We were contacted by the LBJ Expressway maintenance engineering team with an idea. The need was to document and inspect the bearing pads associated with a few of the bridge structures without having to deploy a difficult to schedule and expensive snooper truck to do so. They came to me inquiring as to whether the idea of using drones could accomplish this goal. I am often presented with different ideas, and most of them I don’t think could work very well, but this one got my wheels turning. I had my work cut out for me researching how we could use drone technology to complete this project quicker, safer and more cost effectively compared to alternative inspection methods.
The Research
First, I had to develop an understanding of the construction. Concrete expands and contracts with the temperature changes throughout the year. The long heavy structures must be able to safely expand and contract between supports without damaging the structure or creating an unsafe driving environment. This is where a bridge elastomeric bearing pad comes into play. These pads are constructed of a high strength industrial rubberized material. As pictured to the left, the bearing pad is what buffers the bottom deck of the bridge and the abutment. Often times, the bearing pad is only a few inches tall and flexes forward and back as the deck expands and contracts with the changing environmental and temperature conditions.
Next we had to determine when and how the inspection could be completed with the drone. The roads are part of and over 75 and 635 in Dallas. These are very busy and heavily traveled roads, meaning we would have to work at night to safely complete the inspection.
The Faa- The FAA has rules and regulations in place for the purpose of protecting the community while operating drones. One of those rules is we cannot operate directly over moving vehicles. Considering this rule, we needed lane closures to safely complete this operation as most of the structures were in the median of the highway and would be very close to being overhead to ongoing traffic.
TXDOT Approval- TXDOT has a standard for safe drone operations on or around TX road ways. If you want to operate outside of the rules they have in place, you must submit a flight and safety plan noting the rules you need to operate around. In our case, one of the rules was that you cannot fly within 6 ft of a structure and we had to be inches from it at all times during the operation. We had to submit a request to complete this project as we needed to literally almost bump up against the structure to perform the inspection
Weather- We needed a fairly calm and clear night operate. During our testing, we learned our operating limitation was roughly 12mph of wind. When wind moves around the structure, it vortexes, pushing the drone in often unpredictable ways. We needed there to be no precipitation and temperatures ideally needed to be above freezing to help with flight time efficiencies.
Road Closures- We had to pre-plan and get permits and schedule Buyers for our proposed lane closures for an 8 hour work period each night.
Drone Capabilities & Limitations- We have multiple drones in our fleet, but had to figure out what could safely fly under the structure, light up the area being inspected, and document it in a high enough resolution to review the structure for problems.
Organization- We had to ensure we knew which files from the field associated with each bent or portion of the structure.
The Drone for the Job
There are a lot of drones on the market with varying capabilities. We considered really 3 different UAV’s for this job and narrowed it down to the DJI Mavic 2 Pro with a custom built cage designed for the bird and mounted lights to get the job done. Other drones with cages built around it were either
With a good understanding of the construction, now I needed to think through how I could safely and effectively deploy drone technology to accomplish the goal of documenting the structural condition of hundreds of bearing pads per bridge. I needed a small drone that had a small vertical distance between the propellers and its camera. We only had a few inches of space below the deck of the bridge to be able to see the bearing pads in full view. Some of the other drones had a spherical cage built around it that wouldn’t give us the perspective required to inspect the bearing pads. The Mavic 2 Pro Specs included a 3.3″ vertical height, 4k video resolution, 20mp still shots, capabilities of easily mounting different types of lights to the drone in different ways and the ability to design a cage around the top and sides of the proportion system.
Execution
With a collaboration between the team at Tarillo Vue, LLC and a retired engineer who worked for TXDOT for over 30 years, we were able to successfully see this inspection through to completion. We laid out a field plan and timeline for completion that correlated with road closures in sections based on our expected efficiency. We were able to complete the project under budget delivering drone photos and video footage of each bents associated abutments and bearing pads along with a detailed report that rated the structural condition and highlighted concerns we observed from the inspection.
The biggest challenge we experienced on the project by far was the difficulty of safely operating the drone around the bridge structure with a little bit higher winds at altitude. It was vital to the operation to be attentive to the drones every move. The vortexing winds around the bridge created a stressful and unpredictable flying environment. While this has been one of the more challenging projects I have been involved with, its been a favorite. I enjoy the complexity and challenges that come with projects like this.
