SAN FRANCISCO -- Caltrans will replace 96 giant seismic shock absorbers on the western span of the Bay Bridge that have worn out twice as fast as expected.

Installed in 2004 between the tops of the piers and the deck as part of a major earthquake retrofit of the double suspension span, the silicon-filled steel dampers absorb energy during earthquakes.

But the 17- to 19-foot long and 2-foot-diameter devices also "move with the daily traffic and the wind," said Caltrans spokesman Will Shuck. "The rubber seals are wearing out much sooner than anticipated."

As a result, about a third of the dampers leak.

Maintenance crews keep the devices charged and working, but rather than continue to repack the silicon, the agency has opted to replace them in the next several years at a cost of $13 million. The dampers were originally scheduled for replacement in 2024. The money will come from bridge tolls.

Dampers of various types are commonly used to earthquake-proof buildings, and were first installed in bridges in the early 1990s. Engineers are designing the new dampers to absorb less daily movement, Shuck said.

Contact Lisa Vorderbrueggen at 925-945-4773, lvorderbrueggen@bayareanewsgroup.com, politicswithlisav.blogspot.com or Twitter.com/lvorderbrueggen.