The 82 year old Anthony Wayne Bridge (AWB) in Toledo, Ohio is undergoing an extensive rehabilitation in two phases starting in construction season 2014. The plan is to first replace the approaches and rehabilitate the superstructure. Upon completion of the superstructure rehabilitation, steps to preserve the main suspension cables will be taken. Prior to taking action to preserve the cables, however, it is necessary to evaluate the condition of the cables. Therefore, as part of cable condition evaluation, an acoustic monitoring system was installed on July 2011 and has been continuously monitoring the main cables since then.
Acoustic emission (AE) is a non-destructive technique which is practical for monitoring elements of bridges where invasive inspection is either difficult or costly. The AE system can be accessed remotely in real time and it does not cause any interruption to traffic. In the case of a suspension bridge, main cables are of primary concern as their condition cannot be assessed externally unlike other bridge components and they are fracture critical. This paper presents a case study on the application of the acoustic emission technique to the main cables of the AWB. Several laboratory experiments were planned and executed to develop understanding of the potential AE sources. Wire breaks were the primary AE sources under concern. Rain and frictional activities induced by traffic and wind events would create secondary and/or noise sources. The rain, friction and wire break were all simulated in the laboratory and it was verified that, by using a combination of parameters along with signal signatures, a wire break signal can be discriminated against other secondary or noise sources.
The AE monitoring system on the AWB uses a series of 7 algorithms that analyze the parameters of each detected AE event. For each feature that meets or exceeds the value of the classification, it is assigned a source type ranging from 0 to 7. Thus for a wire break, the AE signal would meet or exceed all 7 criteria and would receive a `source type’ classification of `7’. The analysis of the data collected on the AWB during January 2013 to June 2013 (excluding May 2013) showed very high acoustic activity near sensors 1, 2, 14 and 15. After further examination it was found that those activities were of frictional nature caused by weather events and traffic induced movements. Many AE events were classified as high as `source type 6’ that occurred during extreme weather events and there was not any `source type 7’ event. This suggests that no wire breaks have been recorded so far. No wire breaks were discovered during the 2012 invasive inspection too, which supports the results from AE monitoring. Meanwhile, the system was unable to capture the signal produced by cutting of wire samples during the invasive inspection and this challenges the reliability of the monitoring system. An `Auto Sensor Test’ performed in March 2014 indicates that there has been degradation in the system’s performance. Many of the sensors do not seem to have proper coupling, thereby causing difficulty in effective signal-source interpretation. Very few AE events were observed in a review of data from the bridge closure period that started on March 17, 2014.