The Bay Bridge: ready for the next challenge

The bridge across the San Francisco Bay is rebuilt and retrofitted for the next seismic event

It might take up to a thousand years for the next earthquake to hit, but when it does, the new and retrofitted bridge across the San Francisco Bay will be ready for it.

The San Francisco-Oakland Bridge, better known as the Bay Bridge, was one of the longest and most expensive undertakings in recent memory in California. Stretching 13.5 kilometres (8.4 miles) to unite the cities of San Francisco and Oakland, it underwent major improvements after being damaged by the Loma Prieta earthquake in 1989. It took nearly a decade of work at a cost of $6.4 billion before the self-anchored suspension bridge reopened to traffic in 2013. Of all the work done to make it resistant to the next earthquake, the most extensive was the replacement of the East Span, which stretches from Oakland to Yerba Buena Island, the mid-point of the bridge.

“Despite the journey's length, it has been completed before the arrival of our next big earthquake,” Steve Heminger, executive director of the Metropolitan Transportation Commission, which is responsible for transportation planning in the San Francisco Bay area, was quoted as saying by the Associated Press (AP) news agency at the opening ceremony. “And thank goodness for that.” Californian state Lieutenant Governor Gavin Newsom said he hoped it would inspire young engineers to pursue similarly ambitious projects. “This is more than just connecting two land masses,” he was quoted as saying by AP. “I hope that the progress that's being represented at this moment is for a generation to dream big dreams and to do big things.'

Although the original bridge had stood proud since it was erected during the darkest years of the Great Depression, it was laid low by the 1989 earthquake that registered high on the Richter scale at about 7.0. The tremor was so strong that a 250-ton (226.7 tonne) section of the East Span’s upper deck collapsed. The scale of the damage was such that officials eventually decided to replace the entire span.

Construction of the new Bay Bridge

According to the California Department of Transportation, engineers made sure its replacement would resist the next earthquake by using so-called “rock motions” to calculate the maximum seismic forces it would experience. Rock motions are the vibrations that travel through the bedrock when a fault deep under the Earth’s surface slips. “The new East Span has been designed to withstand rare seismic events,” reads a department website dedicated to the bridge. “Specifically, the span has a 1,500-year return period. This is defined as the largest rock motions expected to occur at the bridge site once every 1,500 years.”

The new East Span includes 1.2-mile (1.9-kilometre) Skyway; a Self-Anchored Suspension span consisting of a 525-foot (160-metre) tower supporting a bridge deck connecting the Skyway to the Yerba Buena Island Transition Structure; and the Oakland Touchdown, the east end of the bridge connecting to the toll plaza.

Meanwhile, the West Span, the part of the bridge that links the island to San Francisco, escaped the wrecking ball but underwent significant repair and retrofitting. A one-mile (1.6-kilometre) stretch of Interstate 80 and three on- and three off-ramps were replaced. The double-deck feature of the West Span was heavily reinforced, and a column was added to support each deck on the West Approach, according to the website.

Although the West Span still features double decks, the East Span no longer does. The two decks are side by side to give drivers more expansive views.

When the original bridge opened in 1936, it was the longest in the world. Its builders, American Bridge Company for Columbia Steel Company - both subsidiaries of United States Steel - defied critics who thought it was impossible to build the bridge due to the turbulent waters and strong winds, among other factors.

Cars drove in both directions on the upper deck, while trucks and trains travelled in both directions on the lower deck. This double-deck feature allowed the bridge to support a large flow of traffic. In its first year, it saw the passage of nine million vehicles. Traffic would inevitably increase in subsequent decades, rising to more than 100 million a year.

The structure was the most dangerous and expensive project in the country at the time. On the western side were two suspension bridges joined in the middle by a massive central anchorage 30-storeys high with more concrete than the volume used to build the Empire State Building in Manhattan.

Speaking in a documentary film commemorating the bridge’s 75th anniversary, Bob McKee said he was awestruck when he joined his parents for a drive across the structure on the opening day. “It was very exciting for (an) almost five year (old),” he said, remarking that the excitement of so many other drivers had gotten the better of them. “There were a lot of cars and a lot of accidents.”