Any one of a dozen reasons could explain why huge high-strength steel rods — 9 to 24 feet long — on the new Bay Bridge snapped. But there are no excuses for allowing the fasteners to break in the first place, say two California metallurgical engineers.
“There are plenty of explanations as to why steel behaves in a brittle way, but there are no excuses to have them behave in a brittle way,” said veteran UC Berkeley materials sciences professor Tom Devine. “We simply know too much about these processes for that to be the case.”
The rod failures could have been prevented, agreed Newport Beach-based private engineering consultant Louis Raymond. Raymond was chairman for 20 years of the ASTM International’s subcommittee on hydrogen embrittlement, the steel industry’s standards and testing protocol group.
“There is a lot of technology out there that could have been used to prevent this from happening,” said Raymond, referring broadly to a wide range of steel materials, coatings and production methods.
But these engineers, along with other experts queried about the broken bolts, have more questions than answers: Did Caltrans specify the latest steel technology for the rods? If defective, how did the rods pass muster with supplier Dyson Corp., its subcontractors and the state’s quality control testing protocols? Were the appropriate tests done and what were the results? Did the contractor mishandle the bolts during installation?
Until those details emerge, the outside experts say they cannot point to a specific test, material, coating or process that would have prevented the failure, although they say that proper procedures, materials and testing all down the line should have either prevented the failure or caught it before the rods were installed.
Caltrans is scheduled to deliver some answers Wednesday morning when it briefs the Metropolitan Transportation Commission on the investigation and possible fixes.
“The outside experts’ observations speak to the fact that we really need to absolutely understand what happened and have a comprehensive solution in order to have full confidence in the integrity of the bridge,” said commission Chairwoman Amy Worth, also an Orinda councilwoman.
In early March, contractors began tightening nuts affixed to 3-inch threaded rods in two shear keys, massive steel and concrete boxes sandwiched between the bridge deck and the concrete pier cap just east of the suspension span tower. Once clamped into place, the shear keys help counteract movement during an earthquake.
Within the first week, a third of the 96 rods snapped. Caltrans has suspended tightening on the remaining 192 bolts pending the outcome of its investigation.
High-strength steel is highly desired for its combined strength and ductility, or its ability to retain its function after a blow. But it can become fragile under certain conditions, experts say.
Forgers discovered the deformation phenomenon in 1875 and nearly 140 years later, worry over brittleness remains a factor in everything from nuclear power plants to airplanes.
One of the most famous brittle steel cases is the SS Schenectady.
On Jan. 16, 1943, while moored at a Portland, Ore., dock, the tanker’s hull cracked almost in half. A dozen or so ships in the U.S. military fleet would fracture in similar fashion, but the Schenectady was visible from the city and attracted considerable attention.
Critics blamed bad welds, but research later showed the steel had turned brittle when the temperature dipped too low.
Modern high-strength steel is much higher quality, but it is highly susceptible to what engineers call hydrogen assault or embrittlement.
This occurs when hydrogen atoms — one of Earth’s most abundant elements — work themselves into the spaces in the steel’s crystalline-like molecular structure and weaken the bonds, UC Berkeley’s Devine explained.
Steel contains a mix of carbon and elements such as manganese and nickel that is subjected to complex heating, reheating and coating processes.
As Devine explained it, hydrogen could contaminate steel in any number of ways — a mold could be chipped, the item could have been left too long in a hydrochloric acid bath or mishandled during installation, for example.
Steel can also deform if the molten zinc used to galvanize, or protect, the surface is the improper temperature.
But Devine doesn’t think any of these glitches damaged the Bay Bridge rods.
He’s leaning toward another theory — a brittle and cracked inner layer where the zinc and steel melted together during galvanization. If this occurred, he said, the bolts could snap after the nuts were tightened.
Contact Lisa Vorderbrueggen at 925-945-4773, lvorderbrueggen@bayareanewsgroup.com, politicswithlisav.blogspot.com or Twitter.com/lvorderbrueggen.
