Golden Gate Bridge
Golden Gate Bridge - a beautiful view with a glimpse of the Marin Headlands
When the Loma Prieta earthquake struck on October 17, 1989, the Golden Gate Bridge, one of the most recognizable and revered engineering wonders in the United States, suffered no observed damage. To ensure this result in future earthquakes, the Golden Gate Bridge, Highway and Transportation District engaged a team of structural engineers to plan and execute an extensive seismic retrofit program.
The first step was to conduct a vulnerability study of the 1.7-mile-long span. The study concluded that was that a magnitude 7.0 or greater earthquake with an epicenter near the bridge could cause severe damage and close the Bridge for an extended period of time. This would significantly disupt the flow of the more than 40 million vehicles that use the bridge annually.
Any stronger quake could result in the collapse or impending collapse of several parts of the structure, including the San Francisco and Marin approach viaducts and the Fort Point arch as well as the main suspension bridge.
In 1992, the District hired engineering consultants to develop seismic retrofit design criteria. The recommended seismic retrofit measures consist of various methods of structural upgrades and include both the strengthening of structural components and the modification of structural response of the structures so they can better respond to strong motions without damage.
In the end, the total steel and concrete required for the retrofit project included 22.3 million pounds of structural steel, 24 thousand cubic yards of structural concrete and 5.3 million pounds of reinforcing steel.
In 1996, the three construction phases were established. Construction for Phase I on the Marin (north) approach viaduct began on August 19997 and was completed in April 2002.
A primary challenge in this phase of construction was to construct the retrofit measures under continuous traffic. The most visually dramatic Phase 1 work was the removal and replacement of the four steel support towers with footprints of 50 feet by 75 feet and heights of up to 150 feet. The team sequentially replaced the existing towers with new ones that very closely imitate the appearance of the original towers.
In June 2001, the second construction phase began, and it is the most complex part of the project in terms of design and construction. Without closing the Golden Gate Bridge to traffic, the steel support towers and bottom lateral bracing of the south approach viaduct are being entirely replaced, and seismic isolation bearings and joints are being installed at the roadway level. The historic architectural appearance of the external surfaces of the pylons will remain unchanged.
The Phase II construction site has presented its challenges. Bound on the west by the Pacific Ocean and on the east by very steep slopes, the site is pounded by constant adverse weather including strong winds and high waves. Access consists of two narrow roads that must be shared with thousands of tourists visiting the Golden Gate Bridge and the historic Fort Point site located directly below the Fort Point arch structure of the Bridge.
The $160 million third and final phase of construction will retrofit the main suspension span and the two main towers. Once funded, the project can be completed in approximately four years.