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.