The Science of Bridge Collapse Prevention

toddatcw writes "In the wake of the Minneapolis Interstate 35W bridge collapse this week, Computerworld investigates ongoing research which could someday help to prevent future disasters. Acoustic emissions detection systems, which listen for the sounds of metal snapping on structures, are already sold and fitted. Likewise, a new generation of detector systems that monitor for tilting of bridge columns and piers are being designed, prototyped, and researched. 'Sound waves move more efficiently through solid objects than through air, making any sounds easier to listen out for, Tamutus said. "It's not amazing. It's simple. Doctors use stethoscopes all the time. If you put your ear on a train track, you can hear a train approaching from far away... The Sensor Highway II systems, which are portable and can be moved from bridge to bridge as needed, usually cost between $20,000 to several hundred thousand dollars each. Typically, evaluations take between one day and a week.'"

Re:Barriers/Lights

[choongiri]

Meanwhile, engineering research projects, including one at the University of Missouri-Columbia, were already under way long before this week's bridge collapse to advance the science of bridge monitoring. At the school, work is being done on a large-scale sensor system that would be fastened to several concrete bridge piers below a span to alert officials about even the slightest tilting or swaying of critical piers supporting a bridge.

Re:Some of the locals seemed to know...

[Paktu]

There's conflicting reports about it. The Feds inspected it a few years ago and said it was in immediate need of repair, but the state sent in people who claimed it would be viable until 2020. While it might appear that the state just didn't want to spend money, keep in mind that Minnesota has the third lowest percentage of structurally deficient bridges [statemaster.com], so it's not like there were other major priorities that were sucking up funding.

ironic

[circletimessquare]

there's a rant i read a few days ago from a what seems to be a bitter old time engineer who says that ancient styles of bridge design fare better than more modern ones because of redundancy: if something fails, the damage is localized, rather than the whole bridge going because of just one of many of its elements. he points to something called "value engineering"- aided by computer analysis, that is the source of this kind of bad nonredundant bridge design that was the I35W bridge

what's ironic is that modern technology has therefore made bridges less safe, by empowering those from the middle of the last century who wished to save money by losing less materials, at the expense of safety by sacrificing redundancy. just read what he says, saying it better than me [nytimes.com]:

14.August 2nd, 2007 1:39 am

Compare the collapsed steel truss bridge with the reinforced concrete arches of the intact bridge in the background of some of the photographs. The concrete bridge consists of inherent stable arches, a design which has stood the test of time since the Roman Empire. Even if one arch of this bridge had fallen, the remaining arches would have remained intact and loss of life and injury would have been limited to the failed section.

Compare this with the more recent bridge, composed of steel trusses which held up a concrete deck. The entire 1000 foot long section was tied together structurally to save money. It had no tolerance for partial failure. If one section failed, the entire section would go down. This more modern bridge was ugly as well as a poor design. This bridge was designed by modern engineers who have no sense of beauty and think they can calculate every decision on the basis of cost/benefit. They practice a destructive type of design called value engineering - taking out the expensive stuff if it's redundant or optional.

We don't yet know which piece of the structure failed, but it may have been a small one - such as rusted steel, steel which looked OK on the surface but had deteriorated in its carrying capacity, perhaps in tension. The connection between concrete rebar and the supporting steel space frame.

This poor design based primarily on cost considerations has been required all over this country in countless projects for the past 50 years.

One section of the old San Francisco-Oakland Bay Bridge failed in the 1989 Loma Prieta Earthquake because there wasn't enough "give" for the shaking due to the quake. Two lives were lost - one by a woman who tried to drive her car across the gap and who would have survived had she waited for help. However, the rest of the bridge remained and will be used until this fall when it is destroyed after the new bridge opens.

The new San Francisco-Oakland bridge which is replacing the old bridge has the same basic flaw as the bridge which collapsed in Minneapolis today: If any one piece failes, the entire bridge will fail catastrophically! The new Bay Bridge is designed to look elegant and be a landmark - but it has no redundancy in an area with severe earthquakes. It too was designed by modern engineers. It will be a disaster waiting to happen, just like the World Trade Center and the Route 35 Minneapolis Bridge, and the New Orleans levies. America no longer has the leading structural engineers of the world designing its infrastructure. How many of them owe their jobs to our failing political system?

It is ironic that the lack of redundancy in any structure also makes it inherently more susceptible to terrorism - witness the collapse in the World Trade Center.

America is in bad shape, and we seem to be addressing our problems in a piecemeal and ultimately stupid way.

-- Posted by MJ

Re:The bigger problem

[Anonymous Coward]

It was rated 'structurally deficient' [msn.com]:

The Minneapolis bridge's deck, or driving surface, was rated in "fair condition." The superstructure was in "poor condition," and the substructure in "satisfactory condition."

It looks like the 'satisfactory' substructure is what failed. Repairs to the driving surface and the trivial superstructure were ongoing. There was no indication from inspections that the substructure was in need of immediate repairs.

The classification of structurally deficient means that either the surface, the superstructure, or the substructure was rated poor. In this case it was the superstructure which for this particular bridge did not provide support. A little bit of repairs to the superstructure and this bridge would have been cleared of its structurally deficient rating.

Re:Some of the locals seemed to know...

[Thorrablot]

Yes - there's a good writeup here [wikipedia.org] already.

The bridge is a truss arch bridge [wikipedia.org] built in 1967. The design doesn't interfere with river traffic (well, up until two days ago anyways) - but I did hear an interview with a Berkeley professor describe how such bridges are no longer built due to their lack of redundancy in case of span failure.

Re:Some of the locals seemed to know...

[moosesocks]

Likewise, you're not exactly going to be able to attract funding to fix or replace the bridge if you're going around telling everybody that everything's just peachy.

Personally, I sort of doubt that this could have been prevented. It's one of those one-in-a-billion sort of odds that unfortunately caught up with us...

I'm more than a bit irked at the media for taking the "structurally deficient" term, and plastering it all over the news without a very clear understanding of what it means. There's no cause for a panic or a rucus -- our bridges are no more dangerous today than they were last week. Hell, we don't even know what caused the bridge to collapse, and ordering all sorts of emergency inspections (which has been done in many many states so far) is pointless considering that the bridge that collapsed was previously deemed to be safe on multiple occasions.

Of course, other recent incidents such as the con edison steam explosion in NYC reek of criminal negligence.

Re:The bigger problem

[flyingfsck]

Well, people were actually busy doing repairs when the bridge collapsed. It is possible that the hammering of the repair activities contributed to or hastened the collapse.

The data was already there

[michaelmalak]

From yesteday's New York Post [nypost.com]:

A 2001 evaluation of the bridge, prepared by the University of Minnesota, reported that there were preliminary signs of fatigue on the steel truss section under the roadway, but no cracking.

The report said there was no need for the Minnesota Transportation Department to replace the bridge because of fatigue cracking.

But a May 2006 report by the department noted that inspectors saw fatigue cracks and bending of girders along the span's approaches.

I.e., in 2001 they barely passed it because they said, "at least there's no cracking." In 2006, they saw cracking but kept the bridge open anyway. At minimum, they should have closed it to heavy truck traffic, scrapped the idea of doing heavy construction (repaving) on the bridge, and started construction of a replacement immediately.

For more info, see today's Minneapolis Star Tribune article [startribune.com].

Re:Bridge Engineering Isn't What It Used To Be...

[r_jensen11]

Those bridges weren't constructed with having nearly 150K vehicles regularly going over them every 24 hours. The ancient civilizations had marvelous constructions, no doubt. There's no structure built in the past 100 years that will last as long as half of what was build in Rome lasted. However, we abuse our infrastructures a hell of a lot more than they did.

Re:Some of the locals seemed to know...

[DerekLyons]

I'm more than a bit irked at the media for taking the "structurally deficient" term, and plastering it all over the news without a very clear understanding of what it means.

Here in WA, the WA DOT has essentially admitted that "structurally deficient" [kitsapsun.com] is a scare word used to boost priority in asking for federal funding.

Re:The bigger problem

[hey!]

Apparently, a few years ago they had found that the some of the steel had been distorted in unexpected directions. In retrospect this was ominous. It wasn't an acceleration of an expected wear process.

This bridge was designed with a pair of steel arches which balanced on slender concrete piers on either side of the river. The load from the deck was transferred to the arches by a truss system: a network of triangles that reinforce each other. The problem with this design is that the failure of a single element puts the whole system out of balance. Such designs aren't used any more, after a bridge over the Ohio river collapsed in 1967 -- ironically the same year this bridge was completed.

One of the important lessons, I think, is that if you have a complex piece of engineering that would kill people when it fails, anything unexpected is a serious, serious concern.