Extradosed in BC.pdf

7/29/2019 Extradosed in BC.pdf

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CIVIL ENGINEERING NEWS

BRIDGESHybrid Extradosed Bridge Under Way in British Columbia

The Golden Ears Bridge, w hich w ill span the Fraser River in British C olumbia, is a low-p rofile, six-lane structure incorporating the characteristics of cable-stayed and pure ex tradosed bridges.

E ngineers designing a new bridgeacross the Fraser River in Brit-ish Columbia had to develop a cost-effective solution that would traverse968 m, accommodate two navigationchannels, and have no tnore than fivespans. Height restrictions imposed bythe presence of a nearby airport lim-ited theheight of the pylons, rulingout such traditional solutions as cable-stayed and suspension bridges. So theengineers devised a 968 mlong con-tinuous hybrid extradosed bridge thatwill accommodate the long spans with-

low-profile, six-lane structure incor-porating the characteristics of a cable-stayed bridge and a pure extradosedbridge, says Don Bergman. P.Eng.,a vice president of Buckland &Tay-lor Ltd., ofNorth Vancouver. BritishColumbia, which is responsible for thedesign and construction engineering ofthe bridge and the approach viaducts.Bergm an notes that in a cable-stayedbridge the vertical loads are deliveredback to the towers primarily throughthe cable stays. In a true extradosedbridge, the deck participates signifi-

ticularly near the main piers," he say"But farther ou t in this bridge, m uch the load is being carried by the staysjust as in a true cable-stayed bridgThe low-profile towers, their steepsloping stays suggesting a harp, algive the appearance of an extradosebridge.The bridge will consist of thre242 ni long main spans and t121 m long side spans and will b32 mwide. The deck was designeto be as light as possible in order tminimize the foundation costs as we


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a longitudinal steel box girder alongeach edge of the bridge. The girderswill be connected by transverse steelfloor beams spaced 5 ni apart from thecenter of one beam to tbe center ofthe next. Tbe two box girders will be1.2 m w ide and w ill vary in dep th from2.7 to 4.5 ni. the deepest sections con-necting to the main piers. Tbe deckwill be composed of precast-concretepanels witb cast-in-place infill stripsplaced betwee n the precast panels overtbe top flanges of tbe floor beams andmain girders. Tbe bridge will be sur-faced witb a bigh-density concreteoverlay.

The pylons will rise just 40 m abovethe deck. Ten cables, eacb comprising31 to 55 strandseacb strand being15.2 mm in diameterwill extend inbarplike fashion at an angle of app roxi-mately 18 degrees from botb sides of

the pylons and connect to the maingirders at the deck.Twin wall piers composed oi rein-forced concrete were selected for thebridge because of tbeir flexibility in thelongitudinal direction, whicb enablesthem to tolerate tbe large longitudinalmovements associated witb such a longcontinuous structure, says Bergman.The lower portion of tbe pier legs willbe protected from direct ship impactsby concrete collars.Because tbe bridge will be locatedupstream of an island where tbe riverbreaks into two distinct navigationchannels, tbe design creates two ves-sel navigation envelopesone on tbenortb 170 m wide and 36.9 m bigh forthe primary n avigation channel and theother 120 ni wide and 16.4 m bigb.The foundation system, which basbeen designed to resist lateral ship

impact loads, will be composed o2.5 m diameter reinforced-concretsbafts. Steel casings will be used for tbeupper portions ofthe shafts. The loweportions will be uncased. Th e sbafts wilreach a depth of approximately 85 to911 ni and will pass through what Bergman describes as soft river depositsThey will be installed tbrough precastconcrete templates that will later befilled with concrete and reinforced toform the pile caps for the main piers.

The bridge is long enough fowind loads and aerodynamics to bof concern, says Bergman. Designeremployed wind tunnel tests to finetune the shape and extent of the leading edge ofthe deck to ensure tbat thbridge will perform aerodynamically.In the longitudinal directionthe cable stays, main box girdersand pylons will form two vertica

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structural planes. The main girders,the upper pylon cable anchor shaft,tbe twill wall legs of the piers, andthe main transverse crossbeams forthe pylons all intersect at what Berg-man calls a knuckle joint. Accordingto Bergman, ensuring that all of th estructural elements fit together .it tbislocation has been a significant chal-lenge, and the design is currentlybenig fin^ilized.Tbe north approach to thebridgewill be appro.xiniately (1.6 km long, andthe south approach will be appro.xi-

niiiteiy 1.2 km. Tbese approaches willconnect to the main spans at a set oftransition piers.According toBergman, tiie bridgeis a low-profile adaptation of tbe AlexFraser Bridge, acable-stayed structure

that also spans the Fraser River andwas designed byBuckland ik Taylor.The new bridge will be located near aKatzie First Nation tribal reservation,and its design will reHect the presenceot this coninuiiiity through aestheticthemes. Railings with elements shapedlike salmon will be incorporated intotliL- design, along with sculptures ofgolden eagles mounted near the top oftbe main pylons.

Tbf Golden Crossing General Part-nership (GCGP) has been contracted forCSl.l billion (U.S.S


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Extradosed in BC.pdf