Cast-in-Place Reinforced Concrete Slab Bridges: - Convey

C A S E H I S T O R Y R E P O R T • N O . 5 9Cast-in-PlaceReinforced ConcreteSlab Bridges:M AKING A G REATS OLUTION G REATERCast-in-place reinforced concrete slab bridges continue to be the most economical,durable, and easy-to-design bridges in use today. These bridges are oftensupported on different types of foundations, or cast in various shapes, or faced withdifferent materials, but the basic function remains the same: the slab serves as themain superstructure member, requiring no other beams or girders to carry the loads.Reinforced concrete slab bridgesspan roads and railroad tracks, riversand streams across the United States,in turn carrying cars and trucks, pedestrians,and trains. Their popularitystems largely from their low cost ofconstruction, their durability, theirsimplicity, and the versatility theypresent to bridge owners anddesigners.Despite the long history, relativesimplicity, and economy inherent inslab bridge technology, bridgeengineers are building new designand construction innovations into this“workhorse,” thereby creatingstronger bridges that can be builtfaster, longer, and more slender.The cast-in-place reinforcedconcrete slab bridges presented hererepresent recent innovations and showtheir exceptional constructibility.“We almost always usereinforced concreteslab bridges when wehave span lengths ofaround 25 feet; we nolonger used T-beamand girder bridges atall. Design charts fortraditional slabs—andfor post-tensioned andvoided slabs—makeour bridge design verystraightforward.”— Michael Cullen, P.E.Senior Bridge Engineer, CaltransStruve Slough Bridge:FASTER ReconstructionThe October 17, 1989 Loma Prieta earthquake shook the two deck-and-stringerStruve Slough bridges on Highway 1 in California so intensely that the bridge pierspunctured the 6.5-inch-thick deck slab as the stringers below the deck separatedfrom the piers. Both bridges partially collapsed.The Struve Slough Bridge carries thousands of cars and trucks per day betweenSanta Cruz, Watsonville, and Monterey. The bridge collapse caused a major inconvenienceto area travelers, and it was imperative that the bridge be rebuilt in theshortest time possible.After evaluating several alternatives, the California Department of Transportation(Caltrans) concluded that a cast-in-place reinforced concrete slab was the mostcost-effective and fastest way to rebuild the two bridges. (continued inside)

Cleveland Road Bridge Replacement:LONGER Single SpanA number of conflictingrequirements faced thebridge owner and engineeron the replacement bridgeover Biscayne Point Canalin Miami Beach, Florida.The replacement bridgerequired increasednavigational clearancefor boaters below, yetrequired decreased horizontalprofile to improvevisibility for traffic above.The solution was ashallow (16-inch-thick),cast-in-place reinforcedconcrete slab bridge.Traditional concrete slabbridge spans are typicallyless than 50 feet long and have a typicalspan length-to-depth ratio of 43:1.The key to this exceptionally longsingle span (65 feet) is two-way actionachieved by combining conventionalreinforced concrete design with posttensioning.The slab structure distributesthe loads to both the parapetsand abutments through #10 (#32) steelreinforcing bars at 6 inches on-center“We were very pleased to be able to satisfy thepublic’s preference for a bridge with no centersupports that could impede boat traffic in thecanal.”— Israel Magrisso, P.E., Senior Project ManagerFlorida Department of Transportation(top and bottom), post-tensioning tendons,and concrete with a compressivestrength of 5,500 psi. The parapets aremonolithic with the bridge deck slaband serve as structural elements.PVC pipes were placed 12 incheson-center in the deck slab to createvoids and thereby reduce the weightof the structure. Large-diameter PVCsleeves embedded within the side-walks serve as sleeves forthe water, gas, and electricutilities.Photo © Kim Walker StanberryThe bridge has becomea true visual asset to thesurrounding communities.But one of the biggestadvantages of the longspan length was that thisbridge was built withoutimpacting the endangeredJohnson’s Seagrass withinthe channel, or the manateesthat frequently swim there.Cleveland Road Bridge ReplacementMiami Beach, FloridaOwner: Florida Department ofTransportation, District 6Engineer: Stanley Consultants, Inc.,West Palm Beach, FloridaContractor: Lanzo ConstructionCompany,Coral Gables, Florida(continued from cover)Struve Slough Bridge...“Caltrans has standard design charts for the superstructure,”says Michael Cullen, P.E., Senior Bridge Engineer withCaltrans. “Therefore, we only needed to custom-design thesubstructure and the connections to bring them up to thencurrentseismic standards. This saved a tremendous amountof time.”Cleveland Road Bridge ReplacementMiami Beach, FloridaOwner: Florida Department ofTransportationEngineer: Stanley Consultants, Inc.Contractor: Lanzo ConstructionCompanyCaltrans designed the 2 replacement bridges and preparedthe plans and specifications in just 10 days. Caltransthen offered an incentive/disincentive construction contract.General contractor CC Meyers removed the old bridges,built the new bridges and approaches, removed a temporaryaccess road, and restored the site in just 55 days, a full 35 days earlier than thecontracted schedule. Of that time, it took 27 days to drive 200 new support piles,

Woolf Avenue Bridge:THINNER Works BetterThe old Woolf AvenueBridge over U.S. Highway 6and the CRANDIC Railroadin Iowa City, Iowa, wassupported on 30-inch-deepsteel I-beams, which onlyleft a 14.5-foot clearancebetween the bridge andthe highway below.The bridge had been hitseveral times by trucks(twice closing the bridgefor extensive repairs). Ithad also deteriorated tothe point that traffic loadinghad to be limited.“The slender cross section you can achieve witha concrete slab bridge is aesthetically pleasing.It also greatly improves the functionality forhighway crossings like the Woolf Avenue Bridgeand for water crossings subject to flooding.”Because of the 71-footspan length, bridge engineersNNW, Inc. originally— Rick Fosse, P.E., City Engineer, City of Iowa Cityconsidered replacing the bridge inkind.“But we thought it would be a encased steel piles) that the cast-in-inclining the support piers (concrete-shame not to improve on the clearance place slab bridge became technicallyproblem,” says Steve Jacobsen, P.E., feasible. Inclining the piles shortenedPresident of NNW. “We knew we the span length to 63 feet. But thisneeded a thinner structure, so welength still exceeded the practical spanperformed some structural modeling to length for the traditional solid reinforcedconcrete slab bridge. Therefore,see if a reinforced concrete slab wouldwork.”to reduce dead weight, a “voidedIt wasn’t until engineers triedslab” was designed to be a mere28-inches thick, and allowedthe vertical clearance toincrease to 16.5 feet.The voided slab, ineffect, acts structurally asbeam elements, in whichthe reinforced concrete atthe top and bottom of theslab resists the majority ofthe bridge forces. Casting16-inch-diameter cardboardtubes 24 inches on-centercreated the voids.Because Iowa City usesdeicing chemicals on itsroads and bridges in thewinter, epoxy coating of thesteel reinforcing bars wasused to increase the longtermdurability of the structure. Thenew bridge now creates a slender,elegant gateway into Iowa City.Woolf Avenue BridgeIowa City, IowaOwner: City of Iowa CityEngineer: NNW, Inc., Iowa City, IowaContractor: Iowa Bridge andCulvert, Inc.,Washington, Iowa...FASTER Reconstructionthe most difficult part of the project. During steel reinforcingbar placement, ironworkers worked vigorously to place thebars for both bridge slabs in just 11 days.Besides working around the clock, the contractor implementedseveral other time-saving innovations. For example,bridge camber was increased by 50 percent so that formworkcould be removed in 7 days, in lieu of the 10 days in thecontract.The contractorStruve Slough BridgesWatsonville, Californiaalso suspended Owner: California Departmentof Transportationthe formworkEngineer: California Departmentfrom the piles, thereby eliminatingof Transportationthe need to erect a more elaboratesupport system.Contractor: CC Meyers,Rancho Cordova, Calif.