Bay Bridge designer fears leaks are damaging main cable

A worker walks on the Bay Bridge eastern span’s main cable, which the bridge’s lead designer says is vulnerable to corrosion because of leaking rainwater. A worker walks on the Bay Bridge eastern span’s main cable, which the bridge’s lead designer says is vulnerable to corrosion because of leaking rainwater. Photo: Justin Sullivan, Getty Images Photo: Justin Sullivan, Getty Images Image 1 of / 85 Caption Close Bay Bridge designer fears leaks are damaging main cable 1 / 85 Back to Gallery

The cable that holds up the new Bay Bridge eastern span is vulnerable to corrosion because of rainwater leaking into its anchorages, the bridge’s lead designer has warned Caltrans — a threat that the agency has previously downplayed on the $6.4 billion project.

Although bridge officials have been preoccupied with the possibility that rods at the base of the span’s tower could be corroded by water, lead designer Marwan Nader of the T.Y. Lin International design firm in San Francisco said the bigger concern is the cable — specifically, the twin steel boxes where the cable is anchored inside the span’s deck.

It is those anchorages that are being exposed to water during storms. Nader’s fear is that if nothing is done, the water will corrode the cable at its base.

The cable — a bundle of 137 strands of steel that form a figure-eight over the top of the tower — is critical to the bridge’s integrity. On the span, it is protected from the elements by a steel jacket. But in the anchorages on either side of the eastern end, the strands fan out and are exposed — and if water gets to them, they can corrode and fail.

“In a suspension bridge, the cable is what holds the whole thing up,” said Russell Kane, a corrosion expert in Texas who has advised companies in the oil and aerospace industries. “The issue here is corrosion damage — any kind of pitting or loss of steel weakens those strands.”

Kane added, “The bridge wouldn’t stay up if enough metal is lost from those cables. Anything that reduces the load carrying of those cables decreases the durability of the design, reduces safety factors and reduces the service life.”

Open to elements

The corrosion danger is very real, Nader told a Caltrans-convened peer review panel in July. According to minutes of the meeting, released only recently, Caltrans officials acknowledged that rainwater has been flowing into the two anchorages because of design problems with the guardrail system.

The anchorages are located inside the suspension span’s hollow deck. The enclosed area was supposed to be kept sealed from rainwater, but was left open during construction between December 2011 and December 2012, Caltrans officials have acknowledged.

Project officials warned about the problem at the time.

“Water is entering and ponding in the interior,” Caltrans senior bridge engineer Darryl Schram warned in May 2012 in a memo to Tony Anziano, then the head of Caltrans’ toll bridge program.

“To prevent corrosion of the bridge interior, water intrusion and ponding must be reduced and/or eliminated,” Schram wrote.

Experts from outside the agency who have examined photos taken last year say rust was visible at the time and that there were signs that the protective zinc coating on the cable strands had degraded.

“The rust is proof the zinc coating is gone,” said Lisa Fulton, a Berkeley engineer specializing in corrosion who has studied the photos. “There is active corrosion, and the source is hydrogen from the marine atmosphere or standing water.”

The deck was later fully enclosed, but the water issue persists. The problem, Caltrans has said, is that holes drilled through the deck to anchor the span’s guardrail are allowing rainwater to drip into the belly of the bridge and around the cable’s anchorages.

Nader told the peer review panel that “as the official engineer of record” on the eastern span project, he is “more concerned about the water inside the east anchorage than the tower anchor rods,” according to minutes of the panel’s July 9 meeting in Oakland.

“He noted that the cable structure is the most important element in this bridge and needs to be protected,” the minutes say. Although each anchorage has a dehumidifier, that system can’t handle water flooding in from rain, Nader said.

“The dehumidification is not effective if rainwater is allowed to seep into the east anchorage,” Nader told the Caltrans panel, according to the meeting minutes.

Rust can degrade the steel strands, and hydrogen in rainwater can infiltrate the rods used to anchor the cable, putting them at risk of snapping, corrosion experts say.

Nader did not respond to requests for comment.

Caltrans has yet to say how it plans to deal with the leaks. Brian Maroney, the bridge project’s chief engineer, has said the idea is to wait for a storm that lasts several days and exposes all the spots where water is getting into the deck and anchorages.

“What’s going on right now is not acceptable to us, and we’re going to fix it,” Maroney said in March. He said at the time that he had only recently learned that a key element of the bridge drainage system had been dropped, contributing to the leaks.

Caltrans confident

Caltrans has downplayed the risk of corrosion at the cable anchorages, saying the dehumidifiers can take care of any amount of water that gets into the twin boxes, each 90 feet wide, about 120 feet long and 25 feet deep.

Photo: Paul Chinn, The Chronicle The lead designer of the new Bay Bridge eastern span is worried...

The agency has repeatedly asserted there is no “active corrosion” on the span, and that any rust visible in the deck structure is the product of a breakdown of steel grindings and shavings left behind during construction. Agency officials did not respond to requests for comment for this story.

Inside the anchorages, the cable strands are attached to steel sockets that then are secured by 25-foot-long rods. Similar rods elsewhere on the span corroded and even broke after being exposed to water, but Caltrans says the cable rods are at comparatively low tension, putting them at far less risk of sudden failure.

Nader agreed that there was little to worry about from the well-publicized flooding of some of the sleeves that contain more than 400 high-strength steel rods at the tower’s foundation.

Small cracks found in many of the rods are “inconsequential” and likely to be “mechanical” in nature, Nader said at a bridge meeting in May, according to minutes of that session.

An expert hired by the bridge’s oversight panel, Steve Christoffersen, concluded that the cracks were caused by hydrogen embrittlement and could be the first sign of corrosion of the rods. But like Nader, Christoffersen concluded that the cracking “may be inconsequential” if the rods are kept protected from water.

Fearing more failures

However, bridge officials have delayed implementing such a plan, which has a price tag of between $15 million and $25 million.

One of the tower rods broke after being exposed to water. Maroney said last month that there was no evidence it snapped because of corrosion, but several outside experts interviewed by The Chronicle said the steel rod almost certainly was weakened by water.

Experts say that without protection, the problem will get worse.

“Some rods will fail over time — hopefully the bad rods will be spread at random, but this is something they need to find out,” said Yun Chung, a retired Bechtel engineer and specialist in high-strength steel fasteners. “They need to think about what real problems they have and how many more potential failures will occur in the tower rods.”

As for the corrosion risk to the cable, Chung said he is concerned that tiny corrosion pits will develop on the steel wire because of the water exposure.

“When you lose the volume of metal, you lose load carrying capacity and, this is the worst case, you can’t hold the bridge up,” Chung said. “It’s only a single cable.”

Jaxon Van Derbeken is a San Francisco Chronicle staff writer. E-mail: jvanderbeken@sfchronicle.com Twitter: @jvanderbeken