OAKLAND — How California handles the catastrophic failure of giant steel rods on the nearly finished Bay Bridge will help other states and countries avoid a similar and possibly fatal mistake, say international engineering association leaders.
“Engineers working in this area from throughout the world are waiting with bated breath to hear the outcome of the forensic and failure analysis,” said Kevin Garrity, engineer and the immediate past president of NACE International, the Corrosion Society. “My hope is that once everyone gets to the bottom of the major issues, the lessons learned will make a significant contribution to a more global understanding of the problems that can occur with high-strength steel.
“In this case, at least, it was detected before the bridge went into service.”
Caltrans and American Bridge Fluor Joint Venture, the suspension span contractor, have said they’ll soon release findings into why a third of 96 bolts — 3 inches in diameter and 17 to 24 feet long — snapped after workers tightened them in early March.
The threaded anchor rods vertically connect three critical pieces of the bridge at the pier just east of the main tower: bridge deck, shear keys or bearings, and columns. Shear keys help control side-to-side sway, and bearings counter seismic uplift.
The broken rods were manufactured in Ohio in 2008. An additional 192 anchor rods fabricated in 2010 by the same company and installed in two adjacent shear keys have not failed but are undergoing intense testing.
On Wednesday, Caltrans, the California Transportation Commission and the Metropolitan Transportation Commission — three agencies the Legislature appointed in 2005 as co-managers on the $6.4 billion replacement eastern span, will also provide:
It’s been 23 years since the magnitude-7.1 Loma Prieta earthquake struck and sheared pins on the Bay Bridge’s cantilever span. A large section of the upper deck fell onto the lower lanes, and one motorist died.
Dithering over design, alignment, bike lanes and who would pay for the massive cost overruns took years. Construction finally started in 2002, and like every mega public works project, it has experienced problems big and small. Now, just months before the scheduled opening, the specter of snapped bolts has weakened public confidence in the new span’s much-touted ability to withstand a big temblor.
Whether or not the Bay Area celebrates the opening of the world’s largest single-tower, self-anchored suspension span on Labor Day will hinge on what the bridge team’s investigators say about how fragile rods ended up on the new span.
They know the rods became brittle through a well-known chemical reaction, where hydrogen atoms invade the spaces between the steel’s crystalline structure and weaken it. But the team has yet to state publicly whether the hydrogen contamination occurred during fabrication or while the rods sat in their casings for nearly five years in pools of water, or both.
Based on what metallurgy experts from around the country have suggested, the most likely culprits are excessive hardness and galvanizing: The stronger the steel, the more susceptible it is to hydrogen intrusion, and galvanizing can trap hydrogen atoms beneath the zinc coating.
Out of 14 tensile strength tests Caltrans performed on rod samples in 2008 — part of the same batch that broke — two were stronger than what American Institute of Steel Construction chief structural engineer Charlie Carter called the “critical zone.” Another five were within 2½ percent of levels considered at high risk for hydrogen-induced brittleness.
ASTM International, an industry standards organization, has warned for decades against galvanizing fasteners made out of the same material and grade of steel used for the 288 anchor rods installed in the new bridge. The same galvanized steel — called A354 BD — was also used for 932 other pieces on the bridge. The bridge team has said it is inspecting all the galvanized high-strength steel used on the span.
“Research conducted on bolts of similar material and manufacture indicates that hydrogen-stress cracking or stress cracking corrosion may occur on hot-dip galvanized Grade BD bolts,” according to ASTM.
Portland Bolt and Manufacturing in Oregon declined to submit a bid quote on the Bay Bridge bolts when Caltrans put out bid specifications in 2005, chiefly because of the galvanizing requirement, said company quality assurance director Dane McKinnon.
“We won’t galvanize A354 BD bolts for any reason,” said McKinnon, who is also a member of ASTM’s fastener standards committee. “We’ve chosen to take ASTM’s warning as more of an absolute.”
Caltrans’ bridge design manual prohibits galvanizing this type of steel, but engineers opted to do it anyway for reasons they haven’t yet explained. Generally, galvanizing is a relatively cheap and long-lasting anti-corrosive treatment.
Agency representatives said in late April they took measures to reduce the hydrogen risk by requiring the galvanizer to clean the steel surface before through dry grit blasting rather than an acid bath.
Learning how Caltrans came to its decision to deviate from both its own policy and long-established industry guidelines is key for the global engineering community, said Carter, also chairman of the Bolt Council.
“This will be a good object lesson for all of us,” Carter said. “If you knew about (hydrogen impacts) and forgot, this is reminder. If you didn’t know, well, unfortunately, engineers learn a lot from experience.”
Contact Lisa Vorderbrueggen at 925-945-4773, lvorderbrueggen@bayareanewsgroup.com, politicswithlisav.blogspot.com or Twitter.com/lvorderbrueggen.
