New Bay Bridge shows signs of rust in critical areas
04/12/2014 8:40 PM
04/17/2014 6:54 PM
Some of the most vulnerable and integral cable sections and rods on the new $6.5 billion San Francisco-Oakland Bay Bridge are rusting.
The bridge suspension span and iconic tower rely on an unusual design: A single cable, comprising 137 steel strands, loops over the tower and under the bridge to hold it up. That cable is secured on the eastern edge of the span, inside chambers designed to keep out water and marine air to prevent corrosion.
But a Sacramento Bee investigation found that inside one of the chambers, where the suspension cable is attached, the cable strands and rods show rust. Lab tests confirm the rust, alarming independent engineering experts who warn of severe long-term implications. They urge the California Department of Transportation to move quickly to fix the problem.
Experts said that if corrosion worsens, it will lead to structural damage well ahead of the planned 150-year service life of the bridge. Among The Bee’s key findings:• Laboratory tests of corrosion residue from two locations inside one chamber confirmed steel rust in addition to salt deposits, which accelerate corrosion.
• The chambers, designed to be sealed against water and dehumidified, were drenched for about a year between December 2011 and December 2012 during construction, according to Bill Casey, a top Caltrans engineer for the bridge. Cables and anchor rods were exposed to more than 21 inches of rainfall during that period, as well as the mist and humidity from the bay’s marine environment. Ongoing leaks through bolt holes at the bridge surface have caused high humidity and puddles of water during recent storms, despite efforts to keep the chambers dry.
• Three engineering experts, who have independently analyzed other problems with the new bridge, said the corrosion is significant because it makes the rods and the cable strands vulnerable to cracking. If the strands crack where they are attached, continual vibration caused by trucks overhead would worsen the cracks and jeopardize the strength of the span’s single main cable, said University of California, Berkeley, engineering professor Abolhassan Astaneh-Asl, a well-known expert on steel bridge engineering.
“The implications are structural – and very serious,” said Astaneh, referring to the lifespan of the bridge.“This bridge is fracture critical, which means if any important element of this bridge fails ... the bridge is going to collapse.”
In an email, bridge spokesman Andrew Gordon said “there is no concern” about corrosion in the chambers, attributing the rust to “metal shavings/particles generated by grinding and other work” that scraped the steel decks and their protective layer of zinc paint.
“We have seen no signs of degradation of the strands or rods,” Gordon said.
Gordon also noted work continues on the bridge. “The opening of the bridge in September did not mean that all work was completed,” he said “...We are still buttoning up the bridge.”
Gordon said that bridge officials hope to seal the leaks “before the next rainy season.”
“The best hope is still dehumidification,” said Yun Chung, a retired Bechtel Corp. engineer and metallurgist. “The longer they delay in achieving the (low) humidity on a continuous basis, the worse it will get.”
The problem lies in two chambers, about 130 feet long and 15 feet high, on the eastern end of the bridge. Inside, the main cable that holds up the bridge divides into 137 strands that are inserted into assemblies called sockets, which are attached to threaded rods that look like giant screws without heads. In each chamber, the rods pass through a wall and are locked in place with a nut.
White and red-brown rust can be seen in patches on the threads of dozens of rods. Rust encrusted the cable strands at the junctions of dozens of sockets, and crack-like indentations are apparent in a few cases.
Many of the rods are considered potentially vulnerable to cracking, due partly to manufacturing errors that let hydrogen enter the steel. A combination of hydrogen, rust and stress can create tiny cracks in the steel that grow and cause the rods to break. A separate breakage problem surfaced last year, when 32 rods that secured a shear key – a seismic device on the eastern pier of the suspension span – snapped. Caltrans adopted a costly retrofit to secure the shear key. It also began a testing program, which is still in progress, to determine if similar rods on the bridge must be replaced.
Chung and Lisa Thomas, a metallurgical and materials engineer at Berkeley Research Company, a forensic firm in Berkeley, have co-authored technical reports about the broken rods and related issues. Thomas testified before a state Senate committee on the issue, and, at a recent meeting about the problems, Caltrans acknowledged that some concerns raised by the two independent experts have merit.
Chung and Thomas reviewed Sacramento Bee photographs taken inside the chamber on the westbound side of the span, and photos taken by a colleague of Thomas’ during a tour of the eastbound chamber. They said the photographs show steel rust in dozens of locations.
“Everything got wet during construction. We got water in here,” said Casey, the engineer who manages the primary contract for the suspension span, standing in a chamber recently. “This was open for a good year.” He described the white residue as “zinc corrosion” – proof that the “sacrificial” zinc layer is doing its job, protecting the steel.
“You’ll need years and year and years of that white before you get through the zinc,” and start to corrode the steel, Casey said.
Lab tests show rust
During a tour and interview with Casey, The Bee collected samples of the rust-colored residue in two locations – a junction of the cable strands and sockets, and an anchor rod – using cotton cloth to rub off corrosion deposits.
Thomas examined the samples with a scanning electron microscope. Along with zinc, she found abundant iron – evidence that the protective zinc layer had been breached and the underlying steel was corroding, Thomas said. She also found sodium and chlorine – signs of salt, whose chemical name is sodium chloride. Some samples showed magnesium, another component of seawater.
The presence of these elements, Thomas and Chung said, represents “absolute evidence” that bay mist or humid, salty marine air infiltrated the chambers and corroded the steel. “Chlorine was practically in every spot that I analyzed” from the anchor rod, Thomas said.
She examined every particle type present in the samples, and a few were as Gordon described – steel shards and spheres from the grinding and metal shavings. But their sharply defined features and composition differed dramatically from the majority of particles, which showed a more amorphous combination of zinc, iron and other elements.
Combined with visual evidence from the photos, the tests led Thomas, Chung and Astaneh to conclude the steel in the main cable strands, socket junctions and anchor rods corroded.
Caltrans dehumidifies the anchorage chambers and a few other areas that contain sensitive rods or cable sections to prevent rust from forming in the marine environment.
The experts said Caltrans should quickly solve water leakage and humidity problems, and seal and dry out the chambers to stem further corrosion.
Casey said the chambers are “not perfectly airtight” and that there is no practical way to prevent some outside air from entering. “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,” he said. During rains, water enters and pools on the floor, he said.
That’s why Caltrans sets dehumidifiers to 40 percent relative humidity, below the level that water can condense on the vulnerable steel, Casey said.
“You dehumidify the heck out of it, because the bridge will live and die by the cable,” he said. “In 150 years, water will find its way to that cable somewhere.”
Gordon said that due to ongoing ventilation needs for construction and painting, humidity “readings have varied.” He said the “data does not reflect the final conditions inside the suspension span.” Casey said the humidity level rises when it rains.
Caltrans’ difficulties with dehumidification are reflected in its Feb. 26 list of remaining tasks.
It shows that ducts between dry and wet zones were not sealed until last November, and the dehumidification system was not balanced to ensure proper results until late January. The system still lacks the correct louvers to control air flow, and ducts were installed improperly, according to the list.
Asked if those problems had been corrected, Gordon wrote, “we are fine tuning the dehumidification units.”
The top of the tower and its base also require dehumidification and the Caltrans list showed the top dehumidifier had fixes in January. The dehumidifier at the bottom, needed for vulnerable high-strength anchor rods that secure the tower to its foundation, has not yet been installed due to construction requirements. Caltrans expects to do so this spring.
“My main worry is that the corrosion can’t be arrested because the humidity can’t be controlled well enough,” Thomas said.
She said her concern encompasses the cable strands and the anchor rods that secure them to the bridge. Chung and Thomas said the rods that secure the cable strands have characteristics that make them more vulnerable to trouble. For example, their threads were “cold rolled” – a manufacturing process that increases surface hardness – which boosts susceptibility to cracking.
Unusual design raises risk
Astaneh said his concerns about corrosion in the chambers stem partly from the new span’s unique design. Main cables for most suspension spans, including the Golden Gate Bridge, are secured on land, inside deeply recessed anchorages. No vibrations can reach there, Astaneh said, describing a typical anchorage as “the darkest, the coldest, the quietest place in the world. The cable is dead. Absolute stillness.”
Typically, such anchorages place the socket assemblies behind a steel plate and damper, to prevent vibrations that can affect the cable and socket, Astaneh said.
On the new Bay Bridge, the main cable is anchored to the bridge itself, and sockets are suspended in the middle of each chamber, rather than behind a steel plate. Due to this design, the strands and sockets move a fraction of an inch each time a truck passes overhead, thousands of times every day. Such movements are evident inside the chamber.
“It’s like you made a machine to study the fatigue behavior of the cable,” Astaneh said, comparing it to hammering the junction of the strands and sockets seven times every minute.
Even if corrosion continues within the chambers, it might take a long time for many individual wires to crack and break.
But the anchorage design adds to the likelihood of failure, Astaneh said. After each strand was inserted into a hollow steel socket, that socket was filled with molten metal to meld the strands into a solid assembly.
Corrosion and drip patterns in the chambers show that water collects where the strands end and the socket begins, Thomas said.
Ongoing corrosion would cause cracks at the strand-socket junctions, according to Astaneh, and would be aggravated by the vibrations from trucks, which concentrate stress at that point. That combination of risk factors eventually could break a junction at its cross section, he said.
“Eventually it’s going to fracture,” Astaneh said, referring to these cable-socket junctions. With graduate students and other engineers, he is studying Sacramento Bee photos and video images, and other data, to calculate how many years the junctions can withstand the continual stress.
Gordon called the vibration issue “an incomplete hypothetical situation that we cannot answer.”
The independent engineers suggested several ways to address corrosion inside the chambers. If Caltrans seals the chambers effectively, hot, dry air could be blown on the cable strands to drive residual moisture from crevices between the wires, arresting corrosion at its current level, Chung said, although not fully alleviating concerns about cracking and traffic vibrations.
Mockups of the strand-socket assemblies could be tested in a lab, or one assembly could safely be removed for destructive tests to see if cracks have already formed, the experts said.
If needed, the assemblies could be replaced with better-designed alternatives that dampen the vibrations, Astaneh said. This would require cutting and splicing the strands, a technique he has explored and begun to design.
For the anchor rods, it would be impractical to sandblast the corrosion and then apply protective zinc paint. The space is too confined, the engineers agreed. But if officials determine that the corrosion is worrisome for the long term, those rods can be replaced.
Chung compared Caltrans’ situation to challenges that preceded the explosion of the space shuttle Challenger in 1986, which occurred after engineers failed to address concerns about one simple gasket.
“Caltrans has built a cutting-edge bridge,” he said. “The anchor rods for the bridge are what the O-rings were to the Challenger.”
Bay Bridge terminology
Here is a guide to help understand the terms and details related to the examination of corrosion on the new San Francisco-Oakland Bay Bridge:
Main cable, strand, wire
The new Bay Bridge suspension span uses a single main cable, nearly a mile long and “self-anchored” to the bridge itself. That cable is 2.6 feet in diameter and has 137 strands, each comprising 127 high-strength steel wires.
Each strand of the main cable is melted into a steel socket, part of an assembly to anchor the cable to the span’s eastern end.
Cable anchor rod
High-strength, threaded steel rods, 3½ inches in diameter, attached to the socket, then to a chamber wall, to secure the cable strands.
Two sealed chambers on the eastern end of the suspension span that hold the cable strand ends, sockets and anchor bolts. Designed to be dehumidified to prevent corrosion, Caltrans has so far been unable to keep out all water and humidity.
A device that protects the bridge against the shear forces of earthquakes.
A rod that meets certain technical specifications, including high strength, specified by ASTM International, an international standard-setting group. When rods of this kind, which secured a shear key on the new span, snapped last year, concerns arose that other such rods might prove unreliable. Caltrans is conducting tests on the matter.
Creation of a zinc coating on steel to protect against steel corrosion. Many large rods on the new bridge were galvanized by dipping them in molten zinc – a process now thought to have introduced hydrogen into the steel, one reason those rods are considered susceptible to breakage.
Cold rolled threads
Rod threads can be cut or rolled. Threads can be created by rolling a rod through a die that shapes the steel into threads. Rolled threads typically create harder surfaces, a possible risk factor for breakage on the new span. Most of the anchor rods for the main cable were cold rolled.
Scanning electron microscope
A device that can identify different elements within a particle. It operates by bombarding the particle with a stream of electrons, then reading X-rays emitted in response. Each element reflects a signature X-ray wavelength.
Sources: Toll Bridge Program Oversight Committee, Bee reporting
Abolhassan Astaneh-Asl, Ph.D.: A professor of structural engineering, mechanics and materials at UC Berkeley. Author of numerous reports about failure analysis in buildings and steel bridges, and the effects of earthquakes on bridge design and performance. Astaneh has spent thousands of hours studying and modeling the new Bay Bridge.
Yun Chung, M.S.: A retired Bechtel Corp. engineer and metallurgist. Author or co-author of several technical reports that examined Bay Bridge anchor rods and bolts, Chung has criticized Caltrans and the Toll Bridge Program Oversight Committee for their handling of testing and evaluation issues.
Lisa Thomas: A professional metallurgical and materials engineer at Berkeley Research Company , a forensic firm in Berkeley, and co-author (with Chung) of analyses of Bay Bridge anchor rods and bolts. Thomas testified recently before the California Senate Transportation and Housing Committee about materials used in the new Bay Bridge.
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