LANSING, MICH. — The Michigan Department of Transportation (MDOT) recently updated information on the MDOT website at www.michigan.gov/highwaybridgereport about the safety of 4,396 state highway bridges. MDOT has posted bridge safety reports online since August 2007.

State bridge information can be downloaded by route number and/or county, and are current as of the end of 2011. Only highway bridges greater than 20 feet in length are included; ratings for pedestrian, railroad, and locally owned bridges are not included. MDOT is updating bridge condition information on the Web four times a year.

"We are entering our fifth year of putting highway bridge safety reports online for citizens," said State Transportation Director Kirk T. Steudle. "Monitoring and maintaining the safety of highway bridges and overpasses remains one of our top priorities."

National Bridge Inspection Standards require MDOT to inspect bridges every two years. The Mackinac, International, and Blue Water bridges are inspected annually, exceeding federal requirements. These three bridges are managed by bridge authorities, and their ratings are not included in the list MDOT has posted to the Web.

MDOT employs more than 20 bridge inspectors who have specialized training and work in teams of two. Bridge inspectors use a variety of tools to assess bridge safety and structural integrity. The types of inspection performed include bridge safety inspections, fracture critical inspections, fatigue-sensitive inspections, and underwater inspections. Non-destructive evaluation and bridge monitoring also is done as needed. Techniques used include calipers to measure the thickness of steel, ultrasonic testing to check for defects in steel, sounding to detect concrete separation, and monitoring bridge deflections and response to load.

Of the 4,396 bridges included in the Dec. 29, 2011, report, 299 are classified with the engineering term of "structurally deficient," meaning they may require rehabilitation or replacement at some time in the future; and 977 are classified as "functionally obsolete," meaning their design is outdated and may require modernization at some time in the future. Five bridges were removed and four were added. The last highway bridge report was posted to the Web on Nov. 1, 2011, and covered bridge inspections through Oct. 1, 2011. The new report lists 42 fewer structurally deficient bridges and 78 more functionally obsolete bridges than on Oct. 1, 2011.