WEST MEMPHIS --A bad weld is suspected as the cause of the significant fracture found in a 900-foot steel beam on the Interstate 40 bridge over the Mississippi River at Memphis, according to a top Arkansas Department of Transportation official.
The suspicion prompted the agency to accept the recommendation of a University of Arkansas civil engineering professor to perform ultrasonic testing on other welds on all four 900-foot steel beams tied to the twin arches holding up the suspension portion of the Hernando De Soto Bridge when testing resumes next week.
"It's clear to me from the photos that the fracture initiated at a weld," said Gary Prinz, who as an associate professor specializes in fatigue and fractures of metals, particularly steel. "It indicates to me a defect or a stress riser due to the connection welds where the fatigue crack initiated."
Prinz likened a defect in a weld to a piece of uncooked flour in a cookie.
"When you're welding things together and they cool at different rates, there is potential for a defect to happen," he said. "It's kind of like a crumb in the cookie. A piece of uncooked flour baked into the cookie that makes the cookie break a little easier."
Rex Vines, deputy director and chief engineer for the department, brought up the "bad weld" theory in an email outlining a more robust inspection of the bridge that will be undertaken by an outside company, HNTB.
"As there are suspicions that the failure began at a weld, the consultants will be performing ultrasonic testing of the welds along the tie-girders as an extra precaution," he said. "Welds are normally inspected visually, however the ultrasonic testing will allow us to recognize potential issues before they are visible to the naked eye."
Ultrasonic testing typically is used to confirm welds at the time the steel is fabricated, according to Prinz.
"They'll go and look for additional defects along these box girder welds to make sure things are within acceptable levels and there's no additional cracks that are initiating," he said.
The department has staff bridge inspection teams that typically inspect the superstructure and substructure portions of a bridge. Such a team missed the I-40 bridge fracture in at least two inspections in September 2019 and September 2020, which has called the agency's inspection system into question.
"The primary difference in this inspection is a different set of eyes will be used ... since there was some question as to the thoroughness of ARDOT's most recent in-house inspections," Vines said.
An inspection team from the civil engineering firm Michael Baker International discovered the fracture by chance May 11. The department hired the firm to inspect the suspension cable portion of the bridge. The Michael Baker team also will resume its inspection, according to Vines.
Not detecting the fracture earlier made it impossible to make repairs without closing the bridge to traffic, but it could have been avoided had the fracture been identified sooner, Prinz said.
"They didn't catch the crack until it grew to the point it was unstable," he said. "There's very clear ways to retrofit small cracks that form at weld connectors. It happens. But when they get to this magnitude, then it's a little more complicated. It's unsafe."
The inspection also takes place after the Arkansas Transportation Department boss, Lorie Tudor, has pledged to overhaul the inspection process to ensure dangerous anomalies do not get overlooked in future bridge inspections.
She said the department adheres to inspection standards set by the Federal Highway Administration. They were adopted 50 years ago last month after the collapse of Silver Bridge in Point Pleasant, W.Va., on Dec. 15, 1967. A total of 46 people died in that disaster.
In cases like the I-40 bridge, the department exceeds the standards by requiring all bridges labeled "fracture critical" to be inspected annually.
The Federal Highway Administration requires all bridges to be inspected every two years. But the department inspects "fracture critical" bridges annually because they have steel elements subject to tension whose failure would probably cause a portion of or the entire bridge to collapse," according to the American Association of State Highway and Transportation.
What redundancies the department will eventually employ remains undetermined, but the department's senior staff members think the robust inspections of the I-40 bridge and the Interstate 55 bridge meet Tudor's definition of redundancy.
Traffic from the closed I-40 bridge at Memphis is being diverted to the nearby I-55 span.
"For immediate redundancy, the I-40 and I-55 bridges are being reinspected," Vines said. "ARDOT is hiring a consultant to reinspect the I-40 bridge. TDOT [in Tennessee] is reinspecting the I-55 bridge. Future inspections will also include redundancies, but we are still developing the exact means and methods."
Work to reopen the I-40 bridge remained on pause for a second day while the contractor retained to repair the crossing, Kieweit Infrastructure Group of Omaha, continues preparations for the second phase of work, Tennessee transportation officials said in an update issued Wednesday afternoon.
"They are extending the work platform and finalizing design plans," according to the update. "The contractor's input provides important construct-ability details. In addition, the contractor is working with steel fabricators to determine steel plating availability to provide an expedited fabrication of the phase two components."
The first phase, which began Friday and was completed Tuesday, attached two steel plates weighing several tons to the weakened area of the beam. The work is designed to stabilize the bridge to safely allow repair personnel and equipment, as well as the inspection teams, on it.
At the same time, Tennessee Transportation Department personnel continue to tweak the primary alternative route across the river, the I-55 bridge, to improve traffic flow ahead of Memorial Day weekend, the traditional start to the summer travel season.
The time it takes to get across the Mississippi River has lengthened to the point that it is costing the trucking industry $2.4 million more a day, according to the Arkansas Trucking Association. A third of the traffic on both bridges consists of trucks, according to Arkansas Transportation Department data.
The I-55 bridge now is carrying 67,000 vehicles per day, a count that "is generated from a radar station located just off the east side of the bridge," according to the update. The I-40 and I-55 bridges each averaged more than 40,000 vehicles daily before the I-40 bridge closed, according to the Arkansas Transportation Department.
"In anticipation of increased holiday traffic, we are also installing an additional camera dedicated to monitoring the interchange, and additional message boards are being set up to assist northbound traffic," Wednesday's update said.