The California Department of Transportation announced this week that it had fixed anchor rods that secure the main cable of the new San Francisco-Oakland Bay Bridge, but a leading expert in engineering failure analysis cast doubts on the adequacy of the repairs.
Chambers on the eastern side of the suspension span enclose the rods, which are more than 3 inches in diameter and up to 32 feet long. Caltrans said last year that many of the 274 rods were skewed off-center inside holes through the steel base plate to which they are secured. Some pressed against the plate, risking damage during regular use or a seismic event.
After months of study, the rods recently were shifted to ensure at least 2 millimeters of clearance and will perform as designed, according to a report provided this week by Brian Maroney, the span’s chief engineer.
Robert Bea, engineering professor emeritus at UC Berkeley and an expert in risk assessment and failure analysis, called the actions “a good move.”
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But he said in an interview that Caltrans had not alleviated concerns about the long-term consequences of design and construction errors that caused the problem. He referenced problems with similar rods on another part of the bridge that cracked due to manufacturing errors and long-term exposure to the bay’s marine environment and rainwater. In 2013, some of those rods – which secured seismic-safety equipment – fractured under tension, leading to a costly retrofit.
The Sacramento Bee reported last year that some of the main cable rods – and the cable itself – had rusted because the chambers were left open to the elements during construction and because the chambers leak during rainstorms. Corrosion can contribute to cracking.
Bea said that the off-center rods shifted by Caltrans were damaged on their threaded portions after contact with the base plate but that Caltrans did not examine them for micro-cracks.
“One would expect to see micro-cracking at the roots of the threads,” Bea said. Such cracks can grow due to corrosion and years of vibrations caused by trucks on the bridge roadway, or by the extreme shifts in a major earthquake. Crack growth can eventually fracture the rods.
Bea suggested more testing to check for cracks and to more fully evaluate the possibility of serious or catastrophic failure of the span if some of the anchor rods fail.
At a meeting Friday in Sacramento, Caltrans officials told the Toll Bridge Program Oversight Committee that testing continues on a different set of rods, those that secure the bridge tower to its base.
Last fall standing water was found in spaces surrounding a few tower base rods during a routine inspection. Tiny cracks and corrosion were found on one rod extracted for testing, leading to questions about long-term reliability, particularly during a quake. On Friday, Caltrans said additional tests showed micro-cracks in a second rod. In that case, the rod had not been surrounded by water.
Caltrans continues to investigate the significance of the cracking, Maroney said.
Recent Caltrans tests also found one of the remaining 422 tower base rods to be shorter than the others. Further tests will determine if that rod is shorter because it has fractured, which would add concerns about the durability of the remaining rods.
Call The Bee’s Charles Piller, (916) 321-1113. Follow him on Twitter @cpiller.