the elements, that actually caused the problem. Last inspection was about a yer ago. Several of the bridges on that section of the freeway have been hit by loads including a boom truck. Having worked on the construction of that part of the freeway, I would be more worried about the site/foundation stability. In any event, it was slated for replacement and it was interesting that the recent dramatic problem occuring within 2 miles of the capitol and DOT headquarters was not a cause for immediate replacement, but a sign that continuing observations for quick problems is needed on suspected or "troublesome" bridges by trained inspectors and not news reporters.

It is interesting that the previuos highway commissioner, 9 not a trained technical person) was fired by the state for not being available during or a month after the I35W bridge problem. The refreshing thing is that the news media reported she was recently hired by the federal Tranportation Safety Board. Apparently, because of the news media, she was terminated last Friday.

What is needed is a sound technical knowledge as a basis for inspection and design for both technical aspects and administrative positions.

On an sidebar, The I35W bridge (8 or 10 lanes) is now 90% completed and should be turned over about 3 months early (9 or 10 months of construction) to earn a $20,000,000 bonus based on a formula of 50% of the extra community costs incurred without the bridge. It was a design build project that encountered good weather and the contractor incurred significant of overtime. The downside is that the infrastructure improvements by the City of Minneapolis and highway department may not be able to match the performance of the design/build consortium, which combined design and construction in one package.

Dick

 

[concretemasonry]

A minor correction - The person noted earlier as the highway commissioner that was terminated may have been the responsible administrator. At one time the Lieutenant Governor was the actual/acting Highway Commissioner if my memory is correct.

In any case, it was a situation where the administators did not posess a technical background, nor was there a technically able person as the "face" for an engineering oriented department that is subject to the "media".

 

[youngstructural]

Some great points from everyone, but I like JLNJ's question about whether or not the problem would have been caught is a great conversation/debate starter...

I suspect we'll all have different views, but here is mine: I feel very strongly that this depends on exactly what type, frequency and level of detail of inspection is carried out. The points raised by msucog fit my view well, but there are two further points I would add...

- Upon code changes, a bridge should be fully re-analysed for any new works to be undertaken. Thanks to the 5% existing structure rule, this is often not carried out. My personal belief is that it should only ever be applied once; IE: If a bridge is to be modified, it should only ever be done once before complete design check is required prior to any further modifications being carried out. This is much less likely, although not impossible, when governments no longer have their own engineering divisions. Who will pay to design a bridge that has already been built? That said, such a procedure would stand a very good chance of catching an undersized memnber...

- When bridges are inspected by the same team of engineers on a routine basis, many subtle senses of what a bridge's colour, texture, water levels, approaches, etc become normal. This inherantly results in a better quality of inspection.

I am not advocating that all bridges must be inspected by the same people all the time, but that much of the time it should be consistent. A fresh eye also has tremendous value, but that could easily be achieved by rotating teams through bridges.

So, my final answer: Yes, I believe that detailed, thorough and comprehensive inspections would have very likely caught this design flaw, as long as they included detailed design analysis.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[Qshake]

I believe the buckled/bowing/bent plate would have been an immediate issue for any bridge engineer familiar with these types of structures. And who better to inspect these bridges than engineers familiar with them. This sort of procedural categorization of bridge inspection is done by a lot of DOTs. In other words, many DOTs realize their capacity and contract with inspection firms that have engineers familar with these or other bridges as is necessary.

I also believe that the bridge industry was travelling down a disasterous road by not checking connections. Based upon the code, the connections are designed for no less than 75% of the member strength.....how can that ever be interpreted to mean "connections are stronger than the members and therefore don't need to be checked"? When you rate a bridge you should rate the whole bridge and not just the parts you understand better than others.


Regards,
Qshake

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

 

[youngstructural]

A quick counter-point Qshake:

- If you are taking about a bent plate, it has already failed. Any bridge maintenance engineer should catch that, if not on a visual inspection, then certainly a detailed inspection. It should never make it to detailed comprehesive before being caught.

- If we are talking about a design flaw, prior to any form of evident failure, that will normally only be caught in a design review. Or, alternatively, I have seen a very experienced engineer simply "know" that a member was undersized by looking at it... Doing that kind of inspection reliably only comes with thirty and fourty years of experience, if at all.

And you're dead-right about connection checking: It really has to be part of the design process, and a wholistic approach to a bridge check is an absolute necessity.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[hokie66]

I doubt that analysis of existing bridge structures will ever be part of routine inspections. There is just not enough capacity in the engineering profession to do that. For major modifications, yes, but not just because a code changes, unless the code change has identified a particular problem.

From my reading of the reports to date, I don't think the gussets plates in question were ever seen to be bent before the collapse.

As to whether this bridge should have been saved by inspection, I doubt that I would have picked the undersized plates by being on the bridge. I think there is much more likelihood that I would have found the anomaly by examining the drawings, where everything is in one place and the eye and mind can gather the information and process it all together. There would have been engineers in Sverdrup's design office capable of, as YS says, "knowing" that these gussets were undersized by looking at the drawings. How it got through we will probably never know.

 

[concretemasonry]

Beyond the original design many years ago, there were many other major factors through the years that possibly contributed to the collapse. - Redecking/modifying years earlier, corrosion of other memebrs and a contruction project underway with questionable predictability/control of the activities and placement of equipment and materials. Rebuilding a brigde deck is sort of a balancing/juggling process. These also could be factors in the eventual report conclusions.

Dick

 

[Qshake]

Hokie66 -

If you've read the reports todate you'll know that there are two photos, one 2003 and one 1999 that show the bend in the plates. Hence in my opinion these should have been red flags and investigated.

No, I don't think that full indepth analysis are part of any past inspection, but i think that they should be as a matter of going forward, for at least two reasons. First, the best time to act on inspection data (section loss, etc)is when it is received. Wait too long and it goes into a file never to be seen again or for at least 1 or 2 years pending the frequency of inspection. Second, between inspection there are instances where loads will change and all elements of the bridge should be evaluated after modifications.

One of the things that clearly went wrong here on top of thin plates is that not all the bridge elements were checked for the additional weight of the overlay or the construction equipment.

Now, I am of a certain age to know that prior to high labor rates, material was a premium and guess what? Weight of bridges were optimized for weight and crane picks. And that was the essence of engineering in that era. What pray tell would any engineer capable of thinking believe they could just throw weight on to a bridge that was optimized without doing an analysis, which should have included the gusset joints.

No, I don't think that our present day or yesteryear's inspectors would have known that a 1/2" plate at that location was inadequate, but I do expect an inspector to make note of it when it is bent.





Regards,
Qshake

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

 

[youngstructural]

As Qshake has said, there are photos that show these plates. Now we're all human, but the metal ruler review of site photos should have been an immediate red flag.

And being human, I suspect Hokie simply didn't remember the photos when posting his reply...

I DO NOT argue that we would have caught this in my old department, or that anyone failed in their duty. I hope very much that whatever investigations are underway report that this was an unfortunate oversight, accidentally caused, and recommend practical, workable, solutions.

I have already shared my opinion regarding re-analysis...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[hokie66]

You may be right about the photos, but I just looked through the 1999 and 2003 reports again, and couldn't identify any showing bent plates at the joints which failed.

In addition to annual maintenance inspection reports, this bridge was subject to several external investigations. The UMinn and URS studies are the ones where the gusset problems could have been found.

 

[LobstaEata]

I hope this comment adds some value to what has been an interesting discussion. Over a 25 year bridge design career as a consultant, I've had the opportunity to inspect over 500 highway and rail bridges. One bridge in particular comes to mind as an example of what happens when conventional wisdom is not regularly challenged.

The bridge was a 1600 foot long, multiple span, truss over a navigable tidal water. (ie. a major structure) There were 540 primary structural members in the truss alone that had not been load rated since the bridge was built in 1936. The bridge had been in service until 1980 and was then subsequently used for only overflow and emergency traffic diverted from a new bridge that had been built along side the truss. The truss was still inspected every 5 years by DOT's bridge maintenance division.

In 1991, we was asked to determine the feasibility of rehabilitating this truss, so we recommended that an in-depth member by member inspection and load capacity rating of the bridge be conducted to establish a basis for appropriate rehabilitaion methods. The DOT argued that our recommendation for a 2 week inspection and full load rating analysis went too far beyond their own maintenance inspection routine of 3 days for this structure. They also had never had performed a load rating on this bridge up to that point as they felt it was not necessary. Our firm insisted that this engineering work was required to provide conclusive recomendations in the study. After much duscussion, DOT reluctantly went along with our receommendation and funded the work.

Our analysis revealed the bridge that originally had been designed for H15 truck loads only rated at H6. The in-depth inspection and subsequent plan research revealed that a fabrication error in construction resulted in two missing strengthening plates for four identical built-up members. Inspection notes and photos also revealed that lacing between these built-up elements had been bent. After reporting these finding to DOT, The bridge was immediately closed to all further overflow nad emergency traffic and has not been re-opened since that time.

The point is that if it were not for performing a load rating prior to inspection, it is questionable whether this defect would ever have been discovered. I'd like to think we were able to avert a potential tragedy, considering the heavier trucks of today compared to those when the bridge was built. It is my belief that FHWA's current consideration of requiring safe capacity load rating and inspection of deck truss structures be performed by qualified design engineers is appropriate and necessary to ensure the safety of the travelling public. I am of the opinion that this receommendation should also extend to all bridges classified as "major" structures.

Thanks for enduring what ended up being a longer winded post that was intended. I hope it offers some clarity for those that are early in their careers and might have developed a false sense of security handed down from superiors or co-workers that do not tend to challenge the conventional wisdom.

Best Regards,
PE in New England

 

[hokie66]

Very interesting, LobstaEata. Just one question. How did you actually find the error? Was it in comparing the structure as built with the design drawings and shop drawings?

 

[youngstructural]

Good follow-up to a fantastic post... I too would be very keen to hear the HOW of the error/omission/problem being discovered!

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[LobstaEata]

In response to the "how" question as it relates to discovery of the error/ommission, here is the rest of the story.

Upon completion of the load rating analysis which revealed that the bridge was rated for H6 truck loading, we started an investigation to first verify that the rating was indeed accurate, secondly to research design plans and shop drawings that fortunately the DOT had kept on file and finally to verify that physical conditions during inspection actually matched our research.

The first part of the investigation confirmed the H6 rating and which members that required further scrutiny. Our research discovered that although the table of member sizes (they used a tabular format in the day) did accurately show the correct configuration of plates in the member section diagrams, that the text description did not include mention of one layer of plates to be installed on the back of each double channel used for the built-up member. Shop drawings carried this error forward, since detailers apparently only were looking at text descriptions, the fabricator/erector manufactured the sections as shown on the shop drawings and of course the rest was history.

It is important to note that this error remained undetected for 55 years, despite numerous inspections by State DOT personnel. Even though these four identical members were deficient, the evidence of damage was not sufficiently clear to inspectors, since the bent laces was not severe enough to warrant concern.

By the way, the State did ask us why the bridge had not showed more pronounced signs of failure since it was built in 1936, so we then were enegaged to perform material testing and research the basis behind the AASHTO safety factors. The long and short of it was that the recommended safety factor prescribed by AASHTO of 2.12 had been reduced to 1.04, based on reduced section and material property adjustments. Way to close for comfort in our book. Overloads beyond design capacity fortunately were infrequent enough to remain within the lower plastic range of the steel, which proved through testing to exceed recommended design strength. However increased legal loads for trucks that were being proposed in the 1990's might have been just enough to tip the balance, especially since it was buckling strength that governed member capacity.

Well I hope this story served as an interesting anecdote to the reason why it should be important to perform a safe capacity load rating of at least major structures prior to conducting inspections.

Best to you,
PE in New England

 

[rmiFL]

Have you guys seen the post-collapse layout pictures? You can definitely see where it failed. IMO this bridge was neglected, It still has lead paint on it, obviously they never completely sandblasted and and painted it since the late 70's.
If they had been stripping the paint rather than overcoating, the gussets that were buckled and had section loss would have been brought to the EOR's attention that point.