THE CONCRETE BRIDGE MAGAZINE

SUMMER 2008

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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ASPIRE , Summer 2008 | 27 SEGmEntAl CASt-In-PlACE ConCREtE / CAlIfoRnIA dEPARtmEnt of tRAnSPoRtAtIon, oWnER drilling SuBContraCtor: Pacific Coast drilling, Petaluma, Calif. EartH Work and minEd SHaftS: ladd & Associates, Redding, Calif. BridgE dESCriPtionS: three-span, segmental cast-in-place concrete single cell box girder constructed by the balanced cantilever method and three-span, cast-on-falsework post-tensioned concrete two cell box girder with inclined piers StruCtural ComPonEntS: South bridge: 68 concrete segments in spans of 348 ft, 571 ft, and 436 ft. north bridge: Cast-in-place on falsework with spans lengths of 175 ft, 229 ft, and 175 ft BridgE ConStruCtion CoSt: South bridge—$37 million; north bridge—$9 million upwards from the river for more than 1000 ft, enveloping the current highway that is benched into the mountain about 240 ft above the river. The landslide area is approximately 3000 ft wide. Geotechnical studies have concluded that the slide is progressively losing strength and there is a high probability that the complex will continue to move in the future. This highway relocation project is an emergency project that is fully financed with federal emergency relief funds. The expedited delivery of this $65.7 million dollar project only took 28 months from the initial planning study phase until award of the construction contract. The complete project includes two bridges, two tieback retaining walls, and a large cut between the two bridges. The bid for the structures work was $49.4 million. The contractor started work in June of 2006 and is expected to complete the project in 2009. north Bridge The smaller north bridge is a concrete inclined leg frame arch with a 229-ft center span and 175-ft end spans. This bridge type fits this particular site well by meeting the requirement to keep the piers out of the ordinary high water limits of the river while still maintaining balanced span ratios. This configuration actually kept the piers out of the higher 100-year water level, which simplified the environmental process even more. The river at this site was contained within a relatively narrow canyon with steep rock walls, which provided suitable foundation material to anchor the inclined piers. The superstructure of the bridge is a cast-in-place, post- tensioned, two-cell box girder with sloping exterior webs. The box varies in depth from 13.8 ft at the piers to 5.9 ft at the ends and midspan. The deck width is 42.8 ft and the bottom slab width varies from 16.8 ft at the piers to 24.7 ft at the ends and midspan. The bridge was cast on falsework that was up to 140 ft tall. The solid concrete tapered piers are anchored into the mountain in 17.5-ft-wide by 6.9-ft-high by 80-ft-deep mined shafts to develop the full probable plastic moment of the piers during a seismic event. The shafts were excavated through very hard rock that required blasting, as well as weathered and fractured rock regions that required rock bolting for stability. C o n c r e t e c o m p r e s s i v e s t r e n g t h was specified as 6100 psi for the superstructure and 5100 psi for the piers. The concrete strength required at time of post-tensioning was 3600 psi. The total post-tensioning jacking force applied at abutment 1 for the full-length tendons was 8776 kips. The tendons consisted of 0.6-in.-diameter, 270 ksi, low relaxation strands. South Bridge The larger south bridge uses a cast-in- place, segmental concrete box girder with normal weight concrete. Like the north bridge, the piers were not only outside the required ordinary high water limits of the river, but also the 100-year flood levels. The south bridge was selected to be segmental because its height above the river was too high for economical construction using falsework. The span lengths of the bridge are 348 ft, 571 ft, and 436 ft. The south bridge, which is 275 ft above the river is a cast-in-place concrete segmental box girder constructed by the balanced cantilever method. Photos: © Jon Hirtz, Caltrans. The north bridge, which is approximately 150 ft above the river, was built as a cast-on-falsework post- tensioned concrete box girder. The lower portion of the falsework (table top) can still be seen below the bridge as well as the temporary access trestle behind the bridge. Aspire_sum08.indb 27 6/24/08 1:40:35 PM

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