EuroWire Sept 2015

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September 2015 30
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,” 10th 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.
A detailed description of that design, published on ,
illustrates the situation confronting Mr Maroney and his team of
investigators as 13th 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
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Image: www.bigstockphoto.com Photographer Zsolt Ercsel