Minneapolis bridge inspection photo of gusset pitting
The National Transportation Safety Board has reported that the Minneapolis bridge that fell into the Mississippi River probably failed because the critical gusset plates apparently were a half inch too thin. Gusset plates, for those including your truly who are not bridge engineers, are sheets of metal used to shore up critical parts of a bridge.
The NTSB had zeroed in on the gusset plates in some of their initial investigations, which perplexed some engineers according to an article in the Engineering News Record. Gene Corley, senior vice president of Skokie, Ill.–based CTL Group, said:
It sounds like they are thinking that [the gussets] might have been the wrong size.
Then paragraph that follows this chills me even more now than when I read it in August. Nicholas Altebrando, bridge practice leader for New York–based SYV Group, pointed out:
Gusset plates are part of primary inspection. You would inspect them all.
The article goes on to point out:
Even so, catching a problem with a riveted gusset plate, designed to hold truss joints together, isn’t easy, engineers say, because stress may not show on the surface.
It quotes Richard M. Gutkowski, a professor of civil engineering at Colorado State University:
Welds are checked, for sure, but the state of the strain and stresses in a gusset plate wouldn’t be anything that’s visibly evident.
In the interim report released this week, the NTSB found there was a fundamental design problem in the bridge gussets, warning bridge engineers to recalculate the loads on similar bridges. In a press conference, National Transportation Safety Board Chairman Mark Rosenker stated:
What broke was the U-10 [a node of gusset plates that connect the bridge's beams]. We know why it broke: because it was inadequately designed for this bridge.
When asked if rust and/or corrosion could have played a role he answered:
We did not believe it played much of a role in any way on this issue, It was a weight-bearing issue.
As seems to be the case with everything in America these days politics immediately reared its ugly head. The issue has been a political firestorm in Minnesota in part because the Republican governor, Tim Pawlenty, who is rumored to be a possible vice-presidential candidate, refused to allocate funds to repair the bridge.
The Minneapolis Star Tribune pointed out Rosenker has deep ties to the Bush Administration and the Republican Party. The article went on to note:
Politics have long been a little more than peripheral in Rosenker’s career. Before his appointment to the NTSB, he worked as Deputy Assistant to the President and Director of the White House Military Office. That’s where he worked closely with Cheney.
Minnesota Democrat James Oberstar, Chairman of the Committee on Transportation and Infrastructure, alleged Rosenker had overstepped his mandate by ruling out rust and corrosion. Oberstar told the Star Tribune Rosenker’s actions were:
An inappropriate, unfortunate and uncharacteristic statement by a board chairman.
In a press release Oberstar stated:
We are still waiting to see if delayed or deferred maintenance may have also played a role in the collapse. Additionally, the unusually heavy loads of construction equipment and vehicle traffic on the bridge that day may have also been contributing factors. It’s too early to say with certainty what the exact cause of this tragedy was.
The interim report focusing on design problems that was unanimously approved by the five-member NTSB makes for interesting reading when compared with the inspection report. It was this blog–not a newspaper article or other media report–that identified three problems in the inspection that factor into the NTSB report.
First, it pointed out the issue of bridge inspection. In particular, it zeroed in on the practice of visual inspection that is the way most bridges still are evaluated. A visual inspection would not have identified the thickness of the plates as a problem, nor would it have revealed the types of problems Gutkowski discussed.
Second, it uncovered an issue the media have still yet to understand: the quality of the metal used in the bridges. Here is what I wrote:
Under current law, the steel used in bridges must come from the United States. However, the current law states that the steel used in bridges can be inspected piecemeal. In other words, five pieces of steel in a bridge can all be inspected separately rather than as a unit which is sort of like checking your car out in pieces.
Third, the article pointed to the archaic scoring system used to rate bridges and its unreliability. I likened it to scoring Olympic diving except it was done by only one person. The Minneapolis bridge scored a four on a scale of ten in its last inspection. That meant the bridge was “structurally deficient.” I believe this blog was the only source I know that quoted the following paragraph from the actual bridge inspection report:
The long term plans for this river crossing need to be defined with replacement, redecking, etc. Due to the “Fracture Critical” configuration of the main river spans and the problematic “crossbeam” details, and fatigue cracking in the approach spans, eventual replacement of the entire structure would be preferable.
Maybe I was one of the few who actually read it.
I read it again in light of the new NTSB findings. What leaped out to me were the references to the gusset plates:
Panel Point #4 (East Truss Stringer Joint): Connection gusset plate has a weld overlap. (p. 20)
Panel Point #11 (East Truss): Section loss: at gusset plate bottom chord. (p. 23)
Panel Point #11 (East Truss): Pitting: inside gusset plate connection at L11. Stringer #3 has two bolts missing at the floorbeam connection. (p. 23)
Panel Point #13 (East Truss): Bottom chord gusset plate has section loss, flaking & pack rust. (p. 24)
Panel Point #9′ (West Truss): Truss bottom chord/sway frame connection (gusset plates) has section loss, pitting, heavy flaking rust. (p. 37)
Panel Point #8′ (West Truss Pier #7 Stringer Joint): Truss bottom chord/sway frame connection (gusset plates) has section loss with heavy flaking rust. (p. 37)
Panel Point #7′ (West Truss): Wind bracing gusset plate, at stringer #14 has loose bolts. (p. 38)
Note the number of times the words “section loss” appears.
The NTSB Interim Report specifically focused on the gusset plates at U10 and L11, stating:
The initial onsite investigation of the collapsed structure identified the failure of the U10 gusset plates as occurring early in the event. The L11 gusset plates are detailed similarly to those at U10.
The report found:
The gusset plates at U10 and L11 consistently failed the D/C ratio checks conducted and the U10 gussets also violated the unsupported edge limitations.
It is interesting to compare these findings with the inspection report. Maybe it’s just because I am not an engineer, but it took some time to make the comparisons. The NTSB report is interesting in that it is ambiguous about the location of L11 and U10. The “u” and “l” stand for upper and lower and are numbered consecutively on both of the two main trusses of the bride, so there are actually two L11 and U10s–one on the left side and one on the right. It wasn’t until I saw a picture of the press conference that I realized the report was referring to both sides. So I used the following diagram from the bridge inspection report to illustrate those locations, which are both near the middle of the bridge where it spans the river.
As pointed out above, the last inspection before the collapse noted:
Pitting: inside gusset plate connection at L11. Stringer #3 has two bolts missing at the floorbeam connection. (p. 23)
It also noted the following about L11:
Truss bottom chord member L12′/L11′ has a cracked tack weld at an interior stiffener. (p. 26)
Bottom chord member L12′/L11′ two cracked tack weld at diaphragm tab.
I hate to disagree with the NTSB, it seems to me we have here a classic chicken and egg question. Was the bridge too thin or was it so thin that the defects had a larger impact? Rosenker emphatically stated that rust and other problems could not have played a factor in the collapse, but how can he be so sure? If you have a structurally deficient house without termites and one with, isn’t it logical to assume the one with termites is weaker and that weakness compounds its possibility of collapsing?
But there is another point no one seems to be making. Let us assume Rosenker is right, that the problems uncovered in the bridge inspection did not play a role. The report STILL recommended–and this bears repeating–
Due to the “Fracture Critical” configuration of the main river spans and the problematic “crossbeam” details, and fatigue cracking in the approach spans, eventual replacement of the entire structure would be preferable.
The bottom line is that if the recommendations had been followed and the structure replaced then it would not have collapsed. If the governor had allocated the funds for the replacement rather than rigidly sticking to his no-taxes pledge those people who died would be alive today.
A week ago, my wife and I were having dinner with old friends we had not seen for awhile. I found out that night he was within two hundred yards of the bridge when it went down. He was traveling with a group in another car which had just crossed the bridge. In other words, it went down between them–a distance of maybe a mile at most separating the two cars.
I repeat what I said at the end of the original article because it seems even more relevant now as we move into another Presidential election and the governor under whose watch this occurred–the same governor who refused to ask for funding to remedy the defects in the report–is being talked about as vice-presidential material.
The real question is: Will we learn the larger lesson that government is not the problem but the solution? It’s not just physical infrastructure that has deteriorated under the Counterrevolution and lives are not only being lost by bridges collapsing. Our social and intellectual infrastructure is crumbling and killing people as surely as that bridge.
We all know about our schools, which are suffering under years of budget cuts and trying to fulfill unfunded federal mandates such as No Child Left Behind. But did you know that the third leading cause of death in this country is medical errors, errors in part caused by a health care system as badly in need of repair as that bridge? Then there is the scandal of our inner cities where young black men are dying from guns almost as fast as our troops in Iraq. And now we have a mortgage collapse as dramatic as that bridge falling in the river.
If all that comes of the Minneapolis bridge disaster is more visual inspections and some bridges being fixed, America will have missed the point. If our presidential candidates and Congress continue to ignore the real cause, then you might as well analogize the entire country to the Titanic.
Right now, I feel like a passenger in steerage who happens to look out a porthole to see an ominous wall of white. I yelled the equivalent of “iceberg ahead” in the book The Strange Death of Liberal America–all but predicting another disaster, but few noticed a voice coming from down here. Here’s what I wrote:
In a way the equivalent of many New Orleans already exist, their damages just as real and devastating…The big question plaguing America is whether this very real Counterrevolutionary hurricane will spur the same bureaucratic bungling and indifference that accompanied Katrina.
I’m yelling again. But after awhile you get hoarse. And you get discouraged. And you blame yourself because you could not get people to listen. And people keep dying unnecessarily.
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