Golden Gate Bridge seismic retrofit work moves into final phase

SAN FRANCISCO (KTVU) -- The 20-year-old project to seismic retrofit the Golden Gate Bridge has now moved into its final phase and authorities hope the work will extend the life of the iconic bridge well beyond its current 80 years of service.

The final phase of the multi-decade retrofit project will start next year and will target a section between the two towers. Engineers chose that section to be last because it is more flexible and more earthquake resistant than any other part of the bridge.

The retrofit work will strengthen the bridge into a so-called "lifeline" structure that will remain in operation.

The work also includes custom designed and tested shock absorbers that will cause the bridge to shake a lot less in the event of a large earthquake, including one that would equal or exceed a quake similar to the one that devastated San Francisco in 1906.

That work, however, can only happen once the work platforms are designed and tested so as to be able to withstand winds of 100 miles per hour or higher.

Officials can only move forward by erecting an 8 foot mini-suspension structure inside a wind tunnel to test platform designs that guarantee the safety of workers and the integrity of the bridge.

The rigorous testing is an attempt to prevent a similar collapse that occurred to the Tacoma Narrows Bridge, which failed under the duress of strong winds.

"We expect construction to be complete in 2024," said Priya Clemens, the Golden Gate Bridge District spokeswoman.

The work to retrofit the Golden Gate Bridge began shortly after the 1989 Loma Prieta quake.

The Golden Gate Bridge was not damaged by that quake but the Bridge District initiated a multi-phase project to prevent collapse in the next big event.

"We started with the most vulnerable sections of the bridge," Clemens said.

• Phase One: Completed in 2001, retrofitted or replaced the entire structure on the Marin County side.
• Phase Two: Completed in 2008, did the same for the San Francisco side.
• Phase Three: Completed in 2013, made sure the bridge, even if it were damaged during a major quake, would not collapse.

"So if someone is driving across the bridge and a major quake happens now they won't die," Clemens said. "There won't be any fatalities."

Workers are also adding the suicide barrier net, which was also wind and computer tested. Authorities said the suicide barrier is expected to be completed by 2021.

By KTVU reporter Tom Vacar.