Golden Gate Bridge begins final phase of its seismic retrofit

What happened

The Golden Gate Bridge, Highway and Transportation District, which owns the bridge, calls this work Phase 3B. It is the final part of a retrofit programme that began after the 1989 Loma Prieta earthquake. According to Construction Briefing, the bridge showed no observed damage in that earthquake, but later studies found weaknesses that could matter in a larger one. The district then split the strengthening into phases.

Earlier phases strengthened the Marin approach viaduct, the San Francisco approach viaduct and anchorage housing, and the Fort Point arch and southern pylons. The district says those phases already allow the bridge to withstand a large earthquake safely. The final phase aims for more. It is meant to keep the bridge usable after a large earthquake, so that emergency services can rely on it in the hours and days that follow.

Crews began by lowering temporary access platforms and materials beneath the roadway during overnight closures. At least one lane stays open in each direction. The bridge is about 1.7 miles, or 2.7 km, long and carries up to 40 million vehicles a year, so closing it for the work was not an option.

The district has divided the construction into two contracts. Contract 1 covers the two main towers and the two side spans. Contract 2 will cover the main span. The district's board approved Contract 1 in October 2025 at 863,949,008 US dollars, with a contingency budget of 41.35 million dollars. The total project budget for Contract 1 rose to about 1.01 billion dollars. Construction Equipment Guide reported that this stage is expected to take about six years.

The engineering behind it

A suspension bridge carries its deck on vertical cables called suspenders, which hang from main cables draped over tall towers. The towers carry large vertical loads and must also resist sideways forces. In an earthquake, the ground shakes the foundations, and the towers, deck and cables all move in different ways. Strong shaking can overload connections, buckle braces or cause parts to collide. This is a general description of how such bridges behave.

The board document lists the work in detail. It includes retrofitting the base of the tower legs, strengthening the stiffening trusses under the deck, retrofitting the suspender rope supports, strengthening floor beams and replacing the top lateral bracing. Old thermal expansion joints in the roadway will be replaced with isolation deck joints. Pylon and tower connections will be strengthened, and select deck support pedestals replaced. Structural steel will be cleaned and painted, with old lead paint removed.

One item stands out. The contract includes installing 28 solid state energy dissipation devices. In general, such devices act like shock absorbers. When the structure moves during an earthquake, they absorb part of the energy and turn it into heat, so less energy goes into bending and damaging the main members. Isolation joints serve a related purpose, allowing parts of the bridge to move without forcing each other.

The work also shows performance-based thinking. Rather than only preventing collapse, the design aims for a level of performance after the earthquake, here keeping the bridge open for emergency use. A structure that must keep working after a disaster is often called a lifeline structure. Designing for that goal usually needs more strengthening than designing only to prevent collapse.

What it means in Nepal

The sources do not discuss Nepal, so this section is about the skills behind the project. The main lesson is that much engineering work is on existing structures, not new ones. Retrofitting a bridge while traffic still uses it needs careful staging, temporary works and safety planning. Engineers must understand how an old structure was designed and built, find where it is weak, and design new parts that fit around the old ones.

The project also shows how costs are checked. The district used a contracting method in which the builder joins the project during design. Before the price was agreed, three separate estimates were made by the builder, the designer and an independent cost estimator. The agreed price was within about 3 to 5 percent of the other two. The team also kept a risk register listing possible problems, their chance of happening and their likely cost. These are practical tools that apply to any large public project.

A third lesson is about funding and public responsibility. The district's board document shows that the original budget of 870 million dollars came from federal bridge funds, state funds and the district's own reserves. When the agreed price came in higher, the extra 141.7 million dollars came from district reserves. Engineers on public projects often have to explain such increases to a board in plain language, with the risks and the reasons laid out clearly.