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Case Study

Texas has more than 53,000 bridges, the largest bridge inventory in the United States.

The Texas Department of Transportation (TxDOT) conducts routine inspections of most bridges every two years, ensuring all bridges open to vehicular traffic in Texas are safe and best in class.

Project Details

Services
 SoundPrint® Acoustic Monitoring
Monitoring system commissioned in 2002
Operated continuously since commissioning
Bridge Type
 Fan arranged cable-stayed bridge
Monitored Length
 2473 ft (754 m)
Number of Stays
 192
Stay Type
 Grouted 15mm x 7 wire strands in HDPE tubes

Project Highlights

System has performed in excess of 98% of efficiency over its life

SoundPrint identified individual wire events in stays ranging from 50 to 193 m in length

Allows TxDOT to establish remaining service life of 192 stays

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Challenge

The Fred Hartman Bridge, located in Baytown Texas, opened to traffic in 1995 and is one of the largest cable-stayed bridges in the United States. ­

The cable-stayed bridge portion of the bridge is 2473 ft. (754 m) long, consisting of steel girders and transverse beams, and includes a 1250 ft. (381 m) main span. ­ The bridge consists of two 78 ft. (24 m) wide composite concrete decks suspended from diamond shaped concrete towers using a total of 192 stays. ­The stays are comprised of multiple 0.59 in (15 mm) seven-wire strands grouted inside HDPE tubes.

After the bridge completion, large-amplitude vibrations of the cables were observed. A vibration monitoring program confirmed that the stays are subject to wind/rain-induced vibrations, raising concerns about potential fatigue failure of the strands.

Solution

Following testing by the Ferguson Laboratory at the University of Texas, TxDOT installed a SoundPrint® Acoustic Monitoring System to monitor wire break activity within the stays. ­

The installation consisted of three specially-designed sensors on each stay (one on each anchor and one on the stay approximately 8 ft. (2.5 m) above the deck). ­ These sensors are suitable for cable-stayed bridges and are durable enough to withstand harsh marine environments. ­

The bridge is divided into 16 virtual monitoring zones with sensors from each zone connected to an active junction box using durable coaxial cable. ­ The active junction box outputs are connected to the SoundPrint® data acquisition and management system (“DAQ”) by means of multiple twisted-pair shielded cable. ­ The DAQ is located inside the North-East tower leg at deck level. Data is automatically transmitted from the DAQ through a local Internet connection to the Pure Technologies data processing center in Calgary, where the data is analyzed and classified.

On-demand reports are available to authorized individuals through a secure password-protected area of the SoundPrint® website. As the bridge is located in an area with frequent thunderstorms, the system has been upgraded with state-of-the-art lightning protection technology.

Results

TxDOT personnel have inspected some of the stays based on the reported wire events via anchorage investigations and stay force evaluations. To date, the inspections have not revealed significant changes in the measured stay forces due to the individual wire failures. ­ The rate of wire breaks has given TxDOT confidence in the operation of the stays, as well as the vibration damping system installed to reduce cable fatigue.

Case Study

Highways England (formerly the Highways Agency) is a government-owned company with the responsibility of managing the core road network in England. It operates information services, liaises with other government agencies and provides staff to deal with incidents on the roads it manages. The company managed The Mossband Viaduct, which carried traffic over a roadway and railway until its demolition in 2008.

Project Details

Services
 SoundPrint® Acoustic Monitoring – Bridges

Monitoring system commissioned in 2001

Operated continuously until bridge demolition in 2008
Bridge Type
 Post-tensioned concrete
Monitored Length
 836 ft (255 m)
Number of Spans
 8
Number of Sensors
 210

Project Highlights

System has performed in excess of 99% of efficiency over its life

SoundPrint identified and located 6 specific wire break events

Structure life extended over 7 years via structural health monitoring

Client estimated economic benefits $30 to $40 million

Challenge

The viaduct was comprised of twin decks – one deck was constructed of concrete girders butted up against each other, with a top cantilevered slab, while the second deck was a voided box-girder. Half of the spans were post-tensioned cast in-situ concrete table spans and four were suspended spans supported by the table spans on half-joints.

Conventional visual investigations were performed in 1990, 1995, and 1999.

The first investigation discovered water ingress at all the deck joints and next to the drainage pipes. This was believed to have been occurring over many years. General corrosion of the surface reinforcement in these areas resulted in surface spalling.

The second inspection revealed the presence of several corroded and broken tendons in the in-situ table spans. The damaged tendons were located in the deck over the pier supports, where the cable profiles approached the top surface of the deck. A deck construction joint within a meter of the pier support provided a direct water path to the tendon ducts. The cable profiles descended from this location into the mid-span of the table span and to the half-joint anchorage area.

As is often the case with selective visual investigations, one location often showed severe corrosion while an adjacent location appeared undamaged. In this case, the third inspection showed that some of the longitudinal tendons had all the strands completely corroded whereas only two meters away, they appeared in good condition. Clearly, the tendons at the half-joint locations were at risk at all 14 locations, but the extent of deterioration at every location was unknown.

Solution

The Highways agency implemented a comprehensive bridge management plan starting in 1999 to assess the rate of deterioration of the post-tensioning, and if necessary, to intervene and strengthen the structure. ­The plan consisted of:

  • Monthly visual inspections of critical sections
  • Installation of vibrating wire stain gauges to monitor cracks on the sides of the sections and soffit of construction joints
  • Load testing to compare stain changes
  • Installation of a SoundPrint acoustic monitoring system to monitor the rate of deterioration of the post-tensioning system

In late 2000, 210 acoustic sensors were installed along the 836 foot (255 m) length of the viaduct in three rows to fully monitor all post-tensioning tendons. ­The sensors were multiplexed at local junction boxes to minimize cabling and data acquisition requirements. Data was acquired via a single acquisition unit calibrated to reject the majority of non-wire events, with events of interest transferred to servers in Calgary, AB for analysis.

Results

The wire break rate found was lower than expected. Six wire breaks were detected during the extended life of the structure, giving engineers/owners confidence that the bridge management program was effective. Further, strain readings during the AIL annual load tests showed that the structure was not experiencing severe changes in deflections, and that the serviceability requirements were being met. ­ The crack growth was also monitored, and thought to be consistent with the rate of deterioration observed with the acoustic system.

 

In this case, the acoustic monitoring system was used to extend the life of the structure by seven years and 10 months, until a new bridge could be built as part of the A74 Cumberland Gap. Approximately $1.4 million was spent on the two monitoring systems and the load tests over 8 years, including the system-related inspection and reporting functions by engineers.

 

Client estimated that the economic benefit in delaying the permanent bridge replacement and not fast tracking a temporary structure was between $30-$40 million dollars.

Case Study

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.

September 19, 2011 Calgary, Alberta

Pure Technologies Ltd., (“Pure”) is pleased to announce the introduction of its next-generation SoundPrint® system for monitoring bridge cables and post-tensioning systems. The system, called SoundPrint Light, utilizes Pure’s patented acoustic fibre-optic monitoring technology for monitoring prestressed concrete pipelines, which was first introduced in 2005 and is now deployed in over 1,500 km of pipelines. The technology provides continuous dynamic integrity information to allow operators to manage these critical assets cost-effectively and proactively.

SoundPrint Light has several advantages over the first-generation piezoelectric-based SoundPrint system. As the optical fibre cable acts as both the sensor and the data transmission medium, the need for a separate wired or wireless communication system is eliminated. Furthermore, because it is an optical-based system, it is immune to electromagnetic interference. This reduces installation and maintenance costs and increases the sensitivity and reliability of the system.

SoundPrint Light will be offered in two configurations. For post-tensioned bridges, including box-girder structures, the fibre-optic cables will be attached with adhesive directly to the concrete surface along the length of the bridge in a pattern that provides full coverage for all tendons in the structure. Because the fibres are acoustically sensitive along their entire length, the distance from a wire break to the closest sensor will generally be less than it would be with a conventional piezoelectric sensor array, resulting in lower signal attenuation and increased system sensitivity. For suspension and cable-stayed bridges, the configuration will consist of a fibre-optic communication cable connecting discrete fibre-optic sensors mounted at cable bands or at the stay anchorages. It is also possible, for new bridges, to incorporate the continuous distributed sensor within the cable assemblies during construction.

Commenting on the development, Jack Elliott, Pure’s President, said:

“We are pleased to be able to announce this important new initiative in bridge monitoring technology. We are commercial originator of the concept of continuous acoustic monitoring of bridges. We installed our first SoundPrint bridge system on a highway viaduct in Huntingdon, UK, in 1997, where it is still operating, and we have been providing bridge owners with valuable information on the integrity of cables and tendons in post-tensioned, cable-stayed and suspension bridges around the world ever since. The introduction of SoundPrint Light is a natural outcome of our experience with fibre-optic monitoring of pipelines and of our extensive in-house research and development capabilities. This development takes the concept of continuous monitoring of bridges to the next level.”

For more information on Pure Technologies, please visit our website at www.puretechltd.com

For more information on this new technology and/or the Bridges & Structures Division, contact:

Logan Fesenmair
Business Manager, Bridges & Structures
Tel: +1-403-537-3355
Email: logan.fesenmair@puretechltd.com