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Advances in Bridge Evaluation Using Digital Image Correlation (DIC)

August 2021


According to FHWA’s 2020 National Bridge Inventory Data, 1.5% of bridges in Texas are Structurally Deficient (SD). In addition, 2,841 bridges in Texas are posted with load limits, which restrict the size and weight of vehicles and in turn the flow of goods and services in the state. The state of Texas has identified 11,704 bridges in need of retrofit or repair, at an estimated cost of $6.3 billion.

The Brazos River Bridge in Brazoria County, Texas, was built in 1939. Rated as Structurally Deficient. Source: https://bit.ly/2WhWrwl; Credit: Mayra Beltán/Houston Chronicle

To address this great need for bridge monitoring and upgrade, the Texas Department of Transportation (TxDOT) is looking towards new technologies to accelerate the process, reduce monitoring and assessment costs, while also delivering more accurate and less conservative load ratings for bridges. A recently funded TxDOT research project allowed a team at the University of Texas at San Antonio (UTSA) to deliver a Digital Image Correlation (DIC) system that goes a long way towards alleviating the burden of maintaining an aging bridge infrastructure.

The UTSA team leveraged emerging Digital Image Correlation (DIC) technology to deliver a uniquely capably DIC system, dubbed the Civil Infrastructure Vision (CIV) system for monitoring surface deformations on structures ranging from small-scale material coupons to large-scale field structures such as bridges. The system can track deformations on virtually any point on a bridge surface as small at 1/1,000th of an inch, even when cameras used to make the measurements are over 100ft away.

The system benefits TxDOT by providing deformations measurement during load tests across all structural members of a bridge. The relative magnitude of these deformations provides a reliable estimate of load sharing between them. For example, deformation data can provide the fraction of a truck load that is carried by various girders in its vicinity. For a typical bridge, the load sharing is often found in load tests to extend well beyond the conservative assumptions of the AASHTO code, thereby reducing the burden on each member, and in turn permitting a higher truck load rating on a bridge.

Civil Infrastructure Vision (CIV) system

CIV system monitoring bridge deformations during load testing in Bexar county

The CIV system is an integrated hardware/software system based on principles of DIC and spatial triangulation. The CIV system consists of two high-resolution, low-noise digital cameras triggered and operated by a laptop computer with the CIV system software. The software was coded by the UTSA research team, that pushed the boundaries of accuracy and measurement sensitivity at large distances and over large fields of view. The system can track the 3D spatial movement of as many targets on the surface of a bridge as desired. In this way, measurements can be made for any point on the surface of a structure that is in view (full-field measurements).

The system is robust with respect to target characteristic features, and only requires minimal contrasting features on surfaces to reliably track targets. High Contrast Physical Targets (HCPTs) can also be attached to a surface to both enhance tracking reliability and/or provide fixed identifiable points at key measurement locations. The accuracy of the system is on the order of 1/1,000th inch for large-volume bridge monitoring applications.

Field Demonstration

Middle span of the multi-girder bridge located at Lometa, Texas.  The bridge was monitored under load testing using traditional instruments and CIV system. Scaffolding was required for contact instruments used for validation.

Multiple bridge load testing was performed with collaboration of TxDOT using the CIV system. One notable field test was conducted on a steel continuous multi-girder bridge located at Lometa, Texas, crossing Simms Creek. The bridge has three continuous spans of 60, 75 and 60 feet. The middle span was monitored using the CIV system, as well as traditional instruments such as displacement transducers and strain gages.

The CIV system setup includes setting up cameras, finding appropriate field of view of the span under investigation, and attachments of high-contrast targets to focus measurements to specific points.  Setup of the CIV system was done in about 2 hours. By comparison, attaching the traditional instruments using scaffolding took over 2 days.

The bridge deformation was monitored with the CIV system at approximately 100 ft away from the bridge. (left: Professor Ghannoum (UTSA), project Principal Investigator, right: Professor Diaz (UTSA), Co-investigator)

The static load testing of the bridge was conducted using a dump truck. The truck was positioned stationary at fixed-locations on two predefined paths. In addition, a crawl speed test was conducted whereby the truck was moved along a path at speeds below 5 mph. The CIV system deflection measurements were compared and verified with measurements from transducers installed under the bridge girders. Results indicated very close match within hundredths of an inch between the CIV system and the more traditional instruments.

Figures 1 & 2 (above): The location of the traditional sensors attached to girders and predefined path of the truck is illustrated in these figures

Figure 3 (below): Fixed-location test-path 1 (midspan deflection)- deflection measurements of CIV system is validated with measurements of displacement transducers installed on the bridge girder.

Figure 4 (below): Fixed-location test-path 2 (midspan deflection)

Concluding Remarks

Deflections at midspan of the girders under crawl speed test-path 2

New technologies are rapidly changing the asset management landscape, allowing measurements not possible previously, increasing measurement scope, and resulting in improved outcomes for aging infrastructure. This unique CIV system pushes the boundaries of what can be measured remotely without contact, and the accuracy of such measurement. More specifically, the CIV system has the following advantages:

  • Scalability: the system can be calibrated for use in small-scale material testing applications, to large-scale bridge monitoring applications.
  • Limited need for access: because the CIV system can operate over 100ft away from a structure, it does not require direct physical access. This feature can shift measurements on difficult-to-access structures from impossible to possible; e.g., for bridges crossing water or heavily trafficked lanes.
  • Fast load testing: setting up the cameras and selecting targets can be done in as little as an hour, such that a bridge or other structure load test can be performed in as little as 2 hours. In bridge applications, traffic interruption during a load test is only required for a few minutes while the loaded truck is positioned. By contrast, conducting a bridge load-test using traditional instruments such as potentiometers can take several days.
  • Ease of use: the CIV system allows TxDOT to conduct load tests using in-house personnel, as opposed to hiring firms specialized in instrumentation.
  • Cost savings: by shortening the duration of load-testing from days to hours, eliminating the need in many cases of installing scaffolding to install instruments, and allowing in-house load testing, the CIV system reduces the cost of structural load testing and capacity assessment to a mere fraction of the cost using traditional means and methods.
  • Improved safety: the CIV system improves safety for the driving public by elimination a large number of lane closures. In addition, safety is improved for highway workers with reduced exposure to traffic and working at heights.
  • Availability: this technology is now available to the engineering community. A system can be delivered by UTSA, calibrated to a specific application.

It is envisioned that such a powerful tool will aid a wide range of forensic engineering applications, improve the accuracy of structural assessments, and deliver more targeted and cost-effective infrastructure management solutions.