OAKLAND — Caltrans officials say they don’t yet know how 32 high-strength threaded steel anchor rods in the new eastern span of the Bay Bridge weakened and snapped. State engineers suspect hydrogen embrittlement, a well-known phenomenon where the element infiltrates the spaces between the steel’s crystalline grains and loosens its bonds.
But is that really what happened? Or was it a manufacturing error?
Did water found at the bottom of the rods’ casings corrode the zinc coating and instigate the brittleness? Or did engineers use the wrong steel?
Armchair engineering is fun but we consulted metallurgical and materials sciences experts for answers to a few questions.
Q: Could sitting in water really lead to brittle steel?
A: Yes, water could corrode the protective zinc coating and leave steel vulnerable to hydrogen invasion when contractors tightened the nuts and put stress on the rods, U.C. Berkeley metallurgical engineer and professor Tom Devine said.
But given that an extraordinary third of the 96 bolts broke, it is more likely that a flaw in the steel made the rods exceptionally susceptible to fracturing, Devine said.
Q: Were the nuts over-tightened?
A: It’s possible, said Robert Ritchie, a senior scientist at Lawrence Livermore National Laboratory and a U.C. Berkeley mechanical engineering professor.
“If engineers did the calculation wrong or applied too much torque, this would definitely result in a higher stress in the bolt and with such a high-strength steel, this is a prime recipe for trouble,” he said.
Q: If engineers were worried about corrosion, why didn’t they use stainless steel?
A: Stainless steel is more resistant to corrosion and hydrogen embrittlement but it usually lacks sufficient strength required and it is much more expensive, Ritchie said.
Q. In August 2007, a six-lane steel truss bridge over the Mississippi River in Minneapolis collapsed and 13 people died. Was its steel brittle?
A: No. The National Transportation Safety Board blamed design errors and excess weight.
Contact Lisa Vorderbrueggen at 925-945-4773, lvorderbrueggen@bayareanewsgroup.com, politicswithlisav.blogspot.com or Twitter.com/lvorderbrueggen.
