Drones. Unmanned Aerial Systems (UAS). Unmanned Aerial Vehicles (UAV). Regardless of how they are referred, these highly versatile machines continue to gain traction, expanding beyond recreational use into commercial applications.
The civil engineering industry is a leader, exploring uses for the technology as a means to supplement, and at times supplant, existing job functions, from aerial mapping to construction monitoring to bridge inspections. But while UAS technology has the ability to change the way engineers approach projects and look beyond what was once perceived as limitations, the expectation of some is that the technology be flawless and offer no restrictions. Unfortunately, that is simply not the case. As with any emerging technology, users must account for growing pains and practical limitations. For bridge inspections, UAS technology can prove to be a game changer under the right conditions.
With the continued proliferation of the UAS community and the desire to fly commercially, the Federal Aviation Administration (FAA) has had to make adjustments to its regulations. To ensure safe and proper commercial operations of “small” UAS (sUAS), which are crafts weighing between 0.55 and 55 lb, regulations were put into place that created separate rules for UAS and manned aircrafts.
The FAA’s Part 107 Regulations, released in August 2016, were less restrictive to the structure sites under consideration and included revisions that allowed engineering firms and state agencies to enhance bridge inspection capabilities with UAS. Most notably, the revisions allowed for more freedom when conducting partial bridge inspections under live traffic, allowing UAS to be flown at closer distances with the appropriate supervision and controllability. In theory, the regulations should create a safer work environment for inspectors and travelers, and reduce lane closures and traffic impact.
Wis. inspects bridges, drones
With the new FAA Part 107 Regulations in place, the Wisconsin Department of Transportation (WisDOT), in partnership with Michael Baker International, investigated the capabilities and limitations of using UAS for bridge inspections. WisDOT established three overarching goals for the pilot project:
- Make bridge inspections safer for the traveling public and inspectors;
- Make the inspection process more efficient; and
- Improve the quality of the inspection reports.
William Oliva, P.E., chief of the Structures Development Section for WisDOT, noted, “WisDOT clearly sees the utility and benefit of UAS in our bridge inspection toolbox. We are confident that these tools will improve our ability to efficiently monitor the condition of our bridges and promote the safety of our inspectors and the traveling public.”
To accomplish the program goals and discover the most valuable lessons from the exercise, the team used a UAS for inspections. For bridge inspections, a UAS has to have the ability to inspect the underside of a bridge as well as a bridge deck, which requires a camera with 180° of view. At the time, the UAS that was used was relatively unique in the market because of its camera placement, unlike other UAS that had a camera mounted completely under the mainframe and therefore, had a limited view upwards.
Notable features of this system were its camera systems (or payloads), its eight-rotor setup for redundancy and its stability flying without GPS, which is important when flying underneath a structure. Two main payloads were utilized for WisDOT’s project—a 36-megapixel camera with 12x zoom and a parallel-mounted thermographic 12.1-megapixel camera with a 30x zoom—that when combined provide side-by-side JPGS and georeferenced 14-Bit RAW thermal images. The eight-rotor redundancy was desirable for the project since two motors on each side could fail and the UAS will remain in the air. Lastly, stability without GPS was important since most of the under-deck inspections would lack GPS connection. Flying a UAS in close proximity to a structure or obstacle without the GPS engaged is a highly difficult assignment for even the most experienced UAS pilot.
For the purposes of the study, WisDOT tasked the team with inspecting three unique bridge types to collect a diverse set of takeaways:
- A 12-span, continuous steel deck girder structure carrying four lanes of I-43 traffic over an abandoned rail yard;
- A 12-span, continuous steel deck girder structure carrying six lanes of I-39 traffic over the Wisconsin River; and
- A single-span, overhead steel truss structure carrying two lanes of Highway 179 traffic over the Kickapoo River.
Unlike most routine inspections done from the bridge deck with a snooper truck, most of the inspection with the UAS was done from the underside of the structure. This approach has obvious benefits as well as some limitations. The absence of lane closures was convenient for the traveling public and removed the risks associated with using a snooper vehicle in close proximity to live traffic. However, transporting all the gear needed for the inspection to the underside of the bridge, where there may or may not be a suitable staging area, was challenging. For structures over water, an boat— anchored, so as to not be considered a moving vehicle and therefore in violation of FAA regulations—was necessary to get under each span and maintain visual line of sight during the inspection.
During the pilot project, a few spans of each of the continuous steel deck girder bridges were inspected and a majority of the overhead steel truss structure was inspected. The underside of the continuous steel deck girder bridges was collected with a GPS connection only a portion of the time. This required high pilot skill to adjust for the wind and to keep the collection at a consistent altitude and pace. Holding a consistent altitude makes the post-processing of the data less labor-intensive when creating a model of the structure. This is less important if the main objective of the inspection is to only collect relevant photos. Inspection of the overhead truss structure utilized a different collection method. Since there were numerous gusset plates, multiple passes along the top and each side were necessary to ensure full photo coverage. The inspection team was able to model a gusset plate from the data collected and evaluate it compared to original plans. This, however, would be overly labor-intensive to do for each gusset plate on the structure. The UAS also encountered magnetic interference warnings when flying inside the overhead steel structure. For this reason, only a portion of the inside was flown.
Must keep warm
Operating a UAS for bridge inspections undoubtedly adds a unique element to the process, so understanding the methods, best practices and challenges for data collection is key for future UAS inspections. It is important to remember that all UAS are not created equal, and certain features work best for different scenarios. The quality of the UAS equipment is important for bridge inspections, as well as other applications, since the value of the results is tied to the resolution of the images and the ability to view the bridge elements from proper angles.
Cold weather is not a friend of UAS. When temperatures drop near or below the freezing mark—as the team experienced in December in Wisconsin—the batteries became too cold to operate the UAS. With the cold temperatures affecting battery life, attention was given to issues such as frequent battery changes, trying to keep the batteries warm between flights, and having charging equipment on-site.
The outside temperature directly correlated to the reduction in flight times. For example, the specifications of the UAS used for the pilot projects state that a pilot can expect flight times ranging from 12 to 22 minutes. However, with temperatures in the mid-30° range, the average flight times ranged between seven and eight minutes. Those times dropped even more precipitously—to around two minutes—when temperatures dipped down to 25°.
While some of the newer UAS on the market have battery-warming capabilities, many do not include this feature. In fact, certain models of UAS have a safety feature that disables takeoff unless the battery is above 50°. In these scenarios, the team must be cognizant to have a warming plan in place, such as a warm vehicle for charging and storage, to mitigate issues. In certain inspection situations, like a remote bridge with limited access below, thinking outside the box is key. In Wisconsin, the team relocated all UAS, batteries and attachments to a suitable area on the river bank and launched from there, using a combination of hand warmers and body heat to keep the batteries warm.
One of the key components of the FAA Part 107 Regulations is that pilots must keep the UAS in unaided sight or have a visual observer (VO) present in the event of an unexpected occurrence. Since bridges frequently cross waterways and treacherous expanses, teams must explore available options to maintain visual line of sight. Certain issues can be remedied by anchoring a pontoon boat or similar watercraft in the river as well as move positions between flights in order to stay visually connected to the UAS.
Visibility also can be an issue as it relates to observing specific bridge components. Even on bridges with low traffic volume, the opportunities to fly the inside of the truss can be limited due to the timing of the approaching traffic. Flying in the truss space and near the beam guard also can result in a UAS detecting interference.
Collecting images of features of the underside of the bridge, especially when attempting to access photos of the bearings, also is challenging. Based on how wide the girders were constructed, accessing certain areas can pose a dangerous challenge as air currents can mess with flight stabilization. Since the manufacturer guidance recommends against flying a UAS closer than 15 ft to the structure, trying to capture and model precise details on pieces such as bearings and flanges can be difficult. A smaller UAS or one with advanced obstacle avoidance technology would be helpful for such a task.
Take a day to think about it
It’s nothing new to seasoned inspectors, but the smoothest inspections occur when a site visit is done ahead of time to determine access to the site and plan for areas to stage the UAS and all accessories. By planning ahead, inspection-day operations can begin in a timely manner and instructions for access can be given to any additional personnel arriving at the site. A safety plan also should be developed and shared with the crew.
The plan should not simply account for the day-of, but rather for the entire project—specifically as it relates to data processing. By processing data as soon as possible following the inspection, inspectors can assess if anything is missing and make sure the next collection is efficiently gathering that data, along with anything additional.
Still need a human element
While many DOTs and consultants opt to send their bridge inspection team out with a shiny, new UAS for a full bridge inspection, the technology works best when paired with the hands-on aspect. The idea that traditional physical bridge inspections will go by the wayside is not the reality.
It will be important to continue to test emerging UAS technology as advances and new capabilities are developed that enhance the bridge inspection operation. UAS technology is rapidly changing, but still has some ways to go as simple functions such as clearing a bird’s nest, removing girder rust, or sounding concrete delamination are still not possible.
There is no doubt that when used correctly, UAS can make the inspection process more efficient and safer, produce higher quality images and minimize impacts to the traveling public. However, inspectors should be aware of all limitations. Having the right expectations of the UAS during a bridge inspection, and budgeting time and cost for a complete inspection service package that includes on-the-ground support and snoopers to inspect the parts a UAS cannot collect is imperative for a successful collection.