Bridge Collapse in MN 29

[bimr]

NTSB is suggesting a design flaw in the bridge:

http://www.nytimes.com/2007/08/09/us/09bridge.html?_r=1&hp&oref=slogin

 

[bridgebuster]

The NY Times shouldn't have published that article; it's just going to cause a lot more confusion.

For instance:

"If there was a design error in the 1960s, failure to identify it before the bridge collapse indicates a problem with the federal inspection program, said Thomas M. Downs, who was the associate administrator of the Federal Highway Administration from 1978 to 1980. "

Either he was mis-quoted or expressed himself badly. Rouine bridge inspection does not include a review of design calculations.

 

[BigInch]

BridgeBuster, That's a very literal translation. A bridge inspection might also be expected to verify to some extent that the design is holding up as intended. A bridge inspection according to the methods used today may not measure actual stress levels, but maybe it should, is the message I receive.

http://virtualpipeline.spaces.msn.com

 

[miecz]

Bridgebuster

A bridge inspection often includes a calculated load rating, based on observed section loss. If the gusset plates were underdesigned, and then had some section loss, then an inspection could have picked up a rating deficiency.

 

[bridgebuster]

BigInch, jmiec,

I've performed several hundred bridge inspections, including several long-span trusses. Mr. Downs is mixing apples and oranges or perhaps he fails to see the difference between a Granny Smith and a MacIntosh.

Yes, an inspection is "to verify to some extent that the design is holding up as intended." However, when assessing components the criteria is "functioning as designed." The inspector isn't expected to give an assessment of the stress level in a main member.

Stress levels can be determined by strain gauge measurements but this is not part of a typical inspection; and it's costly.

"A bridge inspection often includes a calculated load rating, based on observed section loss. If the gusset plates were underdesigned, and then had some section loss, then an inspection could have picked up a rating deficiency."

Not all inspections include load ratings; although they should if section loss is observed. Suppose the plates were underdesigned and had no section loss? An inspector wouldn't know there was a design deficiency, unless the plate was glaringly thin when compared to a similar bridge. A load rating might uncover this deficiency. An underdesigned plate doesn't necessarily make the bridge unsafe: It reduces the design live load and the bridge would need to be posted accordingly.

There are two ratings: Inventory and Operating. Inventory is the maximum truck load that can be used for an indefinite period; Operating is the maximum permissible overload.

The thing with bridge inspection or building inspection, or any other structure is understanding how the structure is supposed to behave; how all the parts work together; what happens if one or more parts fail.

There are times when you look at something and know that it's wrong but there are plenty of times when the inspector will have now way of knowing that the girder should have been 52" deep and not 48" deep.

 

[miecz]

bridgebuster,

I've performed several hundred bridge inspections...

I've performed part of a rehabilitation study for a bridge similar to 9340, so your experience far outweighs mine. We did inventory and operating ratings for all the bridge components. Because our rehab included an inspection of the bridge, I was confusing an "inspection" with a "rehab study". So you're right, an inspection would not necessarily include a rating analysis.

As you say, if there was no section loss, then there would be no reason to question the rating of the gussets. I was merely trying to make sense of Mr. Downs remark.

 

[unclesyd]

Here is where the news reports probably originated.

http://www.ntsb.gov/Pressrel/2007/080807.htm

 

[JLNJ]

The level and placement of responsibility can get fuzzy for engineers doing inspections.

If I'm an owner and I have a failure of a structure I had an engineer "inspect" I'm having a hard time understanding why he would miss something so obvious that it would cause a failure - even if there was no "section loss".

This has been a sticky point for our firm in the past. Especially for buildings where the original drawings are not available and/or the original design intent is not clear.

 

[unclesyd]

Just another day in bridge building. This goes with the other one a couple of days ago in California.

http://www.azcentral.com/news/articles/0809mr-collapse0810-CP.html

 

[denoid]

From today's Minneapolis Star Tribune, here is an excellent editorial:

http://www.startribune.com/562/story/1352036.html

Tom G. Peterson - if you're part of this forum, I give you 10 stars!

 

[bridgebuster]

unclesyd - you'd think those guys in Arizona would've learned something when a similar accident crushed a car in Colorado killing a family. After that happened, numerous DOT's were issuing new policies for steel girder erection. So much for policies.

Let's give Tom Peterson 20 stars.

 

[hokie66]

Looks like Mark Rosenker needs to learn how to say "no comment". Giving the media tidbits from the NTSB serves no purpose. If they want to print supposition, they can monitor this site.

 

[darkwing88]

Below is the recommendations from the '06 URS report. Does anyone know whether any of these recommendations were carried out?

 

[jimmynorton]

"Probe to See if Construction Played Role"

I would be willing to bet that a truck of some sort or a piece of equiptment "Fell through" a section of the bridge deck that was previously milled and was being prepped for re-surfacing. I read in a previous post that some portions of the bridge were undergoing a full depth deck replacement and other portions were undergoing partial depth deck replacement.
Perhaps a truck drove over a portion of concrete that was not fully curred or a section that was milled and not capable of supporting the full weight of a piece of construction equiptment.
You should be able to completly remove the deck from the superstructure without failure if the superstructure is stable.
Overloading of the bridge with construction equiptment is a non issue in my opinion. The fact that there were loaders,tankers and trucks of different sorts on the bridge is minimal compaired to the normal traffic that crosses the bridge on a daily basis when all lanes are open.
I dont think vibration from jackhammers would be an issue either. The DOT doesnt allow the use of large hammers on a bridge deck. If im not mistaken the max size allowable hammer is only about 17 lbs. vurses a 60 or 90 lb hammer which is much more destructive and the only reason for the smaller hammer is to lessen fractures in areas not to be re-surfaced.

A previous poster suggested that maybe formwork fell, striking a structural member thus collapsing it. I dont think so, 2x4's and plywood dont weigh that much or at least not enough to collapse a structure of this size unless the structure was in a state of total dis repair. Even if the formwork was filled with concrete, a 9" deep deck spread out over many sq ft dropping on an open girder structure would not cause too much damage. It would be different had it been a box girder type structure.

 

[hokie66]

darkwing,

It has been reported that the state selected the option of inspection only, but I don't see that option listed, unless NDE was interpreted as visual examination only. There was apparently a lot of interpretation of this report by managers and politicians, with the objective of not doing anything. And the report dealt with corrosion only in a peripheral manner. The state's bridge inspectors wrote about corrosion, but again, nothing was done.

 

[msquared48]

My gut feeling screams at me to look to the foundation here being the ultimate cause of the failure - a shift/settlement, causing unanticipated stresses in a critical member in the trusswork. The north abutment at the juncture of the main and approach spans to me is very suspect from the video sequencing I have seen. Time will tell though. I assume that the piers are on large caissons and not on deep raft footings.

I really feel for the pour souls who had a personal moment of H*** as they went down into the river, not being able to get out, or never realizing what happened. Makes me realize how vitally important the work I do is, and how it impacts the lives of others. Note that the root word of vital is vita or "LIFE". Life safety.

Mike McCann
McCann Engineering

 

[bimr]

Effects of bridge work weren't analyzed

http://www.startribune.com/10204/story/1354824.html

 

[bimr]

Media attention to the bridge's gusset plates called overblown:

http://minnesota.publicradio.org/display/web/2007/08/09/gussetfolo/

 

[bimr]

UC-Berkeley's Hassan Astaneh doesn't cliam to know what caused the collapse of the bridge, and does not want to fault MnDOT for its failure. But he says from what observers know about the bridge, there were other warning signs.

At the time of the collapse, 18 construction workers from PCI were on the bridge. The National Transportation Safety Board is looking at the heavy equipment, including front-end loaders, the crew placed on the bridge. It's also asking about the weight, quantities of rock and other materials that sat on the bridge.

MnDOT spokesman Bob McFarlin said on that Wednesday, the crew was half finished with its job. It included resurfacing a few inches of the deck in some areas.

"In some areas of the bridge there was a complete concrete replacement, which means taking out the entire nine inches and replacing it with nine inches. And that was in eight spots along the bridge," said McFarlin.

Ripping off portions of the entire deck, professor Astaneh says, is playing with its structure. He says as a bridge weakens, the deck picks up some of the slack. It starts supporting the weight of the traffic and the bridge itself.

If a slab was removed over a fatigue crack or near an unnoticed crack, that could destabilize the bridge's delicate weight balance.

"That's not good news. That's another little storm coming in," said Astaneh. "So they're going to compound, and you're going to have all these little effects, which by themselves might not be critical to collapse the bridge."

Those little storms, as he calls them, include the heavy traffic of industrial trucks and daily commuters, the additional lanes that may have changed the balance of stress on the bridge, the fatigue cracks that were difficult to see, and the lack of structural redundancy. The bridge also faced environmental problems from water, ice and salt.

http://minnesota.publicradio.org/display/web/2007/08/06/bridgetroubles/

 

[jike]

The proportions in the photo of the gusset plates look a little odd to me. The plate does look a little thinner than I would expect. I am comparing it to the bolt spacing since I do not know what the member sizes are. Does anyone else feel this way?

Obviously, the analysis will prove this out one way or another.

The photo also looks like the plate might be bent like the bottom chord is not straight. Or could this be a distortion of the photograph?