ASPIRE Summer 08 - Aspire - The Concrete Bridge Magazine

For both bridges, the piers were locatedoutside the 100-year flood levels.Looking south at segmental constructionof the south bridge pier 2 cantilever withform travelers. Abutment 1 can be seenin the background. Photo: © Jon Hirtz,Caltrans.The superstructure varies in depth from31.5 ft at the piers to 11.5 ft at theends and midspan. The single cell, boxgirder cross section has vertical websand is post-tensioned longitudinallyand transversely. The box girder has adeck width of 42.8 ft and a bottomslab width of 23.8 ft. The bridge isbeing built by the balanced cantileverconstruction method from each pierand casting on falsework near theabutments. The contractor is utilizingone set of conventional form travelers,constructing the pier 2 cantilever firstand then moving the travelers over topier 3 to construct the final cantilever.The cantilevers on each side of thepiers consist of 17 segments. The firstfour heavier segments are 13.1 ft longwhile the remaining 13 segments ofeach cantilever are 15.4 ft long. Theclosure segments are 12.5 ft long. Theheaviest segment weighs 200 tons. Thepier table length is 45 ft and is 7.5 ftout of balance toward the center spanside of the pier so that during segmentproduction the cantilever will not bemore than one-half of a segment out ofbalance. The first segment cast is on theend span side of the piers.The heights of the piers from top of thefooting to the top of the bridge deckare 200 ft. Because of this height, it wasdetermined that it was more economicalto use a hollow pier section. Thehollow piershave heavilyconfined cornerelements usingw e l d e d N o .10 reinforcinghoops spacedat 4-3/8 in.o n c e n t e r sto get the necessary ductility to meetthe California Seismic Design Criteria.The solid pier footings and caps wereclassified as mass concrete and requiredchilled water to be pumped through theelements to keep the heat of hydrationbelow specified limits. The footingsincorporated eleven 5-ft-diameter castin-drilledhole piles. The piles are upto 136 ft deep and have a nominalcompression resistance of 4382 tons perpile. The footing dimensions are 36 ftlong by 49 ft wide by 10.5 ft deep.Construction of the superstructurebegan at the pier 2 table in October2007. Early on, the contractor was ableto achieve two segments per week perpair of travelers on the first cantilever.The contractor has plans to increasesegment production to three segmentsper week prior to completing this firstcantilever. The bridge deck incorporatesan integral overlay in which an additional1 in. of cover has been provided to thedeck reinforcement for profile grinding.The bridge has also been designed tocarry a future wearing surface.The design of the bridge utilized the1990 CEB-FIP Model Code for ConcreteStructures to model the time-dependentcreep and shrinkage characteristics ofthe bridge. Testing of the contractorsmix surprisingly showed that the actualshrinkage was twice that predicted in theTypical section for the cast-on-falsework north bridge.The depth varies from 13.8 ft at the piers to 5.9 ft atthe ends and midspan. Photo: © Caltrans.Rendering of thecompleted south bridge.Pier 2 is on the right side of photo. Photo: © Caltrans.