SACRAMENTO (CN) — An engineer Tuesday assured California that the San Francisco-Oakland Bay Bridge won’t fail in a major earthquake, despite weak spots in welded steel that holds together the eastern span’s only support tower.
Addressing the three-member panel that oversees the bridge, chief engineer Brian Maroney said yield strength, which measures how much stress steel can take before deforming, is less important than tensile strength and elongation, which gauge how much the steel can bend or stretch before breaking.
“In an earthquake, I'd much rather have that toughness,” Maroney said. “I'd rather be the willow tree, not the oak tree.”
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.
Field tests found some spots in welded steel plates that hold the tower’s base together had slightly weaker yield strengths than required for 50 grade steel. The tests were done after Federal Highway Administration researchers found parts of an unrepaired welded plate that was cut out of the bridge in 2012 had yield strengths characteristic of 36 grade steel, much lower than the required 50 grade.
“The simple fact that three of the specimens had a slightly lower yield strength is of no consequence to the seismic performance,” Maroney said Tuesday.
Independent engineers disagree. They said this month that Caltrans has not studied the problem thoroughly enough to determine that the bridge won't collapse in a major earthquake.
But Maroney said he went above and beyond by conducting field tests on welded steel plates. The Federal Highway Administration recommended only that the state review construction records to confirm a weaker “rogue” plate was not used in the bridge by mistake, he said.
“In my view, this is done,” Maroney said.
The chief engineer said he's not surprised that the yield strengths differ so starkly from those recorded when the plates were manufactured at the steel mill. That’s because the plates were welded together, exposed to high heat, and ladders were sometimes temporarily welded to the plates during construction. Welding alters steel’s material properties and can result in slightly weaker yield strengths, Maroney said.
“The heat from those welds could absolutely change the properties,” he said.
He added that the field tests and Federal Highway Administration's preliminary test results found tensile strength and elongation were much higher than required for 50 grade steel.
“I got plenty of extra,” Maroney said. “In an earthquake, I have no problem at all.”
Maroney's conclusions were backed up by the bridge's Seismic Safety Review Panel, three experts who review safety issues. They are Dr. Frieder Seible, of Monash University in Melbourne, Australia; Dr. I.M. Idriss, of the University of California, Davis; and Dr. John Fisher, of Lehigh University in Bethlehem, Pennsylvania.
However, a 2014 report by the Sacramento Bee found the panel members have financial ties to Caltrans and bridge contractors, raising questions about impartiality.
“If Caltrans brings someone else outside Seismic Peer Review Panel, that would be very valuable,” said Abolhassan Astaneh-Asl, a University of California, Berkeley civil engineering professor who has studied problems with the bridge for years.
Vince Mammano, California division administrator for the Federal Highway Administration, also was on hand at the Tuesday meeting.
Mammano said the federal agency was interested in testing the welded plates only for research purposes. It recommended that the state check its construction records to make sure it didn't get a weaker piece of steel by mistake, he said.
“Federal Highway sees no reason the bridge is going to operate in any way other than designed,” Mammano told the panel.
Nonetheless, Toll Bridge Program Oversight Committee member Susan Bransen asked the Federal Highway Administration to submit a document in writing confirming that it had no concerns about the bridge's safety.
“I would like a letter from the Federal Highway Administration,” she said.
Independent engineers, including UC Berkeley civil engineering professors Robert Bea and Abolhassan Astaneh-Asl, say the state needs to conduct a full analysis on how the bridge will perform in an earthquake, taking into account its actual material properties. Conducting such a test would require cutting out chunks of the bridge and analyzing them, Astaneh-Asl said.
Bea and Astaneh said the bridge has “low-robustness” or “fracture-critical” design, meaning one major flaw could cause the entire system to fail in a major earthquake.
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 30 years.
Caltrans did not immediately provide requested documents on field tests that found lower yield strengths in parts of welded plates, but Maroney said the lower yield strengths were limited to an isolated area.
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