Note: The maps in this article have been corrected.
Though millions of people commute over them daily, most of us don’t give much thought to bridge conditions until something catastrophic happens. (Minnesotans are arguably more sensitive to this particular issue, given the then-unthinkable 2007 collapse of the I-35W bridge over the Mississippi.) Engineers at MnDOT, however, monitor bridges continuously, hoping to avoid the sort of trouble that lead to headlines — like the deteriorating support bridge in Duluth that closed down I-35 in November.
The safety of bridges depends on a number of factors: roadway condition and geometry, amount of traffic, and structural condition, to name a few. Looking at structural condition alone, MnDOT can put the state’s more than 3,000 bridges into four general categories: Good, Satisfactory, Fair and Poor. The map below visualizes the condition of Minnesota’s bridges as of Spring 2013, the latest available data:
Correction: The original data supplied to MinnPost by MnDOT that was used to create the maps in this story contained errors. Changes in the standard MnDOT uses to calculate bridge load ratings caused some bridges to have lower structural condition ratings on the maps. We've since updated the maps with new data from MnDOT. The new data produced changed structural ratings for 112 of the 3,844 bridges appearing on the original map. The overall composition of bridge conditions changed only slightly — 87% of bridges were in Good or Satisfactory condition, 11% Fair and 2% Poor.
In general, Minnesota’s bridges are in good shape, with structural conditions within MnDOT’s targets: 86 percent of bridges are in Good or Satisfactory condition, 12 percent are Fair, and 2 percent are rated Poor. Just because a bridge is in "Poor" condition, it doesn't mean it's unsafe to drive on — it means the bridge is a higher priority for serious maintenance, also taking into consideration other factors, like the volume of traffic that passes over the bridge. MnDOT projects sufficient funding to meet structural goals for the next 10 years. Beyond that, however, the agency sees expenses exceeding revenues and, absent new funding, will need to make choices about which bridge repairs to prioritize.
Of particular concern is the coming wave of maintenance needed for bridges built during the 1960s Interstate build-out. The 1960s saw the greatest number of bridges constructed of any decade so far — 857 bridges were constructed in those years, 22 percent of all bridges in the state. As these bridges age, their maintenance requirements will increase — as will the costs associated.
Good and Satisfactory bridges are candidates for preventive maintenance — like sealing cracks and other minor repairs — which is funded out of MnDOT’s operating budget. Fair and Poor bridges are more likely to need significant repairs funded by capital investments, money that comes from federal and state road construction dollars.
The general ratings visualized above are a summary of a number of aspects affecting a bridge’s structural condition: the state of its deck, the structure above and below the bridge, the condition of the culvert that carries water under a roadway, an overall structural evaluation and the adequacy of the waterway crossed by a bridge. More details on how bridges are classified can be found in MnDOT’s Bridge Performance Measures Summary [PDF].