December 1, 2005

Service: SoundPrint® - Bridges
Client: Maine Dept. of Transportation
Project Date: 2003
Type of Bridge: Suspension bridge
Length: 2,040 ft (622 m)

David Steinman designed the Waldo Hancock suspension bridge and when it opened in 1931 it was the first successful application of shop fabricated strands in the United States. The bridge utilizes two main cables, approximately 9-inches in diameter. Detailed inspections were performed in the 1990s, where several panels of main cable were unwrapped and visually inspected. Only minor corrosion related damage was discovered and the cables were deemed to be in satisfactory condition.

Following the inspections, a significant rehabilitation program was initiated for the entire structure. As part of this program the cable wrapping system was removed and replaced. During this procedure, it was discovered that corrosion was far more advanced than initially observed. In fact, there were several locations that exhibited severe corrosion. In one such area, an entire strand was broken and other strands were loose and not carrying load. As a result, emergency repairs were implemented to extend the life of the structure by several more years, while its replacement is designed and built.

In order to ensure the future safety of the bridge, Maine DOT commissioned Pure Technologies to install a SoundPrint® acoustic monitoring system to detect and locate any future wire breaks. The SoundPrint system utilized wireless sensors to monitor both main cables from anchorage to anchorage. The wireless sensor configuration transmits data to a data acquisition system located in the abutment of the bridge.

The system detected four wire breaks prior to the main cable strengthening. Acoustic events that pass the onsite filtering process are classified daily and Maine DOT is sent an e-mail to indicate whether or not there has been a wire break that day. Data is also available in near real-time on a confidential web site.

This project demonstrates the difficulty of accurately assessing the condition of main cables of suspension bridges. Inspection of selected panel points and extrapolation to the remainder of the cable can be misleading. Corrosion, as demonstrated on this bridge and others, may not produce visual evidence on the exterior of the cable and can be localized. This may not be identified by selective inspection.

Acoustic monitoring can be used prior to the visual inspections to obtain a more comprehensive assessment of each entire main cable. It will detect wire failure activity in problematic areas that might otherwise be missed by selective inspection. Subsequent inspections can then be directed to those areas to obtain a comprehensive assessment of the condition of the cables.

Experience on structures like the Waldo Hancock Bridge demonstrates the challenges in accurately assessing the condition of bridge cables and the value that acoustic monitoring can bring to the process. Had the technology been available and deployed prior to the original cable inspection, the true condition of the cables would likely have been established sooner, with consequent benefits in rehabilitation and replacement planning and execution.