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Bridge Collapse in Minneapolis

parrotslave

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parrotslave
Joined
Feb 15, 2007
Messages
247
The bridge of Interstate 35W in Minneapolis near the University of Minnesota collapsed during rush hour. The bridge was undergoing renovations, but no word yet on what caused it. Lots of cars in the water, a huge mess.

Hope Minnesota JREFers are all OK.
 
Just looked at it on CNN...:eye-poppi

They did say that it was undergoing construction and was down to one lane each way, but still, one reported dead and I fear it will not be the last...:(

The study on this one will be most interesting...
 
Any structural engineers out there, or qualified to read a March 2001 report on the state of the bridge? PDF file there, 89 pages. I am working my way through piecemeal, and finding shocking hindsight quotes. Help talk me down.

In March 2001, the report said that "the bridge should not have any problems with fatigue cracking in the foreseeable future." However the report did recommend an inspection regime, and that is what likely caught the problems they've been trying to fix over the summer.

Lucky them, because one conclusion stands out to me: "Live load stress ranges greater than the fatigue threshold can be calculated if the AASHTO lane loads are assumed. The actual measured stress ranges are far less primarily because the loading does not frequently approach this magnitude. While the lane loads are appropriate for a strength limit state (the loading could approach this magnitude a few times during the life of the bridge), only loads that occur more frequently than 0.01% of occurrences are relevant for fatigue. For this bridge with 15,000 trucks per day in each direction, only loads that occur on a daily basis are important for fatigue."

The first page of the report (pdf 11) says the ADT is 15,000 per day with ten percent trucks. A mistake in this paragraph really doesn't bode well. Actually this mistake is on page 2 of the report as well (pdf 12), where the 15,000 trucks in each direction are ridiculed (the "details should have cracked open soon after opening if the stress ranges were really this high"). That looks really nasty.

There are two standards in play for this bridge. One is AASHO, the one the bridge was built under (called non-conservative by the report). The second is AASHTO, which were developed in the 1970s. Perhaps the AASHTO standard is to calculate stress loads as if all expected traffic were trucks, and that's why they're citing 15,000 trucks each way. Does anyone know? Am I really looking at a bogus figure that may have masked a serious problem?

ETA: If the actual load of this structure was higher, the stresses could have worn out the bridge faster, right? As in actual daily traffic jumping to 20,000 per day with 18% trucks? I haven't seen yet, but are bridges like this subject to periodic checks of actual daily traffic?
 
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boloboffin said:
Any structural engineers out there, or qualified to read a March 2001 report on the state of the bridge? PDF file there, 89 pages. I am working my way through piecemeal, and finding shocking hindsight quotes. Help talk me down.

In March 2001, the report said that "the bridge should not have any problems with fatigue cracking in the foreseeable future." However the report did recommend an inspection regime, and that is what likely caught the problems they've been trying to fix over the summer.

Lucky them, because one conclusion stands out to me: "Live load stress ranges greater than the fatigue threshold can be calculated if the AASHTO lane loads are assumed. The actual measured stress ranges are far less primarily because the loading does not frequently approach this magnitude. While the lane loads are appropriate for a strength limit state (the loading could approach this magnitude a few times during the life of the bridge), only loads that occur more frequently than 0.01% of occurrences are relevant for fatigue. For this bridge with 15,000 trucks per day in each direction, only loads that occur on a daily basis are important for fatigue."

The first page of the report (pdf 11) says the ADT is 15,000 per day with ten percent trucks. A mistake in this paragraph really doesn't bode well. Actually this mistake is on page 2 of the report as well (pdf 12), where the 15,000 trucks in each direction are ridiculed (the "details should have cracked open soon after opening if the stress ranges were really this high"). That looks really nasty.

There are two standards in play for this bridge. One is AASHO, the one the bridge was built under (called non-conservative by the report). The second is AASHTO, which were developed in the 1970s. Perhaps the AASHTO standard is to calculate stress loads as if all expected traffic were trucks, and that's why they're citing 15,000 trucks each way. Does anyone know? Am I really looking at a bogus figure that may have masked a serious problem?

ETA: If the actual load of this structure was higher, the stresses could have worn out the bridge faster, right? As in actual daily traffic jumping to 20,000 per day with 18% trucks? I haven't seen yet, but are bridges like this subject to periodic checks of actual daily traffic?
Click to expand...

AASHTO is pretty exact with loading. Assuming that everything is a truck load is way too conservative and uneconomical. The axle load is based upon what the bridge actually should see. The first number is likely just a typo. The ADT (average daily traffic) is likely recorded on a daily basis and monitored frequently.

A higher ADT means more cycles which means that the bridge fatigues faster. Heavier loading will reduce the total number of cycles available before fatigue. It seems improbable to me that if the analysis showed an infinite number of cycles remaining before fatigue failure that the bridge would suffer a fatigue failure after just 6 years, regardless of the increase in traffic.
 
Newtons Bit said:
AASHTO is pretty exact with loading. Assuming that everything is a truck load is way too conservative and uneconomical. The axle load is based upon what the bridge actually should see. The first number is likely just a typo. The ADT (average daily traffic) is likely recorded on a daily basis and monitored frequently.

A higher ADT means more cycles which means that the bridge fatigues faster. Heavier loading will reduce the total number of cycles available before fatigue. It seems improbable to me that if the analysis showed an infinite number of cycles remaining before fatigue failure that the bridge would suffer a fatigue failure after just 6 years, regardless of the increase in traffic.
Click to expand...

Having read further, they do calculations of 15,000 with 90% cars. I don't know why treating all of the ADT as trucks was even done. And you are right, they did four months of surveying the actual traffic for this paper.

The thing is, the paper states that doing an AASHTO load calculation exceeded the fatigue threshold and that's where the 15,000 truck number is being using in the text. If AASHTO is that exacting (and I trust you on that), I don't understand why they didn't pay more attention to those calculations.
 
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Not a structural engineer, and I didn't read the report, but I am a civil engineer.

My take on it is that the vehicle load during rush hour, combined with some aspects of the renovation construction (and presumably, removal of some features of the bridge that provided a measure of strength in one axis or another....I'm thinking of removal of roadway compromizing the strength of the box structure of the bridge, myself) exceeded the design load of the bridge.

I know this sounds like a lot of supposition, but typically the most stress a structure undergoes is during construction, as all the pieces that act together to provide operational strength that includes a good margin of safety are not there. The pieces in place are under far greater stress than they'd ever be once the structure is complete and fully loaded.

I hope my wild speculation doesn't derail this thread.
 
On NPR's "Morning Edition" program today a reporter mentioned that according to NTSB estimates (I think it was the NTSB; it was some federal agency), there are 130,000 bridges in this country that are degraded and in need of repair for safety. 130,000! The reported mentioned that that's 25-30% of all the bridges (and I presume that the majority of those are fairly small, like little highway bridges, country lane bridges, and overpasses.) We've built an awful lot of bridges, and they aren't getting any younger...

I can easily imagine the death toll will climb much higher, but I have hope that it won't. I hope they pulled a lot of people from the water.
 
Today on the news I heard the figure that 150,000 was the current ADT in both directions. So the typo is there, but it's the first figure missing a zero.
 
Bluegill said:
On NPR's "Morning Edition" program today a reporter mentioned that according to NTSB estimates (I think it was the NTSB; it was some federal agency), there are 130,000 bridges in this country that are degraded and in need of repair for safety. 130,000! The reported mentioned that that's 25-30% of all the bridges (and I presume that the majority of those are fairly small, like little highway bridges, country lane bridges, and overpasses.) We've built an awful lot of bridges, and they aren't getting any younger...
Click to expand...
If you ever drive in Connecticut you may notice that the bridges are well-maintained. That's the result of the I-95 Mianus River Bridge collapse in 1983. I had driven over that section about an hour before it went down.
 
In this post, I wrote:
I used to live in downtown Minneapolis. Several large churches were nearby, of various denominations. Perhaps the most well known of the churches was the Basilica of St. Mary.

Many of the churches had sets of bells. And there apparently had been some sort of keeping-up-with-the-Joneses competition to see which church could have the most bells, the loudest bells, and who could play the bells the longest.

The bottom line was: sleeping late on Sunday morning was not an option because of all the damned bell-ringing that went on, seemingly for hours. It was a damned nuisance.
Click to expand...
Today, however, I read this report from CNN, and I found myself moved by this passage:
On Tuesday, residents observed a minute of silence at 6:05 p.m., the moment when the I-35W bridge collapsed.

The silence ended a minute later when the city's places of worship began tolling their bells.
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I can imagine this bell-ringing was no minor tribute. It must have been a very emotional experience for those present.
 
Gravy said:
If you ever drive in Connecticut you may notice that the bridges are well-maintained. That's the result of the I-95 Mianus River Bridge collapse in 1983. I had driven over that section about an hour before it went down.
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*don's tin foil hat*

Is that because the NWO had alerted you about the bridge's impending destruction, ensuring that you, as an operative, would safely get across in time? :boggled:

ETA: On each occasion that my wife drove across the Housatonic River Bridge in Stratford, she swore it was about to collapse.

DR
 
Brown said:
I can imagine this bell-ringing was no minor tribute. It must have been a very emotional experience for those present.
Click to expand...

I'm numb to it now. Every night on the local news at 6:05PM they make a point of saying, "It was X days ago that the bridge fell at this time." Seems like every church in town has had some kind of memorial service. And today was the flags at half mast day at state buildings.

It is only the actual funerals that seem real. The rest come across as phony affectations because no one really knows what to say or do but feel like they should be doing something to memorialize it.
 
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