OAKLAND -- Spurred by a catastrophic bolt failure earlier this year, Bay Bridge builders flipped the switch Friday on an unprecedented test program that will put nearly every type of high-strength steel fastener used on the span under the microscope.
"No other construction job has done this," said Caltrans resident engineer Bill Casey on Friday morning at the bridge's shoreline construction headquarters in Oakland.
Named for engineer and inventor Herb Townsend, the test accelerates the effects of a corrosive marine environment on high-strength steel under loads simulated to mimic the bridge structure. Over time, rust exposes the steel to hydrogen, which can invade its granular structure and leave it brittle and vulnerable to fracture.
The 32 anchor rods that snapped in March succumbed to a short-term version of the same phenomenon.
They were the only fasteners that broke, and none of the other 2,210 high-strength bolts installed on the bridge show any signs of a similar malady, Caltrans and its consultants have said.
The agencies overseeing the construction have abandoned the 96 anchor rods and are installing a $10 million to $15 million exterior saddle and cable assembly that will take the loads.
But the failure raised broad questions about whether the rest of the bolts could be susceptible to long-term stress corrosion. There are 17 different types of high-strength steel fasteners of varying sizes and loads on the new self-anchored suspension span, including the 32 that snapped. They were among 96 embedded in seismic stabilizers on the large pier east of the main span tower.
Several types of bolts are sealed inside bearing assemblies and cannot be tested, but engineers say these are protected from the elements and unlikely to suffer corrosion.
In the first round of Townsend tests that started Friday, technicians placed the ends of four threaded anchor rods -- 22 feet long and 3 inches in diameter -- against a solid surface and a hydraulic jack on the other end. Their threads have already been submerged in a heavy saltwater solution.
Technicians will increase the load every two days for 17 days. On day 18, they will crank the load up to 85 percent of the bolts' capacity and hold it five days. By way of comparison, the highest tension placed on a bolt on the bridge is 70 percent.
This is a massive test by almost any standard.
Each individual anchor rod in the test rig holds 800,000 pounds -- the equivalent weight of 67 African bull elephants -- and if and when it cracks, the engineers want to make sure no debris flies around.
So they flanked each of the four test rigs with 6-foot segments of concrete freeway rail stacked four deep and then heaped sandbags over them.
In addition, technicians will remotely monitor the strain gage, which measures whether the rod deforms, and the acoustic sensor, which records the sounds of cracking.
If, when and at what level the bolt in the test rig breaks will "tell us whether that particular type of bolt needs to be replaced on the bridge or whether we could solve the problem by reducing the tension or through some other method, like a dehumidifier," said Bay Bridge materials science engineer Mazen Wahbeh. "Remember, it takes all three things to trigger a failure -- tension, susceptible material and hydrogen."
Smaller samples of the tested bolts will then undergo another round of standard analysis for chemical properties, strength and ductility, and metallurgists will examine under a microscope what happened to their grain structures.
Seven more test rigs will be added to the four already in place. Testing is expected to continue for months, and Caltrans will use the results as the basis of a detailed, high-strength steel bolt maintenance plan, the engineers said.