EuroWire Sept 2015

Transatlantic Cable

A detailed description of that design, published on wired.com, illustrates the situation confronting Mr Maroney and his team of investigators as 13 th September 2013 – opening day of the new span – approached. The opening took place, in fact ahead of schedule. But with an explanation for its corroded rods as elusive as ever, the testing continues. ‘Massive, threaded steel shafts’ Most of the bridge’s eastern span is a long, low ramp rising out of Oakland to meet Yerba Buena Island. Two side-by-side lanes are supported from below by huge, T-topped piles. As the bridge approaches the island it switches to suspension – anchored by an eastern and western pile – to enable huge container ships to move through the channel below and into the Oakland docks. Below the roads each pile is capped with seismic safety features called shear keys and bearings. When an earthquake hits, these let the bridge sway with the rolling earth, while anchor rods – massive, threaded steel shafts up to 24 feet long and two to three inches thick – keep it from bucking o completely. The rods in the eastern suspension pile were the ones that corroded, snapping in half during pre-opening-day tests. Mr Maroney and the bridge’s governing council decided to proceed with the opening and continue testing to pinpoint the precise circumstances for the failure of the rods. As noted by Mr Stockton: “Even with the faulty bolts, the new bridge was more seismically safe than the old.” He reported that the rst tests took place on the bridge itself, with earthquake-level loads applied by a huge hydraulic jack to 406 suspect rods. Only two came up short, but Mr Maroney decided that safety concerns dictated removal of the rods for further testing. Of course, sharp impact is not the only threat to a rod’s integrity. More speci c to the failed Bay Bridge rods is the Townsend test, which checks what happens to a water-soaked bolt over time. Here, each end of the rod is attached to a massive jack. “Using these huge hydraulic jacks we stretch to increase the load, then let the rod sit in a bath for 48 hours,” explained Mr Maroney. He chose this test because many of the original 32 failed rods did not break when tested in situ but from one day to two weeks later. Mr Stockton of Wired observed that both tests using jacks are “hugely expensive” because they require pulling whole rods from the bridge’s concrete. A local materials tester helped Mr Maroney to develop the Raymond test, which mimics

Two steel bridges

Despite intensive and inventive testing, the failed anchor rods on the new San

Francisco-Oakland Bay Bridge resist explanation “The arm is the business end of a Charpy impact tester: it swings into a thing and, on impact, measures how much energy it took to break that thing. In this case, the ‘thing’ is steel from a new bridge connecting two cities in one of the most seismically active places on the planet. And the steel broke.” Nick Stockton of Wired went on to state the conclusion drawn from the at, glittery inner surfaces of the test piece. To a metallurgist they showed that, in its short time holding together the new east span of the San Francisco-Oakland Bay Bridge, the steel corroded. (“The Mystery of the Brand-New Bay Bridge’s Corroded Steel,” 10 th June) The signi cance of this corrosion can scarcely be overstated. The Bay Bridge does not just span a bay but essentially connects two active fault lines. To the west is the infamous San Andreas, source of the “bridge-busting, building-buckling, World Series- stopping 1989 Loma Prieta temblor,” as Mr Stockton puts it. To the east lies the Hayward, relatively quiet since 1868. But seismologists give it a one in three chance of producing a 6.8 magnitude earthquake by 2036. The fairly low-tech Charpy V-notch method of gauging toughness is only one of the tests that materials scientists are using to determine why several anchor rods securing the newest portion of the Bay Bridge, the busiest in the Northern California region, failed their earthquake inspections. In 2013, seismic tests found that 32 rods had been a ected by water corrosion. Several were pried out of the concrete for testing, and a broader investigation turned up four more compromised rods. Wrote Mr Stockton: “The bridge’s engineers want to pry them out and ship them to labs in Illinois and Alabama that will bang, pull, beat, and twist out the cause of their failure.” The urgency derives from the necessity for the bridge to not only survive the next quake but also to function immediately afterward. “The city is going to need this bridge after a big event because a big event will bring San Francisco to its knees,” Brian Maroney, the Bay Bridge’s chief engineer, told Wired . The bridge is intended to roll with the rumbling ground and the anchor rods are a critical element of its design.

Image: www.bigstockphoto.com Photographer Zsolt Ercsel

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September 2015