A True Expert

With 34 years of professional experience in bridge engineering, with the last 15 years at world-renowned bridge-engineering specialists TYLin, Atiq Alvi is unquestionably an expert in his field. During our interview he discussed a wealth of topics, especially new technologies, and passed along some sage advice to young and aspiring engineers. It should all be listened to, but in this limited space I’m going to focus on his detailed description of how his firm has been monitoring one single bridge in his locality: the iconic Sunshine Skyway Bridge in Tampa, Fla.

Structural Health Monitoring

The first stage of the process is structural health monitoring to study the bridge’s behavior.

“We’ve got over a hundred sensors on the main span alone, starting with accelerometers on the cables,” notes Alvi. “We post-process that data to determine the tension and displacement in the cables. We also have accelerometers inside the segmental box perimeters to determine displacement from heavy loads or look at tension variations. In the external post-tension system of the bridge, we have visual and acoustic monitoring for the large modular joints that are on each side of the main span of the bridge.”

There also are water-presence monitors inside the segmental boxes as well as gauges on the cables and dampers to monitor their movement. All these sensors provide a monthly data report.

Nondestructive Testing

In the next step, TYLin uses drones to inspect areas that are difficult to reach or to acquire an overall view. Another more-specialized tool is a cable crawler, a robotic device used to inspect cables. The inspection team then reviews the video footage of all 84 cables to assess the overall condition. If something is identified as a crack or other potential problem, then an engineer will go and “put hands on it” to get a better look.

They also use underwater drones to inspect the bridge’s substructure, taking the place of human divers. In addition, ground-penetrating radar helps study the conditions of the post-tensioning tendons. And they use high-speed cameras to map cracks on the deck surface and then evaluate how much cracking is taking place in the concrete.

“It gave us not only the condition, but a quantity from which we were able to design a restoration project,” explains Alvi.
 

Finite-Element Model

After all the data collection, a finite-element model was created, specifically of the bridge’s main span, that accurately modeled all the components, including the loading material and the materials used on that bridge. It also modeled the construction loads, sequencing and any construction records to create a very accurate model of the grid.

Digital Twin

A digital twin is the final step, which takes all these technologies and combines data from as-built plans, rehab projects, inspection reports, nondestructive testing, structural health monitoring and the finite-element model, providing a true replica of the Sunshine Skyway Bridge with all its deficiencies and enhancements through the years.

“This is something we can use and test scenarios and determine the service life of the bridge, the remaining service life of the cables,” says Alvi. “We can look at what-if cases like ‘what if a cable was damaged’ or ‘what if a cable or a component had to be removed,’ what would we have to do? What type of temporary shoring would we have to use?”

The digital twin also can help the client, the Florida Department of Transportation, prepare program funding for repair projects based upon projections from the digital twin.

Alvi notes that this process won’t be precisely the same for every bridge inspection. “It’s not a ‘one size fits all’ process. It all depends on the bridge and setting.” And Alvi is an expert who certainly should be listened to.