baybridgeinfo – excerpt
The eastern pier of the Self-Anchored Suspension Span contains seismic devices called bearings and shear keys. The bearings allow the road-decks to move slightly during a seismic event, while the shear keys prevent the decks from moving too much. The four bearings (two beneath each deck) and four shear keys (one beneath each deck and two beneath the cross beam connecting the decks) are bolted between the roadways and a concrete cap beam with steel anchor rods. When 96 rods were tightened to connect two shear keys to the cap beam in March 2013, 32 broke.
The rods could not be tightened until the completion of load transfer, the process of shifting the weight of the suspension span from the temporary supports to the main cable. The process was completed in late 2012.
There are a total of 288 3-inch diameter rods, which range in length from 9 to 24 feet, anchoring the bases of the seismic devices to the top of the cap beam. Each bearing has 24 rods, and each shear key has 48.
On March 1, workers began stressing the 96 rods fabricated in 2008 for the two shear keys; between March 8 and March 15, 32 fractured rods were discovered.
Engineers and metallurgists have determined that the bolts broke due to hydrogen embrittlement, which requires a source of excess hydrogen, susceptible material and tension. Ongoing metallurgical analysis revealed that the bolts were susceptible due to the steel being harder on the outside than in the middle, or a lack of uniformity in the steel’s microstructure. The steel also showed low toughness and marginal ductility (the ability to stretch).
The excess hydrogen caused the threaded areas to become brittle and fracture under high tension when the bolts were tightened. An ongoing investigation is looking into the source of the excess hydrogen, which may have been both internal (i.e. residual from production) and/or external… (more)