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$15M Approved to Protect Tower Rods

Tuesday, May 17, 2016

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Corrosion of the steel rods used for seismic stability on the San Francisco-Oakland Bay Bridge is once again the focus of attention for the committee that oversees seismic retrofit work for the structure.

The Toll Bridge Program Oversight Committee (TBPOC) has approved a $15 million plan to re-grout the underwater sleeves protecting the steel rods meant to prevent the structure's 525-foot-tall tower from rising off its foundation during an earthquake, the San Francisco Chronicle reported.

Bay Bridge
Images: Caltrans

The Toll Bridge Program Oversight Committee (TBPOC) has approved a $15 million plan to re-grout the underwater sleeves protecting the steel rods used for seismic stability on the San Francisco-Oakland Bay Bridge.

The plan, presented by the California Department of Transportation (Caltrans) and the Metropolitan Transportation Commission in a committee meeting Thursday evening (May 12), is intended to protect the rods from potential future corrosion.

The panel also approved an additional $1 million for steps to monitor the foundation for additional corrosion.

Steve Heminger, executive director of the Metropolitan Transportation Commission, told the committee the proposal to re-grout the sleeves holding more than 420 anchor rods at the base of the tower is a necessary expense, the Chronicle noted.

“Although this is money we would rather not spend, it’s money well spent,” he said.

Recent History

As previously reported, the integrity of these steel rods has been the focus of earlier investigations related to the $6.4 billion Bay Bridge project’s construction.

Two rods had failed earlier testing that simulated strong earthquake conditions. In that period, a group of 424 25-foot-long anchor rods were examined after it was determined that some of the rods in the tower foundation were exposed to rain- and saltwater.

Caltrans had started inspecting the condition of another set of rods in 2013 after 32 of them were damaged when they popped loose just days after workers started tightening them.

At a May 12 meeting, Caltrans officials delivered the results of the agency's testing program analyzing the tower anchor rods and proposed the most feasible option for their future corrosion protection.

More than 2,000 rods, which range from nine to 24 feet in length, are installed in the bridge.

Testing and Analysis

At the May 12 meeting, Caltrans Chief Bridge Engineer Brian Maroney delivered results of a TBPOC-mandated testing program analyzing the tower anchor rods, as well as the most feasible option for future corrosion protection.

In its anchor rod-testing program, Caltrans evaluated a variety of scenarios, including seismic analysis, mock-ups, micro-indication testing and thread measurements, to determine the best solution.

The agency also assessed a cathodic protection system to counteract corrosion in the east span tower anchor rods and foundation.

For the mock-ups, life-sized models of the anchor rods were constructed and tested at the Caltrans Burma Road offices and in the self-anchored suspension (SAS) tower base section of the bridge, the agency reported.

A contractor was brought onboard to construct the mock-ups and perform the tests under the direction of Caltrans engineers, as demonstrated in a Caltrans video.

Caltrans constructed mock-ups, or life-sized models, of the anchor rods for testing at the agency's Burma Road offices and in the self-anchored suspension (SAS) tower base section of the bridge itself.

Six computerized time tests simulated 100-year seismic events on the bridge under various conditions, including having all anchor rods in place, half in place and none in place, Courthouse News Service reported Friday (May 13).

From the analysis and testing period, Caltrans and its team of participating engineers and scientists determined that the rods are “fit for purpose” and should be grouted as soon as possible to keep water from flooding into the sleeves and exposing the anchor rods to the risk of corrosion.

The panel approved the proposed $15 million plan to re-grout the sleeves to prevent further saltwater intrusion.

Structurally Sound?

Maroney indicated that the agency had explored whether it made more sense to replace the rods. And although he initially thought that was the best solution, the Chronicle said, the mock-up testing scenarios indicated that re-grouting would be sufficient to keep the tower relatively stable during a major earthquake.

"An abundance of analysis and testing over the last fifteen months has proven that this tower base connection has layers of protection to provide superior safety and post-earthquake performance," Maroney noted.

In past analysis, Caltrans has made clear that the bridge’s ability to withstand an earthquake is not dependent on the rods and that their purpose is to deliver an extra layer of protection.

The panel also indicated that the rods are “less structurally important now that the tower is connected to the main structure by heavy cables, they may minimize damage to the bridge during an earthquake, and should be left in place,” the News Service reported.

FHWA official Vincent Mammano, present at last week’s meeting, concurred that the rods "add value during a seismic event."

Maroney voiced his confidence in the structure, saying, “Everything in the Bay Area will move in an earthquake, including this tower. I’m not worried about this tower. And the bottom of this tower is not going anywhere.”

   

Tagged categories: Asia Pacific; Bridges; Caltrans; Cathodic protection; Corrosion; Corrosion protection; Corrosion resistance; EMEA (Europe, Middle East and Africa); Environmental Controls; Latin America; North America; Steel

Comment from Tony Rangus, (5/17/2016, 10:14 AM)

“less structurally important now that the tower is connected to the main structure by heavy cables, they may minimize damage to the bridge during an earthquake, and should be left in place,”. So the heavy cables were a construction aid which were to be removed after construction completion? Is that political double-speak,or what.


Comment from Mark Lewis, (5/19/2016, 1:03 PM)

I'm curious about the results of their assessment of the need for a cathodic protection system.


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