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Case Study

Maine Dept. of Transportation – Waldo Hancock Bridge

The Maine Department of Transportation is the office of state government responsible for the regulation and maintenance of roads and other public infrastructure in the state of Maine. The department manages 2,919 bridges and spans in total, inspecting 2,414 in 2014.

Project Details

Services
SoundPrint® Acoustic Monitoring – Bridges

Monitoring system commissioned in 2003

Operated until bridge retired in 2006

Bridge Type
Suspension
Monitored Length
2040 ft (622 m)
Number of Main Cables
9.6 in (244mm) parallel strand cables
Number of Sensors
22

Project Highlights

Rapidly deployed monitoring system allowed resumption of two-way traffic

Identified & located 4 wire break events and 23 wire cut events

Confirmed effectiveness of cable strengthening measures

Estimated economic benefit in range of $25-36 million

Challenge

The Waldo Hancock Bridge, located in the state of Maine, was completed in 1931. Its deck carried one lane of traffic per direction, while two narrow reinforced concrete sidewalks were used for pedestrian traffic.

Partially due to the National Bridge Inspection Standards (NBIS) stipulated by the Federal Highways Administration (FHWA), a number of inspections of the superstructure were carried out starting in the early 1990s. Portions of the main cables were unwrapped and inspected in 1992, 1998, and 2000. Due to signs of stage-3 corrosion during the 1998 small-scale investigation, the 2000 investigation was expanded to include more panel points on the North cable.
This investigation included four openings on the North cable, and one opening on the South cable. The safety factor had originally ranged from 3.0 to 3.2, based on no damage of the main cable. The wire breaks counts observed reduced the safety factor to just below 2.4 at two of the five locations investigated.

Since the cable condition was worse than anticipated, the bridge owner decided to implement a significant rehabilitation program to extend the life of the structure.

The major component was to replace the external main cable protection system. is replacement enabled an extensive visual inspection of the strands, with further wedging performed at select areas. During this exercise, it was discovered that the extent of the corrosion was beyond what the five panel inspection showed. At the worst location, 10 of the 37 strands were not carrying load, with one strand 100 percent corroded. is occurred on the South cable, where previously only one panel was inspected, reducing the calculated safety factor to 1.5 at the posted carrying limit of 12 tons.

Solution
This situation required emergency strengthening measures. First, a SoundPrint® acoustic monitoring system was installed on both main cables. To save installation time, a wireless system with 22 sensors was used. Load restrictions were placed on the bridge, and until the acoustic monitoring system was fully functional, the bridge was restricted to one-way traffi c for a short time. A total of eight supplementary strands were placed above each main cable, connected directly to each cable band with supplementary suspenders. The heavy concrete sidewalk was removed and replaced with a steel-wood combination.
Results

The acoustic monitoring system detected 4 wire breaks in the first 50 days of monitoring the cables (1 on the North cable, and 3 on the more damaged South cable). Once the supplementary cables were installed and the deck lightened, the wire breaks on the main cables stopped. To give all parties confidence, wires were periodically cut to demonstrate the effectiveness of acoustic monitoring system. Nine wires were cut and successfully recorded before the monitoring began, and a further 14 individual wires were cut and recorded over the following two years. In this case, the acoustic monitoring system was used to:

  • Provide utility during the critical period when strengthening measures were required
  • Extend the life of the bridge for an additional three years and four months, until a replacement bridge could be designed and built.

Approximately $1.1 million was spent monitoring the bridge using acoustics over this time period. Client estimated that the economic benefit of removing the load restrictions for heavy trucks, and not fast-tracking the new bridge was in the range of $25-$36 million.