American government investigators are to investigate why bolts that have snapped on the eastern span of California’s new Oakland Bay Bridge were specified despite being banned from other projects.
The Federal Highway Administration (FHA) said it would grill the California Department of Transportation (Caltrans), which owns and operates Bay Bridge, about the troubled project and its proposed $10M (£6.6M) remedial work programme.
Crews working on construction of the all-steel, self-anchored suspension section of the crossing, which makes up part of the link between Oakland and San Francisco Bay, snapped a number of bolts in March as theytightening them
(NCE 11 April).
Engineers have determined that the 76mm diameter bolts, which range in length from 3m to 8m, broke due to the presence of hydrogen during galvanisation, leading to what Caltrans described as “hydrogen embrittlement”.
So far 32 hardened steel bolts have cracked but there are 2,300 throughout the £4.2bn eastern span, including at the base of the bridge’s 160m tower – where they are difficult to access.
Where the damaged bolts are visible on the bridge supports, which support the road decks above, they are part of a seismic structure designed to counter movement generated in the event of an earthquake.
Caltrans has proposed to secure these bolts by strapping on a “saddle” held in place by high tensile steel cables, but this plan is subject to FHA approval.
The department is also under increasing pressure to explain why it specified the galvanised bolts when it had already banned them from use on “typical” bridges.
Last week Caltrans director Malcolm Dougherty and chief engineer Brian Maroney told California’s Senate Committee that they were aware of the risks but decided they were worth taking.
“Absolute due consideration by all the technical teams [happened] before we moved forward with that decision,” Dougherty told the committee. He added that Caltrans had its “eyes wide open” to the potential risks.
Maroney said a Caltrans team of engineers and consultants had signed off on “these rods [the bolts], these loads, these conditions”.
Maroney and Dougherty refused to reveal who made the final decision, but they promised the committee they would provide a timeline clarifying the decision making process.
WSP technical director John Parker said in the UK any engineering solution would have to be signed off by an independent third party.
“An ‘approval in principal’ document is sent to the bridge authority and then an independent check is carried out,” he said.
“So if we designed a bridge, maybe Atkins would be appointed to check it,” said Parker.
“Ultimately if the client insisted on something you disagreed with I suppose you would have to resign.”