Rice engineer shares insight on bridge inspections

Rice engineer shares insight on bridge inspections

BY B.J. ALMOND
Rice News Staff   
 
A Rice University civil engineer who studies the structural integrity of bridges and monitors their health and safety said careful examination and testing of failed structural components coupled with computer modeling might help determine the cause of the Aug. 1 bridge collapse in Minneapolis. But recreating the scenario will be a painstaking process that could take months.

“It will be very difficult to do this after the fact,” said Satish Nagarajaiah, a professor of civil and environmental engineering who chairs the American Society of Civil Engineers’ Technical Committee on Structural Health Monitoring and Control.

RICK MOORE/UNIVERSITY OF MINNESOTA

The eight-lane I-35W bridge that spans the Mississippi River near downtown Minneapolis buckled during the Wednesday evening rush hour.

“By eliminating certain types of causes, such as the piers not failing, engineers might be able to estimate the possible cause of the collapse,” said Nagarajaiah. “Photos show that the two piers of this steel three-span truss arch bridge are still intact, so I suspect that fatigue fracture in one of the bridge’s trusses is likely to have been a contributing factor.”

How bridges are inspected

Because structural damages on bridges are not always visible to the eye, such as fatigue cracks and fractures in hidden members and joints, engineers assess structural integrity by monitoring the health 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.

Data are usually collected at different times during the week to allow for variances in traffic flow and weather conditions. Engineers then incorporate these measurements into a computer model developed on the bridge’s original design.

When calibrated using the measured strains, the global computer analysis can reveal problem areas that warrant closer inspection. These areas are then examined locally with ultrasonic sensors that can measure cracks in a particular part of the bridge. Additional measurements are recorded as needed, and then all the data are reviewed.

Once the inspection of a bridge has been completed, engineers rate the structure’s overall condition on a scale from zero to 9 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 out of service and beyond corrective action. A score of 1 indicates imminent failure, and 2 indicates critical condition. Nagarajaiah said scores below 3 warrant shutting down the bridge immediately.

The eight-lane Interstate 35W bridge that crosses the Mississippi River near downtown Minneapolis received an average score of 5 when it was inspected in 2005. The deck was rated at 5, the superstructure steel truss was rated at 4 and substructures such as piers and foundations were rated at 6.

The overall score of 5 represents fair condition, which 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.

He also noted that some fatigue deficiencies were observed on the main truss and floor truss system in a 2001 detailed study of the bridge by University of Minnesota researchers.

Analyzing a collapse

As part of the investigation into the bridge collapse, engineers will need to inspect the damaged structural components for clues that will help them simulate what was happening to the structure while it was still intact. Video of the actual collapse should help investigators with their analysis.

“It’s very rare for the entire bridge to collapse,” Nagarajaiah said. “Usually only one or two sections collapse.”

He attributes the collapse of the whole bridge to its design, which lacked structural redundancy.

“This design was typical of bridges built back in the ’50s and ’60s,” he said.

The 40-year-old I-35W bridge was built as continuous truss across two supports, forming the shape of a T. The two end spans sandwiched the two supports and the center span. The overhang at each end of the T connected 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 support. 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 is also a professor of mechanical engineering at Rice, was recently appointed to chair the nonprofit U.S. Panel on Structural and Health Monitoring and Control.
He has been intrigued by the behavioral structure of bridges ever since he was in high school in Bangalore, India.

“Every Sunday the BBC showed different types of bridges and how scientists study their vibrations and determine whether they’re safe,” he said. ”One program analyzed in great detail the 1940 collapse of the Tacoma Narrows Bridge in Washington. That really fired my imagination up and made me want to know more about how structures behave dynamically.”

His interest in structural assessment expanded from bridges to buildings and aerospace systems, including the International Space Station in the current NASA-TiiMS rseearch project.

”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 hopes the tragedy in Minnesota will serve as a wake-up call for monitoring bridges more carefully. Currently, federal regulations require that most bridges be inspected every two years.

”The U.S. has about 590,000 bridges, and 162,800 of them have been identified as being deficient,” he said. Structural deficiencies were found in 81,300 bridges, and 81,500 are functionally obsolete.

”If we expect bridges to last 100 years, the federal government needs to spend the money to maintain them,” Nagarajaiah said. ”The government should advocate regular monitoring of bridges more often and more carefully using new structural health monitoring techniques in addition to visual inspection, which is the current practice. It’s not something you can ignore.”