Main span tower Eastern span replacement of the San Francisco–Oakland Bay Bridge

first stage tower segments showing cross section , attachment methods. lower external gray areas covered sacrificial box structures ( mechanical fuses ), while upper covered external flat plates numerous fasteners join segments.

the design employs extensive energy absorbing techniques enable survivability , immediate access emergency vehicles following maximum creditable earthquake (mce), estimated @ 8.5 moment magnitude in 1500-year time span. rather designing rigidity, instead flexible structure, resonant motion absorbed plastic shear of sacrificial, replaceable components. smaller earthquakes impose elastic stresses on components, higher proportion of plastic (and energy absorbing) stresses in larger earthquakes. design philosophy extends other metal components of bridge, including sacrificial tubular end keys align self-anchored suspension approach structures @ each end.

the tower consists of 4 columns. each pentagonal column consists of 4 tapering and/or straight sections, joined end-to-end external plates , internal stringer finger joints secured fasteners. columns joined horizontally sacrificial box structures. these box joins intended absorb earthquake-induced motion elastic , plastic shear deformation tower sways. under severe earthquake, deformation absorbs energy otherwise lead destructive tower motion, protecting primary structure of span. expected design allow immediate use of bridge emergency vehicles, joins being replaced needed restore bridge original condition. uniquely, tower has no direct connection roadbeds, enough space allow swaying under severe earthquakes without collision.

tower construction

march 4, 2011: phase 4 4 columns in place; jack-up crane (to left) used erect , dismantle scaffold, , gantry crane atop scaffold lifts , places tower columns.

the process build sas tower atop foundation consisted of 5 phases. first 4 phases each consisted of lifting 4 similar columns , bolting them place , elements connecting them, while last phase lift final top cap carry crowning main cable saddle. on july 28, 2010, first of 4 below-deck main tower pillars erected, having arrived earlier in month barge china. placed lifting 1 end barge temporary scaffold, carriage on barge allow lower end move place. after columns bolted place, scaffolding extended upward allow next set of above deck columns erected, lifted, , translated position, process repeated each of remaining phases.

tower erection continued when second set of columns arrived in week of october 24, 2010, 3 months after first set placed. second set of columns erected gantry atop scaffold , placed on first 4 columns placed earlier in year. after columns set place, bolted first set of columns. after second phase complete, tower 51 percent completed , stood @ height of 272 feet. third set of tower columns did not arrive until week of december 15, 2010. third set, larger crane, lifted , placed on second set of columns. tower stood @ impressive height of 374 feet , 71 percent complete. erection process did not continue until following year when final set of tower columns arrived valentine s day 2011. these 4 columns, each being 105.6 feet tall, lifted in week of february 28, 2011 , placed on third set of columns. tower stood @ height of 480 feet , 91 percent complete.

april 15, 2011: grillage in place.

the fifth , final tower phase lift grillage (a structure join columns, more commonly used foundation element) weighs 500 tons, lift main 450-ton cable saddle, , lift final tower head completed entire sas tower. of these final pieces arrived @ site same day fourth set of tower columns arrived. on april 15, 2011, first part of fifth , final phase began. 500-ton grillage lifted 500 feet in air , placed on fourth set of columns. tower stood @ height of 495 feet , 94 percent complete. took 1 day lift , place grillage on top of tower.

crowning double cable saddle emplacement

may 19, 2011: near sunset, cable saddle being positioned before final touchdown.

working entire day of may 19, 2011, operating engineers , ironworkers lifted , placed 900,000 pound double cable saddle atop sas tower. while large portion of span fabricated in china, particular piece made in japan, eastern , western deviation saddles , main cable hydraulic jacking saddle.

this cable saddle guides , supports mile-long main cable on tower placed later in year. in december 2011, deck placement of sas span completed , cable construction progress began. however, few months before in july 2011, tower head lifted , placed on saddle in test fitting , removed allow laying of cable. later on in 2012, cables placed on tower saddle , anchored throughout whole sas span. tower head permanently installed final time, along aircraft warning beacons, completing entire sas tower @ final height of 525 feet (160 m).