In March, 32 gigantic anchor rods in the new Bay Bridge failed due to hydrogen embrittlement, which is the Achilles' heel in high-strength steel.
The $6.4 billion project has been managed by the Toll Bridge Project Oversight Committee (TBPOC) since 2005. On July 8, it released a report on the rod failure that conceded Caltrans' specifications for these rods were inadequate and contributed to the failures.
Unlike bolts, these rods have both ends threaded for nut engagement. The 32 anchor rod failures occurred all in the bottom threads; none failed in the top threads. The TBPOC has failed to explain this most peculiar failure pattern.
Thus, the TBPOC has demonstrated that its understanding of hydrogen embrittlement is inadequate. Its conclusion on the cause of the 32 rod failures is skewed. Furthermore, the report itself is littered with errors, contradictory data and questionable test protocols. Most important, its rod replacement strategy will not guarantee new rods will not fail during service.
Caltrans' director said the new Bay Bridge has more than a million bolts. Still, it is surprising to see the state agency has no materials engineer with expertise in the metallurgy of high-strength steel bolts/rods. Such an engineer could have recognized and corrected the errors in the TBPOC report. TBPOC, Caltrans and contractor American Bridge/Fluor Joint Venture fail to recognize the errors in this and other reports.
The problem lies in part in their failure to appreciate the distinctive nature of the Bay Bridge rods. For a comparison, consider that the two towers of the Golden Gate Bridge used to have 1.2 million rivets, each about the size of a thumb. Many of them have been replaced with bolts and nuts. A 1-inch diameter by 2-inch-long bolt with a nut would weigh about half a pound.
In contrast, the 32 Bay Bridge anchor rods that broke last March were 3 inches in diameter (like your arm) and 10 to 17 feet long. Each would weigh 250 to 400 pounds. The anchor rods that hold the 500-foot-high main tower down to the concrete footing are 4 inches in diameter (like your leg) and 26 feet long. Each would weigh more than 1,100 pounds. Many are 32 feet long or as high as a three-story building. The new East Span has 2,300 of these gigantic rods in 17 locations. All were custom made.
It does not take a genius to figure out that these gigantic rods are extraordinarily different from common-size bolts that are mass produced by tens of thousands. Still, the Caltrans specification applied the sampling plan for quality control testing of mass produced common-size bolts to the gigantic custom-made rods.
This is not good enough for hardness, the most important property to control in avoiding hydrogen embrittlement.
Using a portable hardness tester, hardness is easy and inexpensive to test. It is imperative that every threaded area meets a maximum surface hardness requirement. Unlike common-size bolts, long rods can vary in hardness from one end to the other. Only 100 percent surface hardness testing at both ends of each rod can guarantee all new rods will meet the new hardness requirement.
No one at Caltrans seems worried whether the gigantic replacement rods conform to hardness requirement at the surface. The new rod order requirement in the TBPOC report lacks this 100 percent surface hardness test. This is not very smart. This is one of the reasons why I lack confidence in the TBPOC's ability to resolve the gigantic rod problem in the new Bay Bridge.
Yun Chung, a retired materials engineer, may be reached at firstname.lastname@example.org.