As transportation infrastructure continues to age, new methods of noncontact sensing should be evaluated and, if found suitable, used for bridge monitoring and structural health assessment. This study highlighted the use of infrasound monitoring, a geophysical technique utilizing acoustics below 20 Hz, as one possible solution for noncontact, nonline-of-sight bridge health monitoring. The study focused on the technique of infrasound for infrastructure monitoring with a detailed case study involving a steel, two-girder bridge in northern California. Infrasound was used to detect natural modes of the structure from a distance of 2.6 km. The frequencies detected infrasonically were validated with data collected by on-structure accelerometers. The noncontact nature of this structural assessment approach has potential to supplement traditional structural assessment techniques as affordable, remote, persistent monitoring of transportation infrastructure. Implications for use of this technology were also discussed alongside specific applications for scour monitoring and postdisaster assessment.
Infrastructure in the continental United States is often used beyond its design life due to budgetary constraints and logistical hurdles. New structural health monitoring techniques have been developed to mitigate the increasingly constrained resources available by identifying the potential need to repair and retrofit aging infrastructure from modal signatures. Large structures, such as bridges and dams, emit infrasound (acoustic energy below that of human perception) at their natural modes of vibration, which can be related to structural health and capacity; this infrasonic energy can propagate tens of kilometers from the infrastructure. Currently, the determination of structural health requires intensive hands-on measurements of individual elements at repeated intervals. Persistent, remote infrasound assessment provides a method for standoff assessment of the modal behavior of structures for structural health monitoring and damage assessment. Case studies and demonstrations of this technology are presented for water control infrastructure systems, transportation infrastructure systems, and riverine infrastructure systems.
To date, the infrasound community has avoided deployments in noisy urban sites because interests have been in monitoring distant sources with low noise sites. As monitoring interests expand to include low-energy urban sources only detectable close to the source, case studies are needed to demonstrate the challenges and benefits of urban infrasound monitoring. This case study highlights one approach to overcoming urban challenges and identifies a signal's source in a complex acoustic field. One 38 m and one 120 m aperture infrasound arrays were deployed on building rooftops north of downtown Dallas, Texas. Structural signals in the recorded data were identified, and the backazimuth to the source determined with frequency-wavenumber analysis. Fourteen days of data were analyzed to produce 314 coherent continuous-wave packets, with 246 of these detections associated with a narrow range of backazimuth directions. Analyzing the backazimuths from the two arrays identified the Mockingbird Bridge as the probable source which was the verified with seismic measurement on the structure. Techniques described here overcame the constraints imposed by urban environments and provide a basis to monitor infrastructure and its conditions at local distances (0–100 km).
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