Normally, in the wake of a catastrophe, the details become clear only slowly and fitfully. Last night we heard that the 35W bridge had passed all of its inspections and that the collapse completely surprised everyone. Today, the Pioneer Press reports that inspectors had warned of a problem with this particular bridge, although the state overall had done an excellent job in bridge maintenance:
Bridge inspectors had noted structural problems over the years in the Interstate 35W bridge over the Mississippi River that collapsed Wednesday evening, but it was unclear whether obvious warning signs had been ignored. ...
In 2005, inspectors from the Minnesota Department of Transportation deemed the bridge "structurally deficient," in data submitted to the Federal Highway Administration's National Bridge Inventory.
Inspectors gave the bridge a sufficiency rating of 50 percent on a scale of 0 to 100 percent. A rating of 50 percent or lower means the bridge might need to be replaced.
The condition of I-beams, girders and other components called "structural members" was rated four on a scale of 0 to nine. A rating of 0 means failed, nine means excellent. Inspectors look at these for signs of distress such as cracking, deterioration, section loss and malfunction and misalignment of bearings. The deck received a rating of five and the substructure rating was six.
The design of this particular bridge made its inspection an even greater necessity. It had a span of almost 500 feet with no struts. When it was built, the city did not want to impede river traffic; barges still carry agricultural goods south on the Mississippi River. Viewers of the television coverage will see that the newer bridge that paralleled the collapsed section had a different design, with a support in the center of the river, and one would expect the replacement to mimic that.
That long span relied more heavily on the engineering of the steel supports, and for 40 years, it worked. Yet the 2001 study by the University of Minnesota noted "many poor fatigue details on the main truss and floor truss system", the very portions on which the entire bridge rested. The same study also said that Minnesota probably would not have to replace the bridge in the near future. The Pioneer Press report suggests that later inspections reached a different conclusion, or at least gave a stronger warning of problems.
State legislators expressed surprise at that news. Rep. Melissa Hortman (DFL-Brooklyn Park) noted that Minnesota's Department of Transportation (MnDOT) never mentioned that to the state legislature. If MnDOT knew of a poor rating on this bridge, they apparently never considered it a high enough priority to ask for special funding for repairs.
It's a bit ironic, as Minnesota has a good record for bridge maintenance. The US Department of Transportation rated only 3% of Minnesota's bridges as "structurally deficient", which put us as the eleventh-best state in that efforts. Rhode Island, in contrast, has 23% of its bridges rated structurally deficient, and seventeen percent of Michigan's bridges have that rating.
More details will come in the days and weeks ahead. If someone ignored a warning on this, expect Minnesotans to demand some housecleaning.