Placer River Bridge Inspection on the Kenai Peninsula
National Bridge Inspection Standards (NBIS) require that road bridges carrying passenger vehicles receive a routine inspection every two years. These inspections are primarily visual and require hands-on observations in order to check for loose and broken hardware, corrosion, insect damage, and a multitude of other maintenance and safety issues. Meeting these requirements often involves the use of specialized heavy machinery, such as an under-bridge inspection truck, to gain access.
The U.S. Forest Service (USFS) maintains significant road and bridge assets on Forest Service land. The National Forest Road System has 380,000 miles of roads and 7,500 bridges. Seasonal weather and limited transportation options can pose a challenge to accessing these structures, particularly for heavy inspection equipment. As a result, the USFS is currently exploring the use of unmanned aerial systems (UAS) to augment conventional NBIS inspections procedures.
The Placer River Bridge, located within the Chugach National Forest on Alaska’s Kenai Peninsula, was chosen as a test site for the prototyping and testing of a new UAS bridge inspection system. The Placer River Bridge was completed in July 2013, and at 280 feet, is the longest clear-span glue-laminated timber truss pedestrian bridge in North America. The trail bridge is accessible from a whistle stop built near the Spencer Glacier along the Alaska Railroad line between Anchorage and Seward.
When considering using an unmanned aircraft system for bridge inspection, there are many factors to consider. The overarching goal of the project is to collect high-quality imagery in both real-time streaming video and high-resolution still images. The former is beneficial for real-time flight operations and the latter is for analysis after the flights are completed. The analysis process involves computationally intensive modeling and produces spatially accurate three-dimensional models of the bridge.
For this mission, two Ptarmigan hexcopters were flown in close proximity to a complex structure with limited autopilot guidance without damage to the aircraft or structure and with no loss of collected data. New techniques were learned and practical workflows were further developed. The results indicate that such a data-driven approach can lead to UAS inspection systems that are more capable of meeting the needs of bridge asset managers and inspectors. UAS may provide more comprehensive imaging for inspection extending the lifespan of the bridge more effectively than conventional methods. Additionally, the UAS can be used in a wider range of weather and environmental conditions and can provide more effective post-inspection image analysis and measurement. Likewise, inspecting with UAS may provide savings due to fewer personnel, reduced operating costs, and less time on-site while increasing safety and reducing risk.
One of the most important measures of this mission is an understanding of how small UAS can be utilized by the USFS for future bridge inspections on a national scale. But, the broader reaching scope of this project will impact a diverse range of missions that the USFS has as part of its daily activities in road development, safety management systems, field data collection, public safety, fire management, and forestry.