Bay Bridge Info

Overview

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.

Broken rods

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.

Solution

The rods could not be replaced as they were embedded in concrete and there was not enough clearance to remove them, as the rods range in length from 9 to 17 feet. Engineers selected steel saddles as the permanent fix for the bolts. The steel saddle system is designed to exert the same clamping force as the original bolt design to hold down the shear keys during a seismic event.  

The saddles sit on top of the base of the shear keys, with steel tendons inside the saddles spreading down either side of the concrete cap beam. Prior to installation, workers prepped the cap beam for several weeks, chipping concrete on either side of the shear keys to allow room for the saddles, drilling into the cap beam to allow steel tendons to pass through, and cutting into the face of the cap beam to allow a better connection with the new concrete that was poured to encase the tendons.

Photos of work to prepare for the saddles can be seen at BayBridgeInfo.org/mediabar.

The saddles were selected because despite requiring more detailed fabrication, installation was determined to be easier and required less drilling of the concrete cap beam. The other solution under consideration was a steel collar. The steel saddle retrofit was successfully installed and declared operational on December 18, 2013 at a final cost of approximately $25 million.

Other Bolts

There are a total of 2,306 similar anchor bolts on the Self-Anchored Suspension Span. At the east pier, there are 1,264, including that batch of 96 that will no longer be used because 32 broke. Other bolts can be found where the main cable anchors into the roadways, the top and bottom of the tower, and the bike path. Based on regular inspection and ongoing testing, the remaining bolts continue to perform as designed; many have been in place for a few years and are not showing any signs of failure. Extensive testing has been conducted on the other bolts on the bridge, including tests for hardness, tensile strength and toughness. Tests for stress corrosion cracking began in July of 2013. An investigative report into the bolts identified which bolts needed to be replaced before the new East Span opened to traffic, which could be replaced after opening, which bolts were acceptable with reduced tension, which bolts were acceptable with augmented dehumidification, and which were acceptable as is, with ongoing monitoring. Only the broken bolts needed to be replaced before opening.