Devices designed to absorb the violent shaking to the Golden Gate Bridge during a major earthquake have tested well and will be part of the span as a seismic upgrade continues.
The design of the final “Phase 3B” — addressing the center suspension portion of the span — is now underway and bridge officials hope to get bids out on the work next year.
Part of the process involves developing and installing seismic energy dissipation devices near the ends of the suspension bridge trusses, which run in a latticework underneath the roadway.
The bridge board approved $1.5 million for testing in early 2015 and the results were issued Friday. The news was good.
“The design of the (devices) showed energy dissipation at the predicted level,” Ewa Bauer, chief engineer of the bridge, told the bridge board. “In all, it’s a great success.”
The testing was done at the State University of New York at Buffalo and University of California at San Diego.
The devices — which measure up to 16 feet — include two metal pieces clamped together. One of the pieces is stainless steel, the other tin bronze. During the force of a large quake the two pieces rub together and the tin-bronze piece absorbs the energy from the steel, dissipating the destructive shaking to the span.
“We want to limit the energy that will be transferred to the trusses,” Bauer said. “We are lowering the magnitude of forces that the trusses have to carry. The bridge moves and it will move in a quake. But we want to control the magnitude of the movement and we will do it with the help of these (devices). They will stop the trusses from swinging out of control.”
The damage would be limited to the energy dissipation device and can be repaired relatively easily. Bauer said the system works better than dampers, which wear out over time because of traffic and other pressures put on bridges.
Work for the final phase of the seismic effort is pegged at between $450 million and $480 million, with bridge officials seeking funding in Washington, D.C. The design will include 38 of the energy dissipation devices for the span.
It has been a long journey for the project, which began in 1998. Initially the work was to have cost roughly $300 million but has since ballooned to more than $900 million as prices have gone up over time and bridge officials were forced to further gird the bridge after the 9/11 attacks.
The terror attack, which occurred in the early stages of the seismic retrofit project, forced bridge officials to rethink their plan beyond earthquakes.
Officials examined ways to “harden” the towers “to protect against blast” in 2005. Additional computer modeling to determine how strengthened towers would react during an earthquake were among the issues studied.
Because all the funding was not available from the start, the retrofit phases have been staged by importance. The approaches were the most vulnerable to shaking, the suspension portion the least.
Work on the first phase of the seismic project, strengthening the north approach, was finished in spring 2002. There was concern that section of the span could receive severe damage and possibly collapse in a strong earthquake.
Work on the second phase, along the south approach, was finished in 2007. “Phase 3A” was wrapped up in 2014 and saw the strengthening of the north anchorage house — the box-like structure the length of a football field, which holds all the cables coming off the span that tie down the structure.
Presently the bridge is safe, with span officials noting the north and south ends could sustain an 8.3-scale quake. The suspension section could withstand a large earthquake, bigger than a 7.0, but there would be extensive damage and the bridge would likely have to be closed for an extended period.
There was other positive news Friday for the 79-year-old bridge. Federal inspectors rated the structure “good” in terms of condition after recent inspections. Only an “excellent” is a better designation and typically given to new spans.
“For a bridge of this vintage, it’s exceptional to have this rating,” said Denis Mulligan, bridge manager.
