Yerba Buena Island Transition Structure Eastern span replacement of the San Francisco–Oakland Bay Bridge

several construction phases can seen in 2011 image, finished columns falsework erection through formwork completion prior concrete pouring.

left: temporary double deck s-curve (upper deck westbound toward tunnel).

center: southern columns (for eastbound traffic tunnel lower deck).

right: northern columns, falsework, , formwork (westbound tunnel upper deck).

the yerba buena island transition structure (ybits) elevated roadway bridges gap sas span yerba buena island tunnel. oakland touchdown on other side of new bridge, section of bridge end segment, meaning purpose of segment transition portions of existing bridge main spans of new bridge. connecting structure transitions new bridge s side-by-side roadways upper , lower decks of ybi tunnel. in mid-february 2012, northern structure has been poured , formwork being removed. in september 2012, falsework had been removed, modified, , constructed @ eastbound location formwork completion allowing reinforcing , concrete placement.

column design

there number of columns supporting structure. ground level rises shore level of yerba buena tunnel, height of above ground portion of columns varies. since rock structure supporting these hard shale, normal under previous engineering methods dig relatively shallow foundation each column, structural length varying progressively. modern seismic analysis , computer simulations revealed problem such design; while long columns flex several feet @ top (0.6 meter, more or less), shorter columns break, since rigid deck structures cause imposition of similar amount of motion @ tops of columns, imposing more bending stress per unit length on shorter columns. problem solved making columns of similar (but not uniform) length, shorter columns extending in permanent open shafts deep foundations. allows columns of ybits respond in sufficiently uniform manner. space between column , pit covered protective sacrificial cover, forming type of base isolation system @ more sensitive column locations. in addition, western landing of ybits 0 moment hinge, , there no vertical bending stresses @ point.

construction techniques

the construction process build structure consists of several steps, shown below:

ybits construction

the first step construct foundations of large columns support elevated roadway of ybits. above-grade column reinforcing constructed , enclosed formwork , concrete poured. after curing, formwork removed. next step build roadway itself. spans cast in place, using extensive reinforcing, post-tensioned cable tendons. roadways consist of hollow box structures, cast in place in sections using formwork, owing both complex shapes involved , necessity of maintaining traffic flow on adjacent structures during construction.

viewed completed portion of ybits, double-deck tunnel connects eastern , western spans.

the following sequence applied each span between columns:

^ ybits factsheet
^ cbs news video 60 minutes video extra
^ click on yerba buena island cameras fixed see construction of wooden cast