By: Richard “Lee” Floyd, P.E.
Like many state DOTs, the South Carolina Department of Transportation (SCDOT) has considered the use of structural-health-monitoring (SHM) technology to improve its bridge management and repair protocols.
After some initial research and internal discussion, we determined that judicious use of SHM could enhance our ability to meet several key bridge-management objectives, most importantly, safely extending the service life of selected critical bridges and, therefore, resulting in significant cost savings. This article explains how we saved approximately $700,000 in agency costs for one bridge by using a technically appropriate SHM solution.
The Great’s burden
The Great Pee Dee River Bridge, which carries U.S. 378, was constructed in 1955 and crosses the Great Pee Dee River in the Pee Dee region of northeastern South Carolina. It is approximately 4,000 ft long and its original design loading was for HS20 vehicles. The main spans of the bridge consist of a continuous steel two girder/floor beam/stringer system with cast-in-place concrete T-beams for the approach spans. The two girder/floor beam/stringer system also classifies the bridge as being “fracture critical.” The bridge has an ADT of approximately 5,100 with 8% trucks, and the ADT was projected to almost double over the next 20 years. Significant logging operations and small manufacturing concerns in the local area contribute to the majority of the truck traffic.
By some DOT standards, the ADT and truck use were not excessive, but the detour length, if load-restricted, would have been approximately 50 miles, resulting in millions of dollars of additional user cost per year. As a state agency that supports economic development, SCDOT is sensitive to this added cost, particularly during the current economic environment, so overly severe load restrictions are a key concern.
The Great Pee Dee River Bridge has been inspected using both NBIS visual and tactile protocols since that FHWA program started in the early 1970s, and the CoRe element level protocols since the early to mid 1990s. In the late 1990s, the bridge received a sufficiency rating below 50 and was classified as structurally deficient, which made it eligible for replacement using the FHWA Highway Bridge Program (HBP). However, project programming issues and funding limitations delayed the replacement project letting until November 2008. Construction on the new bridge began in January 2009 and the span was opened in March 2011.
Although weight restrictions were not placed on the existing bridge, the range of visual defects was cause for concern, especially if overloaded trucks used this bridge. SCDOT grew increasingly concerned during the construction-planning period and developed a plan for a significant repair program to assure safe operations on the old bridge during construction, allowing legal-load trucks to use the bridge without detouring.
Placed on a monitor
Findings from NBIS and element-level inspections over the years showed a significant increase in the following conditions:
- Severe corrosion and section loss in the two main steel girders;
- Visible cracks in steel stringers; and
- Increase in deflections, vibration and potential for fatigue initiation.
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The 2007 inspection was of particular concern, especially considering that the old bridge needed to remain in service while the new one was constructed on a new alignment. From a historical perspective, SCDOT’s other bridges of the same type had problems primarily in the floor beams and stringers due to water leakage at the expansion joints. In this case, the primary concerns were the two main steel girders, since SCDOT had never designed a repair for this type of girder design. Several repair schemes were evaluated with an estimated cost averaging about $825,000. While considering the repair alternatives, the agency determined that a SHM solution could provide SCDOT with an adequate safety margin for the old bridge while the new one was being constructed.
During 2008, SCDOT initiated discussions with LifeSpan Technologies of Atlanta regarding use of their structural monitoring technology across South Carolina. After thorough investigation of alternative approaches and suppliers, SCDOT concluded that SHM offered what it needed: A simple, rugged, reliable, cost-effective solution that provided SCDOT with information that was easy to understand and essential to provide the enhanced safety the agency was seeking during construction of the new bridge. In addition, the monitoring system hardware was re-usable, which made it even more cost-effective.
SCDOT executed a contract in January 2009 and the monitoring system was installed during the spring. Active monitoring was under way for nearly two years until the new bridge opened in March 2011. The SHM system hardware was removed from the old bridge immediately after the new bridge was opened and will be re-installed on another bridge with serviceability concerns.
Seeing the strain
SCDOT learned that a SHM solution could provide the bridge owner with significant information to enhance overall bridge management. For example, captured strain (both tensile and compressive) and temperature data was available 24/7 to the engineering staff over the Internet and SCDOT also was able to use handheld smart phones to observe captured data.
After several months of data collection, SCDOT was able to filter out the observed strains driven by temperature changes, allowing it to evaluate and quantify residual live-load strains. This was particularly useful in the Fall of 2010, when SCDOT suspected overloaded truck use during the early morning hours. SCDOT alerted the Transport Police, which began monitoring the situation. While the bridge was not load-restricted, some detained trucks were overloaded, which created a significant safety hazard on the old bridge while the new one was under construction. When the safety situation was explained to the detained drivers, use of overloaded vehicles on the old bridge decreased. Some drivers even asked how the overloads were detected.
Their advice to others
SCDOT saved approximately $700,000 by using a SHM solution versus the alternative of temporarily repairing the old Great Pee Dee River Bridge. The return on investment was obviously substantial, which allowed SCDOT to redirect savings to critical uses that were not possible if it had to repair the old bridge.
SCDOT has been extremely pleased with the use of SHM technology and intends to aggressively deploy more solutions to better manage the inventory of structurally deficient bridges and to provide performance and preservation monitoring of other high-value bridge assets. If SCDOT can safely defer only 1 in 3 planned repair or replacement actions, it believes the return on investment will be robust, while reducing local funding demand, enhancing safety and lowering system risk.
SCDOT was financially and technically successful in deployment of a SHM solution because it was careful about a number of key issues. The following recommendations reflect the agency’s experience, both before and during use of the SHM solution:
- Have a clear objective in mind when you purchase a SHM solution; there’s simply no reason to consider this exploratory R&D–it works;
- Don’t overdo the sensor count or even types of sensors; strain (especially peak strain) plus temperature sensors worked fine in this application;
- Simple, rugged, reliable and easy-to-understand information should be an essential trait of any SHM solution–downtime for any reason is not an option;
- Check suppliers and make sure you contract with a firm that has experience, knowledge and commitment; and
- Aim to achieve a return on investment for each project; re-using the Great Pee Dee River Bridge SHM system will boost SCDOT’s substantial return on investment even higher
About The Author: Floyd is the state bridge maintenance engineer for SCDOT.
