A lifeline link: after the wake-up call of the Loma Prieta earthquake, which rocked the San Francisco Bay area in 1989, Caltrans is building what may be the strongest bridge in America

Public Roads, May-June, 2003 by Sybil E. Hatch

The San Francisco-Oakland Bay Bridge officially opened on November 12, 1936, with a 4-day celebration of one of the most remarkable engineering feats of its day. The grand opening was the culmination of 3.5 years of "firsts" in the world of bridge construction: the deepest bridge piers ever attempted, the longest crossing over water ever undertaken, and the world's largest bridge pier, among many other major engineering and construction advances.

"This marks the beginning of the greatest bridge ever erected by the human race," said President Herbert Hoover at the groundbreaking ceremonies in 1933. And, indeed, until October 18,1989, the San Francisco-Oakland Bay Bridge was invincible, carrying hundreds of thousands of cars every day.

The collapse of a portion of the upper deck of the east span during the Loma Prieta earthquake, which registered 7.1 on the Richter scale, reinforced what many already knew: the critical need for seismic safety upgrades on all Bay Area bridges. The deck portion was repaired, but the California Department of Transportation (Caltrans) realized that the east span of the bridge was vulnerable to seismic shaking and the liquefaction that occurs when soils, especially saturated sands, turn into a liquid state.

A subsequent Caltrans study determined that replacing the existing east span with a new bridge would be more cost-effective than seismically retrofitting the existing structure. Thus began the process of creating, designing, and constructing the new east span from Oakland to Yerba Buena Island, which is roughly halfway between Oakland and San Francisco.

Although the new bridge will not boast the number of world records that the original structure did, says Rick Morrow, supervising bridge engineer for Caltrans, "it is equally monumental in its size, complexity, scope, and impact."

Genesis of a Bridge

During preliminary design of the new east span, Caltrans conceptualized a continuous skyway from Oakland to Yerba Buena Island, basically two elevated concrete segmental structures. The design was subsequently modified, based in part on extensive input from public hearings, public comment on the environmental document, and coordination with the Metropolitan Transportation Commission (MTC).

MTC created two project advisory committees: the Bay Bridge Design Task Force and its technical advisory committee, the Engineering and Design Advisory Panel. The task force reviewed the project from the initial stages of choosing the bridge design materials and alignment through detailed development of elements such as lighting and a bicycle and pedestrian path.

A design competition held in 1997 resulted in a number of creative approaches to a "signature span" for the segment over the shipping channel adjacent to Yerba Buena Island. The suggestions came from architects, engineers, and even local schoolchildren.

MTC recommended two combination skyway/cable-supported concepts, each of which included a skyway structure for approximately 80 percent of the entire bridge length. Thirty-percent designs were prepared for both of the concepts, which helped Caltrans and MTC effectively assess and select a final design for the replacement bridge.

Four Bridges in One

In December 1998, Caltrans announced its choices (listed here from east to west):

* A low-rise post-tensioned concrete box girder, called the "Oakland Approach."

* A pier-supported segmental concrete box girder "Skyway"--at 2.4 kilometers (1.5 miles) long, the longest segment of the 3.5 kilometer (2.2 mile) bridge.

* A signature self-anchored suspension bridge, the first to be built in the Unites States and the largest in the world.

* A doubly post-tensioned concrete box girder that will connect to the east side of Yerba Buena Island.

Two temporary steel structures will be designed and constructed at Yerba Buena Island to accommodate traffic detours during construction of the Yerba Buena Island transition structure. Alternatively, a double-deck detour structure may be constructed on the south side of the existing bridge.

What is a self-anchored suspension bridge? Because the geological conditions of San Francisco Bay cannot support economically cable anchor foundations where the new east span is located, the suspension span is self-anchoring, which means the main suspension cables are anchored in the superstructure itself.

A single suspension cable will wrap over the tower and underneath the western end of the span, then wrap over the tower again and anchor in both superstructure boxes at the eastern end. Suspenders will connect diagonally from this cable, which crosses over the roadway to the outside edges of the superstructure. Drivers traveling over the bridge will not only be treated to stunning views of the bay, but also, when reaching the suspension cable section, they will look up to see a breathtaking crisscross of cables overhead.

The new east span will consist of two side-by-side bridges, each with five lanes plus shoulders, and a bicycle/pedestrian path. In addition to addressing seismic concerns, the new bridge will provide critical operational and safety improvements over the current bridge, including shoulders and standard-width lanes. The existing bridge will be dismantled.