Rice Engineer Shares Insight on Bridge Inspections
By B. J. Almond
A bridge collapse, such as the one in Minneapolis on Aug. 1, is the last thing Satish Nagarajaiah wants to see, even though film footage of a famous bridge collapse is what sparked his interest in the behavioral structure of bridges.
When Nagarajaiah was in high school in Bangalore, India, the BBC ran a series on different types of bridges that showed how scientists study their vibrations to determine if they are safe. “One program analyzed in great detail the 1940 collapse of the Tacoma Narrows Bridge in Washington,” he recalled. “That really fired my imagination and made me want to know more about how structures behave dynamically.”
Today, the Rice professor of mechanical engineering studies the structural integrity of bridges and monitors their safety. Careful examination and testing of failed structural components coupled with computer modeling might help determine the cause of the collapse in Minneapolis, Nagarajaiah said, but recreating the scenario will be a painstaking process that could take months.
“By eliminating certain types of causes, such as the piers not failing, engineers might be able to estimate the possible cause of the collapse,” Nagarajaiah said. “Photos show that the two piers of the steel three-span truss arch bridge are still intact, so I suspect that fatigue fracture in one of the trusses is likely to have been a contributing factor.”
How Bridges Are Inspected
Structural damage on bridges, such as fatigue cracks and fractures in hidden members and joints, are not always visible to the eye. Engineers assess structural integrity by monitoring the soundness of the entire bridge and then zeroing in on specific sections. Sensors placed on sample areas of the bridge record strains caused by vibrations and movement of the bridge and any excessive strains or force in structural members. These measurements are incorporated into a computer model developed on the bridge’s original design. If analysis reveals problem areas that need closer inspection, the areas in question can be examined with ultrasonic sensors.
Once the inspection of a bridge has been completed, engineers rate the structure’s overall condition on a scale established by the Federal Highway Administration. A score of 9 indicates excellent condition. A rating of zero is assigned to a failed bridge, which means it is beyond corrective action. A score of 1 indicates imminent failure, and 2 indicates critical condition. Nagarajaiah said scores below 3 require shutting down a bridge immediately.
The eight-lane Interstate 35W bridge that crossed the Mississippi River near downtown Minneapolis received an average score of 5 when it was inspected in 2005. An overall score of 5 represents fair condition and indicates that all primary structural elements are sound but may have minor section loss, cracking, spalling or scour (erosion of soil around the base of the pier that may cause the pier to tilt).
“Obviously there were some deficiencies, but none serious enough to warrant closing the bridge,” Nagarajaiah said. “It’s very rare for an entire bridge to collapse. Usually only one or two sections collapse.”
He attributed the collapse of the whole bridge to its design, which, he said, is typical of bridges built in the 1950s and 1960s. The 40-year-old I-35W bridge was built with a continuous truss across two supports, the overhang at each end connecting to the ramps from the road. These overhangs created negative bending forces to balance the positive bending forces in the center span, but the design did not include redundant spans, components or supports. So when one overhang failed, there was nothing left to hold up the center span.
“The failure probably started on the south-end span and then progressed to the center span and north-end span,” Nagarajaiah said. “The piers look fine, so I suspect one of the trusses failed, causing the domino effect.”
Fascinated by Bridges
Nagarajaiah, who recently was appointed to chair the nonprofit U.S. Panel on Structural and Health Monitoring and Control, is expanding his interest in structural assessment of bridges and buildings to aerospace systems, including the International Space Station.
“The backbone of the space station is a large truss, similar to a bridge,” he said. “NASA wants us to monitor it and come up with a real-time assessment of the structure’s condition.”
Nagarajaiah said the tragedy in Minnesota will serve as a wake-up call for more careful monitoring of bridges. Federal regulations require that most bridges be inspected every two years, but Nagarajaiah advocates more frequent and careful inspections using new structural monitoring techniques in addition to visual inspection.