Does Structural Health Monitoring (SHM) Provide Safety and Maintenance or Confusing Data?

In 2009 a 12-inch long crack was discovered on a fracture critical eyebar on the San Francisco Oakland Bay Bridge. The crack extended from the outer edge of the eyebar head to the pin. Soon after Caltrans explored possible remote monitoring solutions. The design of the structural health monitoring system (SHM) would continuously monitor the entire length of selected eyebars for the earliest possible detection and location of potential cracking in order to provide an additional safeguard for close to 277,000 vehicles crossing the bridge daily. Caltrans initiated a comprehensive study of 77 SHM firms, which could meet their requirements. Caltrans then selected the top seven firms and used a performance based RFP so vendors could select the best method at their disposal. Acoustic Emission (AE) was chosen for remote monitoring as the most viable solution. The AE SHM system would complement the quarterly inspections performed by Caltrans and assist in monitoring several critical areas that were extremely difficult to inspect. Since early 2012 the system has been on-line and collecting data. This unique project represents the largest known SHM application in which only 640 AE sensors monitor the entire length of 384 eyebars for an approximate total length of 3.94 miles. This presentation will discuss the field deployment of the system and the monitoring results from the automated system. One of the triggered alarm events identified a potential mechanism, which if it had gone unnoticed, could have resulted in additional damage to fracture critical eyebar and a potential repair cost of over $14 million. Since September of 2013 the SFO Bay Bridge has been taken out of service and plans for the old systems to be deployed are in process.

References
Average Daily Traffic: 270,000 vehicles http://www.dot.ca.gov/hq/esc/tollbridge/SFOBB/Sfobbfacts.html MinnDOT/University of Minnesota report titled, “Bridge Health Monitoring and Inspections Systems - A Survey of Methods” 2009 MinnDOT and University of Minnesota. Development of an Advanced Structural Monitoring System (MN/RC 2010-39). available at http://www.lrrb.org/pdf/201039.pdf Figure 6: Eyebar material loss due to frictional wear caused by contact with adjacent conduit. 117 Arturo Schultz, Principal Investigator, Department of Civil Engineering, University of Minnesota, 2010 The Journal of Transportation Research Board, published a paper titled “Real Time Eyebar Crack Detection using Acoustic Emission Method” on this proof test. Authors: Johnson, Michael B, Ozevin, Didem, Washer, Glenn A, Ono, Kanji. Gostautas, Richard S Tamutus, Terry A Publication Serial: Transportation Research Record: Journal of the Transportation Research Board ISSN: 0361-1981 Date: 2012 Transportation Research Board: Selected Field Results: San Francisco Oakland Bay Bridge Acoustic Emission Monitoring Program Authors: Michael B. Johnson P.E., California Department of Transportation, Richard S. Gostautas, Mistras Group, Terry A. Tamutus, Mistras Group Report 14-2859, 2014
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