SAN FRANCISCO (CN) – Independent engineers say California has not studied the effect of potentially weaker steel in the San Francisco-Oakland Bay Bridge thoroughly enough to conclude that the bridge will not collapse in a major earthquake.
“The facts on the table are that material in some parts of the bridge is weaker than what it should be,” said Abolhassan Astaneh-Asl, a University of California, Berkeley civil engineering professor who has studied problems with the bridge for years. “The next step should be for Caltrans and the design team to study what effect this weakness will have on the seismic performance of the bridge.”
The bridge’s $6.4 billion eastern span, completed in 2013, was designed to withstand an 8.5-magnitude earthquake, but several defects – including cracks in the foundation, brittle support rods, bolt holes that leak water through the deck, and now, potentially weaker steel – have raised alarms about safety.
Some spots in the welded steel plates that hold together the base of the bridge’s only support tower have yield strengths slightly lower than what is required for 50 grade steel, according to a recent field review.
The bridge’s chief engineer, Brian Maroney, says that is not a big concern because other factors, such as tensile strength and elongation, which measure how much the steel can bend or stretch before breaking, are more important than yield strength.
“The tensile and elongation together give me toughness, and that’s what I want in an earthquake,” Maroney said.
Independent experts say the state cannot reliably say the bridge is safe because it has not tested how a bridge with weaker yield strengths in certain spots would hold up under seismic stress.
“We’ve got to develop a validated understanding of the performance of the entire bridge system when subjected to extreme loading conditions,” said UC Berkeley civil engineering professor emeritus Robert Bea.
Welded plate tests
Concerns about the strength of welded steel that holds together the base of the bridge’s 525-foot tower were first raised in June. That’s when a Federal Highway Administration engineer urged the state to investigate whether a weaker “rogue plate” was used in the tower, according to emails obtained in a public records request.
The highway administration tested a rejected welded plate that was intended for the bridge’s tower, from one of several rejected welding jobs that had to be repaired due to defects. About 20 repaired welds make up the tower’s base, according to construction fact sheets.
A 0.25-inch specimen cut from one of the rejected 4-foot-thick welded plates had a yield strength characteristic of 36 grade steel, much lower than the required 50 grade steel, the highway administration found.
Maroney initially brushed off the report, saying there are several reasons why the yield strength might be lower. The size of the tested specimen, whether it was taken from a heat-affected zone, and the speed at which it was strained could all affect the yield strength, he said.
But experts say it is reasonable to conclude that other welded plates in the bridge tower have similar properties to the one tested by the Federal Highway Administration.
“If they used the same welding method for other plates, you would expect to have the same sort of trend,” said Lisa Fulton, a metallurgist and materials scientist with the Berkeley Research Company.
Maroney assembled a team to review records on every steel plate used in the bridge’s tower. He also conducted on-site hardness tests to estimate the yield strength of welded plates that hold the tower together.
“I went through all of our construction records and designed a field review,” Maroney said. “I don’t see any evidence that I’ve got a concern out there.”
The on-site hardness tests, conducted with a handheld machine that jabs steel and measures its resilience, showed some sections had yield strengths lower than required for 50 grade steel, according to Maroney.
“This does not surprise me, and it doesn’t concern me,” he said. “When the variations are very small and very local, I expect those kinds of things.”
Maroney’s opinion is backed up by Dr. John Fisher, chairman of the bridge’s Seismic Safety Review Panel and civil engineering professor emeritus as Lehigh University in Pennsylvania.
“The material is great material,” Fisher said of the welded steel. “There’s nothing wrong with it. It will fully do what it’s supposed to do in the structure.”
Fisher and two other members of the panel concurred with Maroney’s opinion that the bridge is safe, despite finding that some sections of steel in the tower have lower than required yield strengths.
In 2012, the Sacramento Bee reported that Fisher and other members of the panel had financial ties to Caltrans and bridge contractors, raising questions about the panel’s impartiality. Fisher has emphasized that his business relationships do not influence his judgment.
“If Caltrans brings someone else outside Seismic Peer Review Panel, that would be very valuable,” Astaneh said.
Fisher said other properties of the welded steel, such as tensile strength, are more important than yield strength when it comes withstanding an earthquake. Tensile strength measures how much stress the metal can endure before breaking, rather than bending.
“Nothing will be gained” from further tests, Fisher proclaimed in a phone interview.
But Berndard Cuzzillo, a mechanical engineer with the Berkeley Research Company, says yielding means the material will permanently deform, which could severely impact the strength of the overall structure.
“When the stress goes beyond yield, it means the material does not snap back to its original shape when you release the load,” Cuzzillo said. “So as the tower is swaying, in one sway, it could cause a stress that does not snap back to its original shape.”
One reason potentially weaker steel in the bridge’s eastern span is a major concern is the overall design of the bridge, a design some engineers have called “low-robustness” or “fracture critical.”
“This particular bridge is of more than normal concern because it’s what an engineer would call a low-robustness system,” said professor emeritus Bea. “Robustness means defect and damage tolerance.”
Unlike the bridge’s western span or the Golden Gate Bridge, which rely on two main cables anchored in the ground, the Eastern Span is supported by a single, 137-steel-strand cable that loops over the tower and under the bridge. The cable is supported by chambers designed to keep out corrosive water and humidity, but rust was found on the cable and support rods inside one chamber in 2014.
“If there are defects, you can expect it not to perform well,” Bea said. “If you’ve only got one belt holding up your pants, that one crack can allow your belt to fail and your pants drop to your knees.”
Explaining the concept to the Sacramento Bee in 2014, Astaneh said: “This bridge is fracture critical, which means if any important element of this bridge fails … the bridge is going to collapse.”
Based on the information available, both Astaneh and Bea say the state needs to conduct a more thorough analysis before it can make any valid conclusions about the bridge’s safety.
“You can’t just give an opinion on safety of the bridge,” Astaneh said. “We have software and simulation techniques that we subject a bridge model to earthquake motions, see stresses and see what happens. That is what needs to be done at this point for public safety.”
Astaneh suggested the state cut chunks of steel from different parts of the bridge, including near the tower welds, send them to a lab, and establish the actual properties of that steel.
“Then I put that into my analysis model and analyze the bridge, subject it to simulated earthquake motions, and see what happens,” Astaneh said.
Bea said the state needs to review the entire system, beyond isolated problems like brittle anchor rods and weaker than required steel.
“The ability to analyze such systems exists,” Bea said. “It can be validated quantitatively.”
But state transportation officials say such an analysis is unnecessary because the bridge’s design team and peer review panel have already concluded the bridge is safe.
“FHWA (Federal Highway Administration) lab work was incomplete, and our team, with [chief engineer Brian Maroney]’s lead and consultants did material testing reports, field testing for steel hardness,” said Dan McElhinney, deputy director for Caltrans District 4. “All that shows that the new Bay Bridge is safe. The welds and steel are very high quality. The earthquake readiness is still very high as expected.”
Meanwhile, a recent forecast by the U.S. Geological Survey predicted a 70 percent chance that a magnitude 6.8 earthquake will hit the Bay Area within the next 30 years.
A report on the bridge tower’s steel plate strength is expected to be presented to the Toll Bridge Program Oversight Committee, which oversees the bridge, at its next meeting in Sacramento on Dec. 12.