Minneapolis Bridge Collapse

David W. Fowler, Ph.D.
Forensics Civil Engineer, University of Texas
Thursday, August 2, 2007; 1:00 PM

Rescue crews resumed the grim task of searching for bodies Thursday morning in the wreckage of a collapsed highway bridge in Minneapolis. The eight-lane Interstate 35W bridge, a major artery that carries more than 100,000 vehicles a day, was in the midst of being repaired and two lanes in each direction were closed when it buckled Wednesday.

"There is little available information so far as to the cause, although I have heard at least one theory that it was caused by fatigue of the steel which is due to millions of load applications. Fatigue in steel may be visualized by thinking of what happens when a piece of wire is bent back and forth until it breaks," said David W. Fowler, a civil engineer at the University of Texas, in an interview with

Photo Gallery: Bridge Collapses in Minneapolis

Fowler will be online Thursday, Aug. 2, at 1 p.m. ET to discuss the collapse and its investigation and the safety of bridges in the U.S.

A transcript follows.


David A. Fowler: The failure of the I35W bridge over the Mississipi was a real tragedy. The cause is not known and it will probably be months before an accurate assessment of the cause will be known. We have hundreds of thousands of bridges that have functioned safely for many years carrying thousands of cars per day but even one failure is tragic. I will be glad to try to answer your questions.


Washington, D.C.: What are some of the more common reasons for bridges to lose their structural integrity? Age, materials, overload? I wonder because so many bridges around here are well beyond their capacity as originally designed.

David A. Fowler: For steel bridges it is sometimes corrosion which reduces the cross section or it may be fatigue, which is the effect of repeated loading (millions of load applications) that reduces the strength. Age is involved from the extent that corrosion and fatigue become more likely with time.


Minneapolis, Minn.: Is it possible that some kind of harmonic vibrations caused the collapse of the bridge?

David A. Fowler: It might be possible although I beleive I heard that the traffic was essentially stopped when the failure occurred, and I am not aware of high winds that could have been a cause.


Minneapolis, Minn.: From the photos that you have seen, what kind of joint fixes the steel support structure to the concrete pylons? Is there a possibility that those caused the failure?

David A. Fowler: I haven't seen the photos, but anything is possible at this point.


London, Canada: In your estimate, what percent of the North American bridge infrastructure is in need of critical repair or replacement? As you may be aware, a similar collapse occured in Montreal last year and based on news reports from the Minnesota collapse, it appears that particular bridge was at significant risk. Should the public be concerned?

David A. Fowler: Ihave heard that up to 40% of our US bridges are in need of repair. That doesn't mean that 40% are about to fail; rather it means that preventaive maintenance is needed to keep the briges from further deterioration.


Vienna, Va.: Looking at the before/after photos, this looked like a pretty "lean" truss bridge. Was there no central support pier?

David A. Fowler: I can't answer that--haven't seen enough photos. I got in last night very late and saw only some footaage on one channel.


Fairfax, Va.: When bridges collapse like yesterday's tragedy in Minneapolis and others in Connecticut and Upstate N..Y some years ago, it always seems to be a complete collapse. From an engineer's point of view, can you explain this?

David A. Fowler: It is probably because bridges that partially fail, e.g. develop a hole in the concrete deck, a concrete beam beneath the bridge is hit by a high load, etc do not cause a dramatic failure and do not get national attention. When bridges do fail in the sense that a span falls it normally causes accidents and loss of life and that merits widespread attention.


Boston, Mass.: I've seen reports that a report was written in 2001 that showed fatigue, but it was thought that this was not a major issue, and one not needing immediate action. I assume it will take some time to know the cause -- but is this the biggest suspect at this point?

David A. Fowler: That is the only possible cause I have heard but it is far too early to be drawing any conclusions.


Washington, D.C.: Did engineers underestimate the potential loads the steel bridge would undergo thus resulting in the bridge collapsing before repairs were made?

David A. Fowler: I seriously doubt that. I think it is more likely there was a material failure but I don't want to draw any conclusions.


Washington, D.C.: Should drivers feel unsafe driving on other bridges throughout the country?

David A. Fowler: We have had an excellent safety record when you think about the fact that we have several hundred thousand bridges and they have performed extremely well. DOTs have a regular inspection program that helps insure that problems will be identified before failures such as this occur.


Washington, D.C.: Besides looking at the bridge for structural failures what else would they look at? Ground movement? Sonic vibrations from train tracks nearby? My prayers to all the victims and all of Minnesota. Thank you.

David A. Fowler: All of the above. When failures occur it is important not to leave out any possible cause.


Vancouver, Canada: How frequently and for what purposes are forensic investigations of large structures conducted after an accident?

David A. Fowler: When any major failure occurs, every one of them will receive a forensic investigation.


Silver Spring, Md.: If the bridge had some lanes closed due to construction, wouldn't that mean the bridge wasn't carrying as heavy a load as it would normally? Seems odd the bridge would collapse while bearing less than usual weight.

David A. Fowler: The total load on the bridge might have been less but the girders or foundations on the loaded side would have probably been fully loaded and that could have been the cause.


Greenville, S.C.: The national bridge inventory states that in the 2005 inspection, the I35W bridge's superstructure was in "poor" condition. Does that refer to the fact that the structure was non-redundant or does that mean the structure was actually in bad condition at the last inspection?

David A. Fowler: Good question. Without more details it is hard to know what "poor" meant. It could have referred to spalling in the concrete deck, beginning of corrosion in the steel girders, etc.


Harrisburg, Pa.: How easy will it be to determine the cause of the collapse? Will engineers be able to look at the structure and pinpoint where the failure occurred, or will it be a more lengthy process?

David A. Fowler: My guess is that they will be able to pinpoint the cause. There will be millions of dollars spent to find the cause and this is justified because we don't ever want this failure to occur again. We must learn from our failures.


Columbia, Md.: So why were Homeland Security officials so quick to say "it wasn't terrorism"? Or is this just another example like "the air near the World Trade Center is fine"?

David A. Fowler: You would have to ask them.


Minneapolis, Minn.: Talk radio programs (and particulaly those against President Bush) are now saying "structural deficiencies" mentioned in previous reports on the condition of this bridge mean that the bridge was seriously unsafe and that goverment officials were not willing to raise the money to correct this problem ... outside of the politics involved, were these statements of deficiencies meant to indicate that this bridge was a danger to the public using it? Is this then a smoking gun pointing to real blame, or a political misuse of the terms mentioned in these bridge safety inspection reports?

David A. Fowler: I think we should not rush to judgment. You are going to be reading and hearing a lot of potential causes and assinging blame to a lot of people and agencies but we need to let the investigation proceed.


Boston, Mass.: I heard, and could see, that a freight train was passing under the bridge. Could the vibrations from a train cause or contribute to the collapse of a bridge like this?

David A. Fowler: It is possible that it could have contributed. By itself, not likely that the train causes the failrue.


Washington, D.C.: According to news reports that bridge was inspected a couple of times over the past 2-3 years and found to be safe. How does something so catestrophic happen after such inspections?

David A. Fowler: Inspections are usually visual and we don't have x-ray vision. Defects in metals and cocnrete sometimes can't be seen. And it is not possbile to see behind all of the elements in a bridge so they do the best they can during an inspection.


Stuck in traffic: Do you feel enough of our gasoline and highway fees are being used for maintenance? Billions are collected in gas taxes, where is the money going?

I also notice that every time a bridge or highway project is proposed, the media is quick to interview the one or two people who may be mildly affected by the construction. It is time to use our highway funds more effectively, and for the greater good and safety of all.

David A. Fowler: I personally do not believe that we put back enough of our gasoline taxes to pay for new construction and particularly for maintanance which is not a glamorous area but extremely important. There is constatn pressure to take dedicated trasnportation funds at the state and national level and spend them on other programs. That needs to stop. We also should probably have a tax based on a percentage of cost rather than a flat rate since that doesn't take inflation into account. But, hey, I am an engineer, so what do I know aobut these things :-)


Arlington, Va.: Hello David. Thank you for taking my question. Local officials here in the D.C. area have been quick to assure the public that our bridges are safe. I'm guessing those assurances are based on the same sort of inspections that the I35W bridge was also subject to.

Without knowing yet the details of why the collapse happened, it will be hard to know how to prevent it from happening again. But based on your professional experience, are there improvements to the bridge inspection/maintence regulations that you'd say have been needed (even without considering yesterday's tragedy)?

David A. Fowler: We have some excellent researh going in bridge design and maintenance, and more accurate monitoring methods are receiveing a lot of attention using some of the high teck methods, e.g. fiber optics, that are now available. You will be hearing a lot about "smart bridges" in the future which will have the ablity to monitor themselves and send a warning when defects occur. But overall we have had an excllent safety record and I have not fear in passing over any of our bridges.


To Vienna: There was no center pier. Just one long truss. If you google "I 35W Minneapolis" you can see the fact page maintained (I believe) by the state.

David A. Fowler: Thanks for the info.


Portland, Ore.: Without going into details that could be misused, can you tell us if terrorists could have exploited known structural weaknesses to take down the bridge without explosives?

David A. Fowler: I would seriosly doubt it but let's let the investiagtion take place.

_______________________ Images: Minneapolis Bridge Collapse (Google Earth)


Texas: What kind of monitoring equipment is available and is it possible to make it mandatory for bridges that are either deemed in need of repair or used as a major thruway (both applied in this case). Wouldn't a steel structure in the middle of a metropolis be easier to monitor? I know that some steel structures have post construction blown-on composite to reinforce and prevent corrosive failure.

David A. Fowler: Monitoring can occur anywhere. As we go more and more to high tech monitoring methods in which the bridges will be instrumented we can monitor a bride in remote areas also. All it takes is money. We will probably target high volume bridges first in our monitoring programs because of their impact.


Los Angeles, Calif.: Around here, highway bridges are often concrete. What are the better choices of materials for different uses and environments?

David A. Fowler: When spans are quite long between supports, usually steel is the first choice. But it is a matter of cost, availability of materials. Steel and concrete can be desiged to do the job. But maintenance is important to prevent corrosion and other problems from causing an unsafe structure.


Alexandria, Va.: Security footage that was aired today shows one half of the bridge collapsing, shearing off at almost the exact midway point. The other half remained suspended for some time. What would contribute to such an even break?

Also, an eyewitness and local commented that the 'surface' construction was actually drilling holes all the way through the bridge leaving sizable holes.

David A. Fowler: I really can't speculate on the cause. Drilling holes in the deck shouldn't seriously weaken the structure.


Anonymous: Because of unbalanced loading was the soil value under the pier in the river compromised

David A. Fowler: I have no information on the soil type and wouldn't know if that were a possible cause even if I did.


Washington, D.C.: Do you know how old this bridge is? Are bridges built differently today than they were at the time this bridge was constructed?

David A. Fowler: I beleive I heard on the news that it was built in 1967 which makes it 40 years old. As I mentioned earlier, steel is subjected to fatigue caused by millions of load applications due to vehicles and corrosion can increase with time. But again I can't speculate on the cause.


Minneapolis, Minn.: This bridge had a unique, "model," automatic de-icing system that sensed the temperature and sprayed de-icer, I think from nozzles on the side. Do you know about that system, and how it might cause corrosion?

David A. Fowler: I had not heard that so I don't know what the impact would be.


Washington, D.C.: From the photographs that we have seen, the bridge failure seems to be complete and catastrophic. Could a small failure of a component of the bridge in just one location cause a collapse of the entire span, or would this type of collapse be indicative of a more wide-spread problem throughout the structure?

David A. Fowler: It is possible for a local failure inone location to lead to a progressive collapse. Think of the top shelf on your bookcase breaking and the books drop down on the next shelf, breaking that one and so on.


Reisterstown, Md.: I understand that it's possible for a bridge to run out of expansion room on particularly hot days. Any chance that this was temperature-related?

David A. Fowler: Steel and concrete both expand with temper4ature increases and bridge designers allow for that in the bearing supports for the beams and girders. If girder or truss expanded more than the "travel" available then it could provide a force that could cause damage to a part of the structure.


Hilo, Hawaii: If the cause is metal fatigue, what methods are in place to test for this, and will testing of ALL bridges begin starting today?

Thank you

David A. Fowler: I am not an expert on monitoring for fatigue but I do know that it is not always possible to tell from visual inspections that fatigue failures are imminent. I really can't tell you what the best methods are for monitoring for fatigue but I do know that there has been considerable research underway to provide the answer.


Washington, D.C.: I was shocked to see that the side spans collapsed as well as the main span. What kind of forces might have caused the side spans to collapse? Or was it simply the lack of the main span's tension force that caused them to give way?

David A. Fowler: I wish I could asnwer that but I know very little about the type of structure.


Lexington, Ky: I've been thinking about the collapse of the Silver Bridge in Point Pleasant, W.Va., back in the late 60's. I traveled over that with my parents shortly before it fell.

What sorts of things would have to be found by the DOT in a routine examination of a bridge in order to order immediate closure?

David A. Fowler: I can't give you a complete answer but for example some of the following might trigger closure:

severe cracking or fracture of the conrete piers that support the weight of the bridge

severe loss of cross section of a key steel beam or bolt or other member

fracture in a steel beam or girder

severe scouring of the soil around a pier which would cause a lack of support

I hope this gives you an idea but there could be many more possible causes.


The loaded side would have probably been fully loaded and that could have been the cause. : I am not an engineer, first of all, but I was under the impression that supporting portions of a structure are designed in such a way as to distribute the load evenly, across the entire supporting framework, not just those portions directly bearing the load.

Thanks, and my heart goes out to Minneapolis.

David A. Fowler: That is not always possible. A truss or girder supporting a small part of the bridge is supported by another girder or truss and so on. If one of the members frail it causes load to be shifted to anothe member and that can lead to a progressive collapse.


Silver Spring, Md.: Will laboratory based testing be conducted on the materials in the bridge? If so, what are these likely to be?

David A. Fowler: They will test everything If you think you physical was thorough, you haven't seen anything yet :-)


Kansas City, Mo.: How long would you expect the investigation to take before the probable cause of the failure is determined? Hypothetically, if you were leading the investigation team, what is the first thing you'd do?

David A. Fowler: VEry good questions. You can't put a time table but I would think that they will be very deliberate and it could take at least a year, and perhaps longer before they get out the final report. First thing you do is assemble a topnotch team of experts, meet and together come up with the plan. Collecting the evidence on site is now critical since the site must be cleaned quickly and you need to learn all you can while the structure is still in place, much like investigating a murder scene.


St. Paul, Minn.: Some of the rock types in the vicinity of the bridge failure are susceptible to dissolution (i.e., karst), which has lead to the failure of features such as stormwater basins. I know you are not familiar with the soil and rock types, but could this type of condition cause a failure in the foundation? How deep is the foundation for a bridge of this type usually set?

David A. Fowler: It could be a cause. The depth depends on the bearing capacity of the soil with depth and the need to be sure that the soil cannot scour away from the foundation. Good question.


Washington, D.C.: Since indications of a potential fracture, for example river marks in the steel, are so hard to see without X-rays or SEMs do you think the way bridges are inspected is inadequate?

David A. Fowler: See some of my earlier responses. I am not an expert on ID of fatigue but I think we will have better ways of doing so in the future but it will cost money.


Baltimore, Md.: How frequently are bridges inspected? Is it a federal or state policed policy?

David A. Fowler: There is a schedule and I beieve it is every two years but on some critical structres it may be more frequent.


Boston, Mass.: It looks like the support is on land, but right next to the water. Is this a common design, and what extra consideration of water erosion is taken when designing or maintaining a support like this?

David A. Fowler: This is common and by properly designing the foundation it can be safely built.


Washington, D.C.: Could the Minnesota winters have contributed to any structural deficiency? What impact does extreme cold have on that type of bridge, if any?

David A. Fowler: Minnesota is not the only state that has low temperatures so that in itself should not be a probelm. The use of salt ont the deck in the winter could have led to more rapid corrosion of some steel members, perhaps, That is a possible but uunknown cause.


Pahoa, Hawaii: To what extent are bridges overbuilt ... i.e., 150 percent capacity, and was this bridge exceeding not only its original design, but its safety margin?

David A. Fowler: They normally are designed to accommodate traffic, sind and seismic loads that are specified by the national standards so they are not "overbuilt" except that the standards require a specified factor of safey that may be 50 to 70% greater than the expected stresses.


St. Paul, Minn.:

Good afternoon, sir. As you stated at the start of the chat, the cause of the collapse is still unknown.

So, how can officials almost immediately after the collapse rule out a terror attack?

(Note: I am 'NOT' saying it was; but neither do I think the idea should be totally dismissed.)

Thank you ...

David A. Fowler: You will have to ask them.


Washington, D.C.:

Here is a link to the 2001 MN DOT report on the bridge

Fatigue Evaluation of the Deck Truss of Bridge 9340

David A. Fowler: Thanks.


Washington, D.C.: Do tunnels have the same lifespan as bridges? While it is cheaper to build a bridge, I'm wondering if long term, a tunnel is a better option.

David A. Fowler: They have different problems. I am not an expert on tunnels and can't really address that.


Minneapolis, Minn.: Just a side note here, this Minnesotan is deeply touched by the outpouring of compassion from around the country and even the world. Thank you all for your prayers.

David A. Fowler: Thanks. We all are praying for the injured and the families of the people affected.


David A. Fowler: I have enjoyed receiveing your questions and trying to answer them. You have submitted good questions. We will be finding out much more about the cause of the failure so please give them time to do the investigation. In the meantime, don't be afraid to travel over our highways. We have the best transportation system in the world and if I had to selelct one reason why the U.S.has had the leading economy for over 100 years I would say it is our great highway system that allows us to move people and goods so efficiently thorughout the nation.


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