The world’s leading bridge publication, London’s Bridge Design and Engineering, features RDI Technologies and its BridgeView line of products in its current issue. Their story is below.
Look and Learn
A new technology that enables fast, non-contact assessment of the structural stability of bridges could help prioritize them for further inspection, writes José María Sánchez de Muniáin
The ability to monitor the health of bridges, roads and structures by the analysis of footage from an off-the-shelf, high-definition digital camera offers many benefits for bridge owners who need to carry out rapid structural assessments without restricting traffic. The technology is being pioneered by RDI Technologies in the USA and originated in research funded by the Department of Homeland Security at the University of Louisville in Kentucky. The research project, led by the company’s founder Jeff Hay, aimed to find a fast, noncontact method for the assessment of critical infrastructure that could assist first responders during emergencies.
RDI’s Bridgeview patented software analyses the data collected across more than two million camera pixels, tracking minute deflections of a bridge as it reacts to different loads. “The data acquisition component has been simplified as much as possible so it just entails setting up the camera, tripod and laptop and adjusting the brightness. The general rule is ‘if you can see it you can measure it’, so it doesn’t have to be sunny, but fog would be an issue.”
Distance can be problematic when dealing with large span bridges, adds Hay, but this can be overcome through the use of telescopic lenses or by breaking up the footage into sections. “The interface adjusts to the environment, asking for distance to the bridge or GPS location. The distance to the bridge — or any object of interest — is needed to return an absolute measurement of displacement with our technology. We generally use laser rangefinders which return a distance with high accuracy. GPS coupled with mapping is an alternative as long as the accuracy is sufficient.”
The system can measure the frequency of vibrations of a cable and, if the mass density of the cable is known, determine the tension. The tension measurement can then be used to determine if it is within tolerance as well as compared to plan specs or previous readings to ensure it has not deviated significantly. “In addition, our technology can measure the amplitude of vibration to ensure excessive vibration is not occurring. Finally we can measure the damping parameter to ensure the cable is being properly damped,” explains Hay.
The company has carried out field tests on the East Coast of the USA alongside a number of bridge inspection companies as well as the Kentucky Department of Transport, trialling the software, comparing the results with data collected by bridge sensors, and then identifying the data that is actionable and important for decision-making. “Through our analysis of tensioned eyebars on a truss bridge we were able to compare vibration behavior of like components and see a large deviation in behavior from a single set of eyebars. That set of eyebars contained the single lowest rated component in terms of load capacity. This was independently verified through a measurement of section loss on all eyebars by the bridge inspection company and demonstrates why we are working with bridge inspectors. They are carrying out their normal procedures and side-by-side we are collecting our data. Then we come together and compare them,” says Hay.
One such bridge is the Sherman Minton Bridge, a double-deck through arch bridge spanning the Ohio River between the west side of Louisville, Kentucky and downtown New Albany, Indiana. “This was done as part of testing the technology with a bridge inspection company for validation in order to show the ability to get the frequencies with our technology,” explains Hay. “We measured the frequencies of vibration of a quarter of the cables on the bridge and from that data we calculated the tensions. The bridge recently underwent a major retrofit which added significant mass, likely altering the tensions of the cables. A measurement of the cable frequencies at this point in time could be used as a baseline for future measurements.”
The ability to present data in a visual way is a major advantage of the technology, explains RDI president Bob Wilson: “With the analysis of a pair of eyebars on a truss bridge, we can clearly discern the unique frequency of the bar in the image, due to it standing out. The frequency images allow users to note patterns in the data that typical measurements such as strain gauges or accelerometers are not capable of producing. We also produce motion amplification video which clearly shows the motions of the bridge and helps people to better understand how the structure as a whole is behaving.”
Because the data set comprises information related to measurements on the entire bridge, it is possible to focus on individual components within the field of view during the analysis: “Once we collect a data set we can select a section of the bridge and measure frequencies of displacement. We are not limited to a single location, and can measure and isolate different components. This information is embedded in the data set,” says Hay.
In addition to identifying bridge components that require remedial work, the technology could also play a significant role in helping departments of transport prioritize inspections without having to disrupt road or rail traffic. “When they know they have a large number of bridges to be inspected but no way of quickly determining which should be inspected first, we can go through them and quickly create a hierarchy, saying ‘these are the ones behaving strangely, look at these first’,” says Wilson.
Field trials have revealed a potential application for Bridgeview as a bridge commissioning tool. “Inspectors are telling us it could be used to get the initial baseline of how a bridge is operating against a specification, when first opening a bridge or before and after a retrofit. And from that initial baseline you have something against which you can track how it should be behaving,” says Wilson.
Looking into the future, RDI is researching a solution that would enable data capture to be carried out using cameras mounted on moving drones. The challenge is how to ensure the vibration levels on the drone and hence the camera are low enough to enable measurements to be sufficiently accurate. “We are working on an algorithm to try and factor out some outside influences and certainly, if we can get a baseline of what a drone looks like in terms of data, we might be able to develop this.”
Also on the company’s radar is the possibility of using footage from existing static video cameras that may already be pointed at bridges. As the fidelity of these IP cameras improves, the ability to track the status of bridges on a long-term basis may become another useful tool in the box for bridge owners.