By Robert L. Reid
Engineers recently created a digital twin of Florida’s Sunshine Skyway Bridge based on data from multiple sources, including more than 100 sensors placed on key elements of the structure as part of a comprehensive structural health monitoring system. The goal is to increase the crossing’s service life to 100 years.
The Bob Graham Sunshine Skyway Bridge is a cable-stayed, concrete box girder structure more than 4 mi long. It carries Interstate 275 across Tampa Bay, connecting Pinellas and Manatee counties on Florida’s Gulf Coast. Completed in 1987, the bridge replaced an earlier steel crossing that was damaged in 1980 from a ship collision that resulted in the deaths of 35 people.
Featuring steep approach spans and two 431 ft tall towers along its centerline, the Sunshine Skyway Bridge is a vital link in Florida’s transportation network, one that the Florida Department of Transportation hopes can achieve an ambitious operational lifespan of 100 years, noted Atiq H. Alvi, P.E., a vice president at engineering consultant T.Y. Lin International and the firm’s Florida bridge sector manager. It is a daunting task, given the corrosive marine environment to which the bridge’s key components are constantly exposed, Alvi noted.
TYLin, a member of the Sidara Collaborative, has been working with FDOT since 2014 to use technology to extend the Sunshine Skyway’s service life through a series of measures that include placing more than 100 sensors on key components of the bridge as part of a comprehensive structural health monitoring system (see “Inspection Tech” below). The firm’s engineers also explore difficult-to-access components of the bridge through the use of camera-laden flying drones, a submarine-style underwater drone, and a robotic device known as a cable crawler, which — as the name implies — moves along the bridge’s cables to help identify potential deficiencies.
In March, the engineering firm created a digital twin of the overall bridge that represents “a living engineering model,” said Alvi.
Developing the digital twin
The digital twin was developed using 3D finite element method software that combined information from numerous sources, including the bridge’s original design and as-built plans. The engineers also examined records and information regarding inspections, the post-tensioning system, bridge geometry, material properties, construction sequencing, stay-cable forces, and data collected through the sensors of the structural health monitoring system.
Once developed, the model was calibrated and refined “using measured bridge responses, including strain, displacement, acceleration, temperature, and other monitoring data to ensure it accurately represents the bridge’s current behavior,” Alvi explained.
The digital twin will also help assess how the bridge will respond to critical events, by using complex algorithms and data to simulate various natural disaster scenarios, such as hurricanes, floods, and tsunamis, as well as human-caused disasters such as vehicular impact and fire. “For example, for hurricanes, the model can assess the effects of high winds, storm surges, and wave impact on the bridge’s stability and integrity, helping to identify potential weaknesses in the structural design or materials used,” Alvi said.
The simulations will allow engineers to identify vulnerabilities such as structural weaknesses in the bridge’s design, areas susceptible to erosion or scour from water flow, and the possible certain forces.
“By identifying vulnerabilities early, we can recommend necessary adjustments to the original design or reinforce specific areas,” Alvi noted. This will provide FDOT with the time to secure funding for the repair or replacement of affected components, instead of attempting dangerous and expensive emergency repairs, he added. “The overall goal is ultimately enhancing the bridge’s resilience and safety.”
Improving inspections
Federal law requires highway bridges to be inspected every two years. For the Sunshine Skyway Bridge, these inspections typically involve an engineering team of 8 to 10 people who set up maintenance-of-traffic measures, close lanes, and use access equipment — such as bucket trucks or rope systems for climbing — to physically reach the various components of the bridge, “looking at and touching and testing the concrete or steel,” Alvi said.
But advanced bridge inspection technology could someday make it possible for a single engineer to do that work, using a drone, sensors, and other tools.
In Florida, at least, the only inspection technology fully authorized by the state is the cable crawler, Alvi added, which means that the in-person biannual inspections are still required.
Thus, TYLin uses advanced inspection technology primarily to track the bridge’s performance and condition and identify areas of concern for an engineer to physically inspect.
These measures improve safety for the engineers by enabling the inspectors to monitor the bridge remotely, reducing the number of times they must physically access the structure to investigate a problem — a problem that the technology helped discover, Alvi noted.
Moreover, these tools can identify deficiencies and potential problems that on-site inspectors might have missed. The cable crawler, in particular, features a number of cameras that often provide a more comprehensive view of the bridge’s cables than engineers using bucket trucks or climbing gear can provide.
The various sensors on the bridge can also provide a continuous and consistent stream of data. For example, TYLin has attached permanent accelerometers to the cables that continuously measure frequency and vibration.
This enables the firm to send FDOT a monthly update on whether the cables are losing tension — “a red flag” that can now be discovered much sooner than was previously possible, Alvi said.
The number and variety of sensors on the bridge can also help verify what is a real problem versus a false alarm, or identify the true nature of what might appear to be a structural issue but is actually caused by something else. For example, TYLin is monitoring an expansion joint on the Sunshine Skyway Bridge via acoustical sensors, strain gauges, and even temperature gauges. These devices indicate how much the expansion joint opens and closes, as well as the temperatures within the concrete and other factors.
Individually, the sensors might provide uncertain data. But together, Alvi said, these devices can confirm that a loud noise picked up by the acoustical sensor is not a structural problem, but just a heavy vehicle moving across the joint, and that the joint itself, according to the strain gauge, is working properly.
In one particular incident, accelerometers on the bridge detected a one-time “huge spike in acceleration” at the midspan of the bridge, Alvi said, but the cause was initially difficult to determine because “everything else was functioning properly.” When engineers examined footage from a series of cameras on the crossing, however, and “correlated the cameras to when the spike occurred, we discovered it was a large, unpermitted truck carrying a load” above the legal limit, Alvi said. So TYLin was able to reassure FDOT that there was no problem with the bridge itself.
Human supervision
Advanced technology might eventually reduce the number of engineers sent to inspect a bridge, but it is unlikely to eliminate the need for human supervision and intervention. For one thing, the drones TYLin uses are able to find and report back data about possible deficiencies, but they are not yet able to conduct nondestructive tests on bridge components. “So, if the cable crawler finds a defect or another system finds something wrong on a tower, then we have to get the access equipment and go out and look at it ourselves,” Alvi explained.
Moreover, the drones are not autonomous, which means a highly skilled person — who has earned a Remote Pilot Certificate from the Federal Aviation Administration — is needed to operate the device. It takes considerable expertise to fly a drone inside the narrow confines of certain bridge components, Alvi noted. And although drones can operate inside those components when the Florida heat would make it difficult for a person to work, the drones cannot fly when winds are too strong.
Drone pilots can be either senior engineers with years of personal experience inspecting bridges or junior engineers whom the more experienced inspectors have sent out “with specific instructions on exactly what they want examined, where to photograph, and what to bring back for the senior engineer to assess,” Alvi noted.
Proactive preservation
Alvi credits the FDOT Districts 1 & 7 Structures Maintenance Office for a proactive and forward-thinking commitment to preserving the Sunshine Skyway Bridge. This commitment stands in contrast to similar precast segmental bridges in the nation that are in substantially poorer condition and now require replacement, largely because a comprehensive preservation program was not established and implemented early in their service lives, Alvi explained.
Indeed, as ASCE’s 2025 Report Card for America’s Infrastructure noted, more than 49% of the nation’s 623,000 bridges are in “fair” condition, and nearly 7% are in “poor” condition, while only 44% are in “good” condition. “As bridges in fair condition continue to age — presenting the possibility of being further downgraded — they also exemplify an opportunity because they can be preserved at a lower cost than bridges in poor condition,” ASCE stated.
“To enhance the longevity of vital bridge infrastructure,” Alvi concluded, “it’s essential to integrate advanced technologies that streamline monitoring and maintenance. This will limit potential bridge downtime due to lengthy repairs while increasing community safety,” hopefully giving the bridge many more years of service.
SIDEBAR
Inspection tech
T.Y. Lin International’s structural health monitoring system for the Bob Graham Sunshine Skyway Bridge in Tampa Bay, Florida, features a wide range of advanced technologies, tools, and techniques. Many of these are already in place, and others are planned for future inspection work.
These devices include:
- Accelerometers at the top of the bridge’s two towers, on the stay cables, on the bridge deck, and inside the superstructure. The deck accelerometers, in particular, feature triaxial gyroscopes that measure angular velocities along three axes to monitor torsional vibration.
- Expansion joint sensors that provide real-time monitoring of joint movement and include acoustic monitoring for continuous noise-level data. Cameras also provide video for visual confirmation of alarms and time-lapse replay of joint movement. Sensors monitor internal temperatures of the web and the top and bottom slabs of the bridge’s box girders.
- Sensors at the ends of each stay cable that monitor movement of the stay cable dampers. A pilot program will also track stress and strain at the stay cable anchor block spacers to help track changes in cable tension.
- A deflection monitoring system for the bridge’s main span that is being developed to track changes in the span’s deflection caused by live loads and temperature fluctuations.
- Wireless Internet of Things sensors that report on the presence of water near the bridge’s post-tensioned tendons within the box girders, which are subject to corrosion damage.
- Other IoT sensors that track environmental conditions, air quality, and vibrations, transmitting data to alert operators through a 5G cellular network.
- A remotely operated submarine device, owned by the Florida Department of Transportation District 7’s Drainage Design Department, that examines submerged bridge components.
- A high-speed imaging system that maps and documents cracking on the bridge deck, which serves as the top flange of the precast segmental box girders. The collected data are compared with previous inspections to identify new cracks, monitor the propagation of existing cracks, and determine when crack sealing or other maintenance measures are warranted.
- An artificial intelligence mapping system that assesses traffic impacts and monitors the number and speed of vehicles passing over the bridge, watching for accidents that could compromise the bridge’s efficiency and structural health.
- Data from the various sensors that are incorporated into the digital twin and used to train an AI machine learning model. The AI system then accurately predicts the bridge’s parameters for any given external condition, compares them in real time with other data, and immediately flags any abnormal parameters.
The description of the sensors and other bridge inspection technology includes information from the paper “Data-Driven Preventive Maintenance and Structural Monitoring of the Sunshine Skyway Bridge,” by Atiq H. Alvi, P.E., Ivan A. Gualtero, P.E., and Steve D. Womble, from TYLin, and Vincent S. Collie from the Florida Department of Transportation. The paper was presented at the 2024 International Bridge Conference, held in San Antonio.
Robert L. Reid is the senior editor and features manager of Civil Engineering.
This article first appeared in the September/October 2026 issue of Civil Engineering as “A Living Bridge Model.”
