Abolhassan Astaneh-Asl has been portrayed as a Cassandra, but these days he merely seems prescient. The UC Berkeley engineering professor was one of the earliest and most vocal critics of the new Bay Bridge design. The self-supported span was inappropriate for a seismically active area, he said, insisting that what was needed was a more prosaic, anchored span fixed securely to the mainland and Treasure Island.
Caltrans, the state transportation department responsible for the bridge’s construction, countered it was all just sour grapes: Astaneh-Asl, they claimed, was piqued because the bridge’s selection panel had rejected his own design due to “…uncertainties regarding its seismic performance, lack of experience with this type of structure, and (an absence of) firm engineering support information…”
That characterization angered the Cal prof, who has insisted his bedrock concern was, and remains, public safety.
Certainly, the years between design and construction have done little to allay Astaneh-Asl’s—or the public’s—trepidations. Among the problems that have surfaced:
- the breaking of 32 of 96 anchor bolts that secured seismic stabilizers known as shear keys
- the discovery of cracked and misaligned welds in the suspension span’s roadway
- concerns that concrete in the tower’s foundation may be in poor condition
- leaks in the span’s deck that will likely lead to corrosion of the main cable and anchor rods
- corrosion of the roadway’s internal tendons.
And now, the latest and perhaps most worrisome revelation: Almost all the 423 bolts anchoring the span’s single support tower are immersed in water at the base, contributing to further corrosion problems.
“It’s analogous to your house not being properly bolted to your foundation,” says Astaneh-Asl. “That’s extremely serious, especially in a seismically active area. I’ve been aware of this problem for some time, and I actually talked (to some reporters) about it a year ago. Since then, I’ve been working on a technical paper about the issue.”
Astaneh-Asl says there should be 424 of the 26-foot-long, three-to-four-inch diameter rods anchoring the tower to its foundation, but that one of them is missing. Each of the extant rods was fitted with a protective collar before it was set in concrete. The collars were sealed with grout—but apparently water still seeped through to the bolts.
“The real issue isn’t improper sealing, though,” Astaneh-Asl says. “The bolts are set in concrete, the concrete is in the water, and concrete is not completely impermeable. The bolts probably would not have stayed dry in any case.”
The big problem, he adds, is the steel composing the bolts (a variety know as A354BD), which are wholly inappropriate for anchoring bridge towers in a marine setting. “Caltrans has a chapter in their specifications for the bridge that specifically addresses the anchor bolts, and the first line essentially says ‘Do not use A354BD steel bolts.’ ”
But that’s not all. Astaneh-Asl notes the bolts were galvanized by dipping them in hot zinc. Every civil engineer worth his or her sheepskin, Astaneh-Asl says, not only knows you shouldn’t use A354BD steel for marine zone bridge support bolts; they also know that if you must use such steel for such a purpose, you should never, ever, galvanize them with a hot zinc dip.
“By doing that, you infuse hydrogen between the zinc and steel,” Astaneh-Asl says. “This causes hydrogen embrittlement—over the years, the hydrogen starts making the steel brittle, leading to cracking and weakening. That’s the same problem with the fractured anchor bolts (that attached the shear keys). They snapped when workers began tightening them because of hydrogen embrittlement.”
Astaneh-Asl notes that the shear key bolt issue was never truly resolved. Caltrans installed a “saddle” device that the agency maintains fulfills the function of the fractured bolts at a cost of $25 million. (Original estimate for the fix: $10 million).
“Caltrans gives the impression that the remaining bolts are OK, that they were tightened up and everything was fine,” Astaneh-Asl says. “But the remaining bolts were never tightened. And because it was demonstrated that the bolts that had snapped had cracks in 30 percent of their area, it tells me that the (seismic support system) can only take 70 percent of its specified load (from seismic shocks).” Even so, the shear key imbroglio pales before the tower support rod problem, Astaneh-Asl says. Aware that the tower’s support bolts were fabricated from A354BD steel, Astaneh-Asl began running computer simulations on what would happen to the tower under various seismic motion scenarios. He found that if the rods were utterly unaffected by hydrogen embrittlement, everything was fine.
“We used Caltrans’ own specifications for seismic stress and bridge capacity,” Astaneh-Asl says. “If the rods are not weakened at the base due to corrosion, you get close to capacity, but you don’t exceed it. But if you have a corrosion problem, the bridge could only withstand 70 percent of (claimed) capacity. Any stress exceeding that capacity could break the bolts.”
In an email response, Caltrans Bay Bridge spokesperson Leah Robinson-Leach maintains that the risk of anchor bolt cracking from hydrogen embrittlement “was reviewed by a panel of national experts in the fields of materials science, fasteners, and corrosion science and fracture mechanics. These experts recently concluded that the tower base rods are not vulnerable to stress corrosion cracking…”
Robinson-Leach acknowledges that “Caltrans Construction field personnel recently observed the presence of water at the bottom of the tower, near a number of A354BD anchor rods. The source of this water shall be fully investigated and addressed. It is noted that this is not a stress corrosion cracking issue, as the rods are pre-tensioned to levels that are lower than their hydrogen embrittlement threshold. However, this may lead to long-term corrosion and needs to be addressed.”
But if the bolts supporting the bridge’s single tower were to break, the tower could very well collapse during an earthquake; and being the single, unanchored support for the structure, the entire bridge would go down with it. That begs the question: How big a quake would it take? The answer is unclear. The bridge is built on deep sediment, which reacts like Jell-o in seismic events. So it wouldn’t necessarily require the Big One for things to get dicey on the bridge if the tower’s support rods aren’t up to snuff, according to Astaneh-Asl.
“Caltrans won’t comment on seismicity,” he says. “They’ll only say that the bridge is ‘service safe.’ And I agree—the bridge is safe under normal service. Even if the tower’s support rods are compromised, the bridge deck pushes down on the tower’s foundation, essentially stabilizing it. But that’s only a half truth. The problem comes when the bridge is not under normal service—like, during an earthquake. When you get any horizontal motion, the bridge is only 70 percent safe. That’s a significant difference.”
Robinson-Leach counters that the bridge is “both ‘service safe’ and ‘seismically safe.’ ” Further, she writes, “we would be happy to have Mr. Astaneh-Asl present any actual analysis for consideration. We have consistently requested that he do so over the years and to date he has refused. All analysis and external review to date that is publicly available has supported the conclusion that the bridge is safe and the strength of the design as constructed far exceeds the shaking that will (be) generated by the ‘Big One.’ “
Posted on October 9, 2014 - 2:38pm