The Michigan Department of Transportation (MDOT) has updated information on the MDOT Web site about the safety of over 4,000 state highway bridges. Updates on bridge conditions will be loaded onto the web four times a year.
Bridge safety reports have been posted online since August 2007. State bridge information can be downloaded by route number and/or county, and are current as of Oct. 1, 2011. Only highway bridges greater than 20 feet in length are included. Pedestrian, railroad and locally owned bridges are not included.
National Bridge Inspection Standards require MDOT to inspect bridges every two years. The three bridges managed by bridge authorities: Mackinac, International, and Blue Water bridges are inspected annually, exceeding federal requirements. Their ratings are not included in the list MDOT has posted to the Web.
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.