During a 2-year study, FHWA took a closer look at the state of the practice for using unmanned aircraft systems (UAS) for bridge inspections.

Unmanned aircraft systems offer inspectors another tool for assessing the condition of bridges.

Bridge inspectors may need to use several access methods and tools to adequately meet the objectives of a bridge inspection in accordance with governing National Bridge Inspection Standards (NBIS). However, some of these access methods, such as an under-bridge inspection truck (UBIT), can be costly to operate because the equipment is expensive to maintain and run and disruptive to traffic because it requires lane closures. Using an unmanned aircraft system (UAS) can be a cost-effective solution to obtaining stand-alone, high-quality visual inspection data, or to supplement standard inspection methods and equipment. Some UASs can also improve inspector safety and enable examination of areas that are difficult to access.

UASs can produce live streaming video, providing opportunity for the inspector to inspect while remaining on the ground. If inspectors identify deterioration in UAS images, they can then decide whether to perform a physical inspection to determine the severity and extent of the deterioration. Using UAS in this manner can provide more efficient use of standard access equipment and physical inspection techniques for assessing deterioration, in addition to increasing safety.

In an ongoing study, the Federal Highway Administration is conducting research to identify types of sensors used in UASs; quantity and quality level of data needed to perform satisfactory inspection using UASs; best practice guidelines for efficient and reliable use of the sensors; and guidance on how the collected data should be assessed, presented, and stored to provide reliable and actionable information to owners to support data-driven decisions. This research study also identifies the minimum requirements of sensors to provide comparable information as other visual inspection techniques.

"We felt it was very important to take a closer look at how State departments of transportation are using unmanned aircraft systems for bridge inspections because of the potential benefits of this technology," says FHWA Executive Director Thomas Everett. "UASs are proving to be incredibly useful to bridge inspection staff to supplement inspection practices."

FHWA expects to conclude the research project in March 2020. What follows are key findings of the research to date.

Components of a UAS

A UAS for bridge inspection includes the unmanned aircraft, control station, sensors, and pilot. A certified pilot is the most important piece of the system, controlling and flying the aircraft in a safe and professional manner. While not always a requirement, a visual observer can aid in scanning the sky to ensure safe flight while the pilot concentrates on the operation of the aircraft. As required by Federal law for all bridge inspections, an inspection team leader must be on site during the inspection.

Optical cameras, infrared cameras, and LiDAR (light detection and ranging) systems are the most common types of sensors used. Depending on the tasks, an inspector can determine the appropriate types of UAS platform and sensor types. Optical sensors capture the imagery data (video as well as still images), which enable inspectors to see deficiencies in an up-close or magnified manner without having to physically access the specific area on the bridge. UAS-captured high-resolution images may reveal defects missed using distant visual inspection techniques. High-resolution imagery can also serve other purposes, from providing a record of surface defects to measuring and tracking some types of defects over time.

Infrared thermography (IR) sensors can detect areas of deterioration in concrete by identifying and viewing temperature gradients. Demonstrations have shown the areas of bridge deck delamination identified using IR sensors correspond well to the areas discovered using traditional sounding techniques.

Inspectors can see irregularities on the bridge deck in this optical image taken by a UAS. The photo quality is sufficient to enlarge areas of interest, as shown on the right-hand side of the photo.

LiDAR sensors actively emit pulses of light—up to hundreds of thousands of returns per second—to accurately measure the distance between the sensor and a target object. The main advantages of LiDAR over photogrammetry are the ability to penetrate vegetation with multiple returns, faster imagery processing times, and improved capabilities to resolve fine features. Inspectors can use a LiDAR point cloud to create a three-dimensional (3D) model of the bridge.

Employing a UAS sensor is beyond simply manipulating the aircraft controls and pointing the sensor at a location. To adequately capture the quality visual information required, personnel must also understand the basics as well as some of the more advanced principals of photography. An understanding of the individual camera's available settings helps to maximize effectiveness.

This infrared thermography image shows possible bridge deck delamination. The yellow and orange areas shown above in the IR map (labeled with circles), indicate possible delaminations.

What UAS Can Do

Typically, bridges that present challenges to gaining access to all parts of the structure for a comprehensive inspection are good candidates for UAS augmentation. For example, on a bridge with an excessively wide sidewalk or tall pedestrian barrier, a UBIT would be limited to access from one side only. A more typical case is a wide bridge where the center is not accessible from a UBIT even when used from both sides. In this case, a UAS could provide imagery from both sides of the bridge.

Example of a LiDAR point cloud of San Francisco Bay and the Golden Gate Bridge in California.

Some State DOTs have conducted research studies or implemented programs employing UASs for bridge inspections to detect certain types of bridge defects. Their efforts have successfully identified bridge defects and collected information important to the bridge planning process. Bridge engineers also have used the imagery captured during bridge inspections to create accurate two-dimensional and 3D models of a bridge for analytical and planning purposes. State DOT efforts have shown that UASs can enhance traffic safety for the public and safety for the inspection team in many cases. For example, during a 2018 study performed by the Minnesota Department of Transportation (MnDOT), contractors flying a collision-tolerant UAS captured imagery inside an enclosed steel arch. Using this type of UAS inside the bridge structure eliminated the need for personnel to enter the potentially dangerous confined space. (Entering a confined space requires specific training for members of the inspection team, and requires the receipt of entry permits in accordance with current safety regulations and practices.) MnDOT reported a potential 66 percent cost savings using UAS compared to traditional methods in 2017 and an average cost savings of 40 percent for the case studies reviewed in 2018.

Identifying which aspects of a bridge inspection are best suited for a UAS according to the needs of State DOTs is useful in determining efficient use. The Oregon Department of Transportation (ODOT) identifies major bridge reporting categories and applies a scale of 1 to 4 to rate the usefulness of a UAS for providing inspection information. ODOT also evaluates how useful a UAS is in conducting various types of inspections. They identified a monetary savings of around $10,000 per bridge and a 10 percent savings in personnel time per project compared to inspections done without UAS.

Summary of Detectable Bridge Defects Discovered with UAS Imagery

This table summarizes the types of bridge defects that inspectors from several States detected using UASs. In these instances, UASs enhanced the inspection process or improved the accuracy of results. The table represents a sample of the defects noted by States and should not be interpreted to mean that these are the only defects that can be detected using a UAS-mounted sensor.

For more information on UAS application in transportation, see "Ready for Takeoff" in the Winter 2018 issue of Public Roads.

Limitations of UASs

UASs can provide many advantages to a bridge inspector. However, they currently cannot replace a person where tactile or other contact inspection methods are necessary or required. For example, inspectors cannot employ only UAS for fracture critical member inspections because of the FHWA requirement for using hands-on inspection techniques. The reason is because today's cameras and sensors still have limited capability to see through dirt, debris, and corrosion that may hide critical defects.

Oregon DOT Assessment of UAS Usefulness

Ratings scale: 1 = not useful 2 = limited use 3 = useful 4 = very useful

"In no way should a UAS be considered a complete solution that will solve all user needs," says Cheryl Richter, director of the Office of Infrastructure Research and Development at FHWA. "It is a tool that may bring efficiencies in time, cost, and safety [of the] bridge inspection process when successfully employed."

UAS operators in both the public and private sectors must adhere to the statutory and regulatory requirements issued by the Federal Aviation Administration (FAA). Public aircraft operations (including UAS operations) are governed under the statutory requirements for public aircraft established in 49 United States Code (U.S.C.) § 40102 and § 40125. In addition, both public and civil UAS operators may operate under the regulations promulgated by the FAA. The provisions of 14 Code of Federal Regulations (CFR) part 107 apply to most operations of UAS weighing less than 55 pounds (24.9 kilograms). Operators of UASs weighing greater than 55 pounds may request exemptions to the airworthiness requirements of 14 CFR part 91 pursuant to 49 U.S.C. §44807. UAS operators should also be aware of the requirements of the airspace in which they wish to fly. The FAA provides extensive resources and information to help guide UAS operators in determining which laws, rules, and regulations apply to a UAS operation. For more information, visit www.faa.gov/uas.

Analyzing and Storing Data

When employing a UAS during bridge inspections, inspectors capture large amounts of data that require storage, post-processing, analysis, and dissemination. For most UASs, the imagery and data captured during a flight is stored on a removable media storage device, such as a secure digital (SD) memory card, integrated into the aircraft platform. The files stored on the SD card are a variety of common file types that are accessible by media-viewing and post-processing software.

MnDOT created this 3D bridge model with selectable image locations using data it collected with a UAS.

Inspectors process the captured and stored data into different products to supplement inspection documentation, better inform decisionmakers regarding the structures, and improve future inspection planning. Common information products include images, video, 3D models, and surface models. Bridge engineers can use UAS imagery of the entire structure to create bridge "plans" for bridges that do not have records of the original structural drawings. Also, inspectors can use this visual information, and the associated geographic position information related to the images, to update the structure inspection records, identify and assess new deficiencies, track the extent of specific defects over several inspections, and update bridge repair recommendations.

In general, an inspector will use the standard inspection report format that complies with the NBIS, supports reporting data to the National Bridge Inventory, and satisfies State DOT policies and standards. When using a UAS to supplement an inspection, the inspector will select the imagery captured by the UAS sensor to include in the report. Thus, using a UAS for inspection purposes should not generate additional paperwork but the information and defects found in the images should be documented in the inspection notes and element condition data, as applicable.

"Data management can be the most challenging aspect of using a UAS," says Joey Hartmann, director of the Office of Bridges and Structures with FHWA. "The substantial amount of data collected requires an appropriate data management plan to ensure the inspectors capturing the data have (1) a standard approach for collecting and transferring the data, (2) a known and secure location and structure for storing and retrieving the data, and (3) a well understood process for sharing the data and inspection products generated by the UAS."

Cataloguing is the process of creating a directory of stored imagery files. It includes identifying where the data are located, identifying the types of data stored, establishing a process for version control, and instituting file naming conventions to which all users will adhere. A more advanced method of cataloging images is using a photogrammetric 3D model of the bridge, which requires creating a photogrammetric point cloud. This method is an alternative that enables all the inspection images for the bridge to be stored as a 3D model. Inspectors can select the bridge section of interest on the model (that is, where a defect exists) to view the image for analysis.

MnDOT tested this 3D modeling method to catalogue images. It enabled MnDOT inspectors to click on a point in the model and view images at that point to view defects. This can reduce the need for a manual photolog because the photogrammetry software will locate the image on the structure.

Future Advancements

As more bridge owners and inspectors incorporate UASs into their processes, the technologies available to improve inspections will continue to advance. For example, first-person view (FPV) devices or goggles are a relatively recent entry to the bridge inspection process. FPV gives the user a unique perspective from which to wirelessly view imagery and control the camera. Some FPV systems provide high-definition 1080p video and enable the user to control the sensor in real time with head movements. The image presented equates to looking at an 18-foot (5.5-meter) high-definition television from about 9 feet (3 meters) away. Some FPV systems also provide inspectors with the ability to digitally magnify the image, making it appear significantly closer and allowing a bridge inspector to see hairline cracks in the structure. For more information on FPV goggles for bridge inspectors, see "A New View for Bridge Inspectors" in the Summer 2018 issue of Public Roads.

Artificial intelligence (AI) is another technological advancement that inspectors may choose to incorporate into the UAS. AI can enable the system to navigate independently without human input throughout the structure (other than instructing the aircraft when and where it is supposed to fly and overriding the system in the event of a malfunction or signal loss). Flying the UAS in the same flight paths using AI can enhance the identification and tracking of defects over time. Inspectors could also use AI to collect and analyze many infrastructure images.

The speed of technological advances and improvements in the integration of new technologies is impacting bridge inspection. More and more bridge owners are employing UAS and exploring new ways to integrate UAS within established guidelines. FHWA is moving forward in partnership with those in the field to find efficiencies in inspection methods, reduce the cost of conducting inspections, enhance the comprehensiveness and quality of collected data, and improve the safety of inspection teams by using UAS, all while assuring the Nation's bridges are safe for travelers.

Hoda Azari is the manager of the Nondestructive Evaluation (NDE) Research Program and NDE Laboratory at FHWA's Turner-Fairbank Highway Research Center. She holds a Ph.D. in civil engineering from the University of Texas at El Paso.

Dennis O'Shea is FHWA's senior bridge safety engineer for the North region. He serves as a technical resource for the National Bridge Inventory and National Tunnel Inventory programs for 13 FHWA division offices in the Northeast. He has a B.S. in civil engineering from the University of South Alabama and is a licensed professional engineer in Delaware and Pennsylvania.

Derek Constable is a bridge management engineer with the FHWA Office of Bridges and Structures. He holds B.S. and M.S. degrees in civil engineering from The Cooper Union for the Advancement of Science and Art.

For more information, contact Hoda Azari at 202–493–3064 or hoda.azari@dot.gov.