Plink goes the pendulum arm as it smashes into a 3-inch section of steel. The sound is high-pitched, which tells any metallurgist with a good ear that the piece is brittle. 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. The flat, glittery inner surfaces show that in its short time holding together the new Bay Bridge, the steel corroded.

This relatively low-tech method is among a battery of tests that materials scientists are using to determine why several anchor rods securing the newest portion of the San Francisco Bay Bridge, the region's busiest, failed their earthquake inspections. The first alarms sounded in 2013, when seismic tests found 32 faulty rods. They'd been sitting in a large pool of water, corroding. Many were pried out of the concrete for testing, and the failures prompted a broader investigation that turned up four more compromised rods. 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 Bay Bridge doesn't just span San Francisco Bay—it practically connects two active fault lines. To the west, cutting up through the city of San Francisco, is the infamous San Andreas, source of the bridge-busting, building-buckling, World Series-stopping 1989 Loma Prieta temblor1. To the east, running through the East Bay, lies the Hayward Fault. It hasn't shaken seriously since 1868, but seismologists suspect it has a one in three chance of producing a 6.8 magnitude earthquake by 2036. And it cuts right under UC Berkeley's football stadium.

It isn't enough for the bridge to survive the next quake; it must function immediately after. "The city is going to need this bridge after a big event, because a big event will bring San Francisco to its knees," says Brian Maroney, the new Bay Bridge's chief engineer. The bridge is designed to roll with the rumbling ground. The anchor rods are a critical part of that design, which is why Maroney is putting them through such intense technological deliberation.

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—so huge container ships can make their way through the channel below and into Oakland's iconic docks.

The Bay Bridge's suspension portion is basically three parts: The middle tower holds the cable high, and the two ends pull it out, providing adequate tension for smaller cables to bear the weight of the bridge's spans. California Department of Transportation. California Department of Transportation

Below the roads, each pile is capped with seismic safety features called shear keys and bearings. When an earthquake hits, the keys and bearings 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 off completely.

It's the rods in the eastern suspension pile that corroded, snapping in half during pre-opening tests. Maroney and the bridge's governing council decided to proceed with the opening (even with the faulty bolts, the new bridge was more seismically safe than the old) and continue testing to pinpoint the precise circumstances for the rods' failure.

The first tests took place in the bridge itself, where Maroney and several others applied earthquake-level loads to 406 suspect rods using a huge hydraulic jack. Only two came up short, but Maroney made the case that it was vital for the safety of the bridge to remove the rods and send them for further testing.

That's where the Charpy test comes in. It's a measure of toughness, "a simple term for the measure of how much energy is absorbed before something fractures and lets go," says Maroney. A paper clip put into the Charpy tester would bend significantly before snapping. This means it is incredibly tough. A piece of chalk, however, is incredibly brittle and would immediately snap. Not tough. Neither were the rods.

But sharp impacts aren't the only threat to a rod's integrity. The Townsend test specifically checks what happens to a water-soaked bolt over time—which seems to be what did in the failed Bay Bridge rods. For this test, each end of a rod is attached to massive jacks. Close to one of the jacks, the rod is soaking in a pool of water. "Using these huge hydraulic jacks, we stretch to increase the load, then let the rod sit in a bath for 48 hours," says Maroney. This test is necessary because many of the original 32 failed rods didn't break at first, but anywhere from one day to two weeks after the in situ tests.

Both tests are hugely expensive, because they require pulling whole rods from the bridge's concrete. So Maroney found a way to test the same types of strain on a smaller scale. "This is when we turned to Lou Raymond in Orange County," he says. Raymond, a veteran materials tester, helped Maroney develop a test that mimics the pressures of a Townsend test at a much smaller scale and using completely different mechanics.

Rather than pulling the rod apart at both ends, the Raymond test bends rectangular cross-sections similar to those used in the Charpy test. To visualize, "hold a pencil between your two hands, and use your thumbs to press up from underneath," says Maroney. Though the Townsend and Raymond tests might seem wholly different, a battery of mathematical calculations confirmed that they essentially test the same types of strain.

These three tests aren't the only ones the rods are going through, just the most kinetic (and most fun to read about). There are a variety of other measures, both mechanical and modeled, that Maroney's materials advisors will use to tease out the cause of the failures. Nor are the anchor rods the Bay Bridge's only problem. Water corrosion threatens rods under the suspension bridge's main tower. There are misaligned deck sections. And inspections have turned up substandard welds in the tower and roads.

But right now, the main focus is on these rods and what caused them to corrode where others did not. Maroney says the failed rods all came from the same batch, which he calls the 2008s—after the year they were fabricated. Though he couldn't yet say for sure, Maroney points to overcomplicated manufacturing techniques. He says that if he were looking at a brand new bridge project, he'd order two sets of rods from the outset: "One to test rigorously, destructively before accepting the second batch," he says. "I would spend a million or two on extra testing before, because we spent 10 million after."

What happens next depends on the outcome of the tests. Maroney says as a careful engineer, he doesn't start trying to figure out solutions until he has data in hand. At the very least, it will change the bridge's future maintenance protocols. And the engineers of the Bay Bridge hope that future will be a long one.

1 Correction: 2:22 ET 06/10/15 Originally, I had written "tremblor." Actually, the word seismologists use is "temblor," which is derived from the Spanish temblor, which means shaking, trembling, or (duh) earthquake.