Steel support beams lined the underside of the I-35W bridge.
The I-35W Bridge that buckled Wednesday evening was first deemed “structurally deficient” back in 1990, officials from the Minnesota Department of Transportation (MnDOT) said Thursday during a press conference on Southeast Main Street in the Marcy-Holmes neighborhood.
The term “structurally deficient” is a “programmatic classification, rather than a level of safety,” one transportation official said at a press conference on the Mississippi Riverfront.
Of the 13,026 state and local bridges in Minnesota, roughly 8 percent—or 1,160 bridges—are currently in the “structurally deficient” category.
An average of 141,000 cars passed over the I-35W Bridge each day, prior to its collapse.
“[It was] our busiest bridge in the state,” Dan Dorgan, the state’s bridge engineer, said.
The bridge was inspected by MnDOT annually since 1993. Before that, inspections were performed on the bridge every two years. At least one engineer is involved in each of MnDOT’s bridge inspection teams, Dorgan said.
MnDOT uses a National Bridge Inspection Program scale in checking area bridges.
The rating scale ranges from one to nine, with a “nine” rating indicating a bridge in excellent condition. Bridges don’t simply receive one rating, but rather, are ranked in various sub-categories. These categories include evaluations of the “deck,” which is the part you drive on, the “structure” and the “sub-structure.” A rating of four or less in any of the three aforementioned categories will result in a “structurally deficient” classification.
2007 study recommended inspections or reinforcement
On Thursday morning, The Bridge learned that a more recent study of the I-35W Bridge had been completed in the past couple of months for MnDOT by URS Inc., an independent engineering and design firm. Calls to URS were not returned, and calls to MNDoT were transfered first around various departments and ultimately to Homeland Security; nowhere did anyone have answers about the study.
MNDoT did comment on the recent report at the afternoon press conference, however. In its recommendation, URS Inc. presented MnDOT with two choices, according to Dorgan. MnDOT could either add steel plate reinforcements to the bridge or conduct a thorough inspection of the bridge’s tab wells to ensure that cracks in them were not proliferating.
Dorgan said MnDOT opted for the inspections, because they thought that adding steel plates might put some of the bridge’s “other members at risk.” Both options became moot on Aug. 1, when the bridge collapsed.
The design of the I-35W Bridge is referred to as an “arc deck truss.” The I-35W Bridge was one of four bridges in Minnesota with that design. Because their structures are so similar to that of the collapsed bridge, the other three arc deck truss bridges in the state—none of which are located in the metro—will undergo immediate “emergency” inspections, Bob McFarlin, assistant to the commissioner of transportation for Minnesota said.
At the press conference, several transportation officials were pressed about how these new inspections would be any different from the inspections that clearly failed in the case of the I-35W Bridge. The most Dorgan would say was that it would allow a “new set of eyes” to inspect the three bridges. It is not known whether the three arc deck truss bridges in the state are part of the 1,160 that have been deemed “structurally deficient” in Minnesota.
Recent resurfacing and plans for reconstruction
Khani Sahebjam, a metro district engineer, said the construction on the bridge at the time of its collapse was a $2.5 million resurfacing project that was slated to be completed on September 30.
“At this time we don’t have any reason to believe those issues are related,” Dorgan said, of a possible connection between the construction on the roadway of the bridge and the bridge’s collapse. “This was a failure in the super-structure.”
MnDOT has indicated projected redevelopment or replacement of the I-35W Bridge wasn’t expected until 2020.
McFarlin also seemed to dismiss reports that a test automated de-icing device used on the I-35W Bridge could have led to increased corrosion of its infrastructure.
“We’re not really looking at the [automatic] de-icing system as a factor here,” McFarlin said.
He went on to say that the same chemicals used in the automatic de-icing process are used when the solution is put down manually.
Sahebjam added that local transportation officials, in conjunction with Minneapolis city officials, are already preparing for reconstruction of a new bridge, pending completion of the investigations by the National Transportation Safety Board (NTSB) and an independently hired group of forensic engineers from Wiss, Janney, Elstner Associates, Inc.