Corrosion Reported on New Bay Bridge


Cable sections supporting the $6.4 billion San Francisco-Oakland Bay Bridge are showing signs of corrosion seven months after the bridge opened to traffic, an investigation by The Sacramento Bee concludes.

The newspaper reported Saturday (April 12) that its investigation revealed rust on cable strands and inside one of the chambers designed to keep out water and marine air.

The California Department of Transportation (Caltrans) says that it is not concerned and that the rust is part of routine, ongoing work.

Independent Experts

The Bee reports that it consulted three independent engineering experts who said Caltrans needed to fix the issue quickly, before the corrosion left rods and cables vulnerable to cracking.

Corrosion, combined with traffic vibrations, could jeopardize the strength of the main cable, experts said.

The bridge opened Sept. 2, 2013, and holds the record as both the largest public infrastructure project in California's history and the most expensive bridge in the world.

'Buttoning Up' A Bridge

The single-tower, self-anchored suspension bridge features a unique design in which the 137-strand main cable loops over the tower and under the bridge, where the strands are inserted into sockets that are attached to thread rods; the rods pass through the walls of two chambers on the bridge's east end, where they are locked in place.

According to the newspaper, some of the strands show rust and "crack-like indentations" where they join the sockets, and "white and red-brown rust can be seen in patches" on several of the rods' threads.

Andrew Gordon, a spokesman for Caltrans, told The Bee in an email that that the rust was attributable to "metal shavings/particles generated by grinding and other work."

"The opening of the bridge in September did not mean that all work was completed," Gordon said. "...We are still buttoning up the bridge."

Soaked Structure Samples

Between December 2011 and December 2012 during construction, the chambers were soaked with water, and cables and anchor rods were exposed to more than 21 inches of rain, in addition to the Bay's marine environment, Caltrans engineer Bill Casey told the newspaper.

Bolt holes at the bridge's surface have allowed leakage into the chambers, causing high humidity, and puddles of water were discovered on the bridge deck under the steel chambers during some of the first storms after the bridge's opening.

The newspaper toured the bridge with Casey and took samples of the rust-colored residue from the junction of the cable strands and sockets and from an anchor rod.

The rods, which secure a shear key for seismic stability, already experienced issues last year, when 32 of them popped loose days after being tightened. Caltrans developed a steel saddle retrofit to secure the shear key and started a testing program to find out if other rods need to be replaced.

The white residue is zinc corrosion, Casey told The Bee, and is "proof that the 'sacrificial' zinc layer is doing its job, protecting the steel."

self anchored suspension bridges
Wikimedia Commons / Caltrans

The Sacramento Bee says the cable strands are showing signs of corrosion, but Caltrans says the rust is from ongoing construction work.

"You'll need years and years and years of that white before you get through the zinc," Casey said.

Lab: Protection Breached

The Bee sent its samples to Lisa Thomas, metallurgical and materials engineer at Berkeley Research Company, for lab testing.

Using a scanning electron microscope, Thomas studied the samples and told the newspaper that she also found abundant iron, which is "evidence that the protective layer had been breached and the underlying steel was corroding," The Bee reported. She also found sodium chloride and some magnesium.

Thomas and Yun Chung, a retired Bechtel Corp. engineer and metallurgist, also looked at photos of the chambers and said steel rust was visible in multiple locations.

According to Thomas and Chung, there is "absolute evidence" that the marine environment has gotten through the chambers and corroded the steel.

Last fall, Thomas and Chung, a retired Bechtel Corp. materials engineer, authored a damning 105-page report detailing concerns about the Toll Bridge Program Oversight Committee's analysis of engineering issues on the bridge's anchor rods.

The committee has managed the bridge's construction since 2005. The panel is comprised of representatives from Caltrans, the California Transportation Committee, and the Bay Area Toll Authority.

Several other reports about the bridge and its oversight have emerged since September, drawing the attention of the state's Senate Transportation and Housing Committee and its chair, Sen. Mark DeSaulnier (D-Concord). The committee has since held a series of informational hearings on improving megaprojects, lessons learned from the Bay Bridge project and, most recently, on reforming Caltrans.

Keeping Out Humidity

Chung told the newspaper that dehumidification was the best bet to prevent the problem from getting worse.

Caltrans dehumidifies the chambers and several other areas to prevent corrosion, but Casey told The Bee that the chambers were "not perfectly airtight" and some air would enter from outside.

"You look for every place you can get air in, and you caulk the heck out of it, but you always learn something new when it rains or something happens," Casey said.

Dehumidifiers are set to 40 percent relative humidity, which is below the level that water can condense on vulnerable steel, Casey said.

suspension bridge design
Screen shot via Fluor

The self-anchored suspension bridge features a unique design in which the cable strands were looped over and under the bridge, as demonstrated in this video from Fluor.

However, ventilation is currently needed for ongoing construction and painting work, so humidity readings have varied, Gordon told the newspaper. Humidity levels also rise when it rains, Casey noted.

'Hypothetical' Vibrations

Abolhassan Astaneh-Asl, an engineering professor at University of California, Berkeley, and expert on steel bridge engineering, said the span's unusual design worried him. Usually, main cables of suspension spans are secured deep inside the ground where vibrations can't affect them.

But on the new Bay Bridge, the main cable is anchored to the bridge itself and sockets are suspended in the middle of each chamber. Every time a large truck passes over the bridge, the sockets and strands move slightly, the newspaper explained.

The strands are installed on the deck anchorage on the east side, loop over the tower saddle and around the west cap beam, back through the tower saddle, and finally into the cable anchorage on the east side, according to the American Bridge/Fluor Joint Venture, which built the east span.

The 137 strands were then bundled together and wrapped with zinc paste and stainless steel S-wire before being painted.

Combine the chamber corrosion and anchorage design, and "eventually it's going to fracture," Astaneh told The Bee, referring to where the cables and sockets connect. Ongoing corrosion would cause cracks at socket junctions, and the vibrations would concentrate stress in those areas.

However, Caltrans Gordon's called the vibration "an incomplete hypothetical situation that we cannot answer."


Tagged categories: Bridges; Corrosion; Department of Transportation (DOT); Design; Program/Project Management; Project Management

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