BergerWorld: 2nd Quarter 2008

The concept of a bridge spanning the San Francisco Bay had been considered as early as the Gold Rush days of the 1840s, and support for a transbay crossing grew significantly in the 1920s with the increasing popularity of the automobile. Finally, in 1933, construction began on the San Francisco-Oakland Bay Bridge, and the structure opened in 1936. At the time, the 10,176-foot bridge was considered a marvel of modern engineering, as it was both the largest and most expensive bridge in the world. Following damage to the bridge from an earthquake in 1989, the need for a new bridge that could withstand stronger seismic ground motions was clearly established.

Caltrans sponsored an international design competition before selecting a consortium led by TY Lin to provide the bridge design. The $1.4 billion signature suspension span project was subsequently bid and awarded to the joint venture of American Bridge and Fluor. KCBL was then retained by American Bridge and Fluor to serve as lead construction engineers for the self-anchored suspension span bridge replacement. The 700-meter self-anchored suspension span deck system will consist of twin steel orthotropic box girders. The single permanent steel suspension tower will be constructed in lifts using a 165-meter-high, temporary steel tower. The temporary falsework structures will consist of a continuous truss founded on seven braced frame towers. Three of the frames at the west end will be located on Yerba Buena Island on "micropile" or cast-in-drilled-hole pile foundations. The other frames will be located on marine-piled foundations in the bay. The falsework will be in service for a relatively long period; and given the seismic risks in the area, the temporary works must be designed for substantial seismic and vessel impact loading. The steel brace frames will be designed as state-of-the-art eccentrically braced frames using tubular sections. The box girders and crossbeams will be delivered in 42 sections weighing up to 1,600 tons. They will be lifted onto the falsework using a marine crane and placed in cradles, which will slide along the truss on teflon bearings to position the girders for splicing to adjacent girders. At the west end, the girders will be slid up to 200 meters along the trusses. The main suspension tower for the bridge will be a single, four-legged, 160-meter-high, steel tower. It will be delivered to the site in four lifts for each leg and lifted off barges and placed using a gantry crane.

Under a separate contract, A&W was selected by the joint venture of American Bridge and Fluor to provide engineering services for the cable installation and load transfer sequence. To construct the bridge as noted, prefabricated orthotropic bridge deck girders will be erected on falsework to their final position. Following the construction of the tower, the cables will be erected using shop-fabricated parallel wire strands. Suspenders will be erected between the main cable and deck girders and will be gradually jacked into position, incrementally supporting the deck on the cables and relieving the load on the falsework, which will then be removed. The deck girders must be fabricated to an exact geometry taking into account the deformations that will occur as the loads are transferred from the falsework to the cable system. A&W's work will include developing the cambered shape of the girders for fabrication and computing the geometry of the main cables and lengths of the suspenders during each phase of the erection and load transfer. A&W will also review the design of the temporary footbridge that will be used to erect the main cables and assist the contractor in monitoring the bridge geometry during the various erection stages.

Upon completion in 2013, the new San Francisco-Oakland Bay Bridge will be the world's first single-tower, self-anchored suspension span. The monumental structure will meet all modern seismic safety standards using state-of-the-art seismic technology; increase public safety and commuter efficiency by improving lane widths and stopping areas; and allow the local community and tourists to access the Yerba Buena Island recreation area via the addition of pedestrian/bicycle lanes.