The National Transportation Safety Board (NTSB) has concluded that the I35W bridge collapse was caused by undersized gusset plates and oversized construction load, and that corrosion did not cause the collapse. I’m as far from an engineer as you can get, but all of that makes logical sense to me.
But it strikes me that the NTSB made an error of ommission. It failed to explore why no steps were taken to address a gusset plate that was known to be badly warped, more than four years before the collapse.
Some terrific investigative reporters at the Star Tribune discovered that Minnesota Department of Transportation (MnDOT) bridge inspectors had a June 12, 2003 photo of a very warped U10 gusset plate in their inspection file. U10 is the plate that NTSB says failed.
That part of the process seemed to work well, and we should be comforted by that. Inspectors spotted and documented a major problem.
But then what? Did the inspector report the problem to superiors? Did the inspectors’ superiors discuss options for strengthening the warped plate? If strengthening or replacing was technically infeasible, did MnDOT consider closing the bridge, as they have in the face of similar problems in St. Cloud and Hastings?
Assuming the plate couldn’t be fixed, why didn’t someone at least warn against parking several tons of construction equipment — reportedly the largest load the bridge had ever borne, equal to the weight of a 747 jet — directly on top of the badly warped U10 gusset?
These are legitimate questions that the NTSB seems to have bypassed.
Think of it this way. Imagine if a doctor spotted a tumor, stuck a PET scan of it in the file, labeled the tumor an unfortunate biological design flaw, and took no further action to prevent further damage from the flaw. The doctor would be 100% correct; the tumor is a design flaw, and not her fault. But the doctor would still need to explore all options for removing, killing or slowing the tumor.
And so it goes with MnDOT. The NTSB seems to have done excellent work examining the strictly technical issues behind the collapse. But for whatever reason, it stopped short of delving into the human and process issues.
I have no interest in villifying MnDOT. They do amazing work that keeps us safe, and keeps our society and economy humming along. I just want to see a great agency get better. There was a gap between inspectors seeing the flawed U10 gusset plate and MnDOT doing anything about it. To prevent future catastrophes, NTSB needs to help us understand the reasons for that gap.