The Yerba Buena Island Transition Structure (YBITS) will connect the Self-Anchored Suspension (SAS) span to Yerba Buena Island (YBI), and will transition the East Span’s side-by-side road decks to the upper and lower decks of the YBI tunnel and West Span.

As with all of the Bay Bridge’s Seismic Retrofit Projects, crews must build the YBITS close to moving vehicles without disrupting that traffic. To accomplish this daunting task, eastbound and westbound traffic will be shifted off the existing roadway onto a temporary detour supported by 200-foot-tall steel towers. The detour will run parallel to the existing lanes on YBI and will tie into the existing bridge and tunnel. Drivers will use this detour, just south of the original roadway, until traffic is moved onto the new East Span.

The first in a series of phases to build the temporary 900-foot-long detour, as well as the YBITS, took place during 2007's Labor Day weekend. During his historic milestone, the entire Bay Bridge was closed - for the first time since the Loma Prieta earthquake - so crews could replace a 350-foot-long, 6,500-ton section of viaduct on YBI. Crews replaced this with a seismically upgraded section of roadway that will also serve as a connection to both the YBITS and detour. This marked the completion of the first part of the new bridge that drivers are already using.

Shifting traffic to this temporary detour will allow crews to attach the western end of the SAS span to the viaduct. This shift to the temporary detour will be the most significant realignment on the bridge to date. To accomplish this, crews will cut away a 288-foot-portion of the existing truss bridge and replace it with a connection to the detour. This maneuver - the most dramatic yet - will involve aerial construction that occurs more than 100 feet above YBI. When the Bay Bridge reopens to traffic, vehicles will travel on the detour until the completion of the new East Span, including the YBITS. The first steel trusses for the detour are already in place, and workers continue to install the 4,000-ton trusses.

•