Long-standing concerns about more than 400 steel rods that secure the San Francisco-Oakland Bay Bridge tower to its foundation gained urgency this week.
State Transportation Department officials told the San Francisco Chronicle that tiny cracks found in two rods – which are about 26 feet long and up to 4 inches in diameter – might also be present in many other rods, placing them at risk for sudden fracture. The cracks appear to be related to corrosion – caused by water leaking into the spaces around the rods.
“As an engineer, if I have these micro-cracks, I have to assume they exist in every rod,” Brian Maroney said at meeting of bridge officials Tuesday.
Stress and corrosion affected similar rods on the eastern pier of the $6.5 billion span that opened in 2013. Corrosion caused hydrogen to enter the steel, making them brittle. The rods snapped under tension, necessitating a multimillion-dollar retrofit to secure seismic gear at that location.
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The Sacramento Bee reported in March that officials discovered the problem at the tower base in 2013 but didn’t begin to examine the issue until last fall. Robert Bea, engineering professor emeritus at UC Berkeley and an expert in risk assessment, called Caltrans’ failure to respond right away “egregious” and “unacceptable.”
Maroney initially blamed the water contamination on a failure by the bridge contractor to fill spaces around the rods completely with grout. This allowed rainwater or wash water to leak in. Now, officials say that salt water from the bay might be entering through cracks in the cap of the foundation.
Salt accelerates corrosion and possible cracking. That makes the rods more vulnerable to failure in a major earthquake. Corrosion also could affect the integrity of the foundation cap, which is strengthened with rebar.
A Caltrans expert panel began meeting earlier this month to develop tests and analyze the problem.
If the rods must be replaced, Caltrans will face a difficult challenge, because their bottom portions are inaccessible. Last year, Bea called that design choice a violation of a fundamental engineering principle – “to engineer explicitly for repairability.”
A Bee report last year detailed findings by Yun Chung, a retired metallurgical engineer who has studied the matter since the anchor rod problems first came to light. He warned that thousands of rods on the bridge are vulnerable to cracking – particularly if contaminated by water. He said the tower anchor rods are particularly problematic.
At the time, Caltrans said it was confident that Chung’s concerns overstated any possible risk to the rods and the bridge.