A team of bridge experts are currently studying hundreds of anchor rods on the San Francisco-Oakland Bay Bridge's main tower to see if any action is needed to further protect the bolts from possible seawater corrosion.
The team, some members of the National Academies of Science and Engineering, were gathered by Toll Bridge Program Oversight Committee (TBPOC), the multi-agency body that oversees construction of the bridge. The goal is to determine the condition and strength of the tower anchor rods, many of which have been exposed to sea water, and to "design an environmentally friendly system to protect them from corrosion far into the future."
The tower base anchor rods are part of a seismic system designed to ensure that during a seismic event there is no uplift during a massive quake.
During recent workshops, the expert panel was shown projection screen images of steel grain magnified 1,000 times. “We are literally putting this bridge under a microscope,” said Dr. Brian Maroney, Bay Bridge chief engineer, in a recent news release.
The anchor rods became the focus of attention when Caltrans inspectors found that they were improperly grouted by the bridge contractor and had been sitting in standing water.
Caltrans says that subsequent testing showed that nearly all tested rods withstood major earthquake-level force, while also revealing two had stripped threads and that one stripped to the breaking point. Despite this 99 percent performance, the panel was also shown advanced engineering analysis of the bridge in a huge quake with only half or even none of these rods performing as designed.
Even in these extreme hypothetical's, this analysis shows the bridge would perform as designed, and be ready to carry truckloads of emergency assistance soon after a major seismic event, says Caltrans. The transportation agency emphasizes that the rods are part of a "belt and suspenders approach." In addition to the more than 400 anchor rods at the center of this analysis, the tower cannot slide on its base because of 150 6-inch diameter dowels, it says.
Maroney recently asked TBPOC to move forward with recommendations such as designing a dehumidification system to more thoroughly dry most of the anchor rod holes below the iconic tower; acquiring special jacks to facilitate the cleaning of the rods; and conducting a custom steel-assessment known as the Lou Raymond Test, named after its inventor. This will help determine if micro-indications present within the specimens would have any effect on the useful life of anchor rods.
The panel will release its findings and formal recommendations to TBPOC at its next meeting on August 24, says Caltrans spokesperson Leah Robinson-Leach.
On August 4, TBPOC released an analysis of the self-anchored suspension bridge's tower without anchor bolts. In the analysis, conducted by T.Y. Lin International, computer models were employed to study various seismic conditions and effects on the span.
The 14-page analysis concludes that the effect of no anchor rods "on global response to the design SEE (safety evaluation event) event is minimal. The paper also states that "the seismic response of the SAS to an SEE event is not diminished in the event that no anchor rod is effective."