SubmissionfortheNEWZEALANDCONCRETESOCIETYCONCRETEAWARDS2015

OKURARIVERBRIDGE

SubmittedbyDavidHepburnBridgeConceptsLtd

SUMMARYMasonryarchestransmitstructureloadstotheirabutmentsandfoundationsbytransferringcompressionforcesthroughthearchring.

Archbridgeshaveprovedthemselvestohavelivestypically10timesgreaterthanother types.Somehavebeen inuse for2000years.Theybecameexpensive toproduceandtheabilitytodesignthemwaslost.Believingthattheycouldbere-engineered for economy and speed, the Bridge Concepts team set aboutdevelopinganewarchbridgeforNewZealandmarkets.

ThebridgeatOkuraRiverrepresentsamajorstepinthatprocess.Itisbuiltfrom6x1mwidelightweightprecastconcreteringsectionswhichareliftedtogethertoformanarchwhichthenhasadenseconcretesaddleapplied.Theconceptofmakinglightweightsectionsthatarestillstrongenoughtocarrytheloadofthesaddle is entirely new. So is the system of analysis created for dealing withearthquakeloading.

Aloadtestshowedthebridgetodeflectabout1mmundera10-toneaxleload.Thatis1/7000ofthespanorabout1/20thofthedeflectionofnormaldesigns.

THEPROJECT

The original conforming design for the bridge crossing comprised aconventionalboltedsteelarchculvertfacedoffwithhandplacedandgroutedrockwallswithtenderpricingcominginrelativelyhighduetositeaccessandunderlyinggroundconditions.

Fraser Geologics Ltd in conjunction with Gibbons Contractors proposed arange of alternatives and settled on the Smart Arch alternative of BridgeConcepts as providing the developer with a more cost effective, similarlooking structure while addressing underlying ground conditions throughpiling the arch structure anddetailing the flanking spandrelwalls forminordifferentialsettlementandgroundmovement.

Construction proved relatively straightforward with spandrel walls andabutments constructed prior to arch installation, sidewall placement andbackfilling.Providedtheinitialgroundworkiscompletedfirst,theSmartArchsystem lends itself to rapid construction and backfilling and is considered

particularlycosteffectiveasanalternativetoprecastboxculvertsorshorter(nominally 7-12m) span bridges, particularly where the client needs tominimize in stream works and wants to achieve an aesthetically pleasingstructureaspartofoveralllandscapingdesign.

The Smart Arch comprises a precast arch systembased onmodern precastconcrete replacing the masonry blocks with adjacent precast faces fullytouching and avoiding the need for mortar and other detailing to transfercompressiveforces.

Construction

Constructionbeganwiththeinstallationof4boredpilestocreatefoundationsinsoftground.Aroundthese,StoneStronghollowretainingwallblockswereplacedonasimplefoundation.Thepilereinforcementwasbentovertolocktheblockstothepilesandtheblocksfilledwithconcretetoformabaseforthearches.

The arch unitswere delivered flat to site and lifted by an 80-t crane into archshape prior to placement. Timber wedges were used temporarily to providereactionforthethrust.Precastspandrelwallswereassembledfrom3piecesofprecast elements and tied across to provide short-term resistance to pressurefrom the concrete saddle that was then placed in a low slump low strengthconcretemixcapableofbeingrammedintoplacewithoutslidingoffthearch.

Finally thewalls were filled to level with normal compactedmaterial and twomorecoursesofStoneStrongblockswereplacedandfillplacedbetweenthemtobringtheroadtothe levelrequired.Thebridgecouldbeusedfortrafficonly7daysafterthelightweightarchringswereplaced.

StatementofFeatures

Longhollowplanksthatusedtoformarchringswerecasttofittoformafacetedarchcapableofcarryingsubstantial load.Theyhavewallsonly50mmthickandaremadefromself-compactingfibrereinforcedconcrete.

The hollow planks of the arches have no conventional steel reinforcementallowingforlonglastingmaintenancefreedesignlife.

Thearchesweredeliveredflatandwerelightenoughforthewholebridgetobecarriedonasingletruck.

Eacharchringwasliftedandplacedinasingleliftbyan80-tcrane.Thesamecraneliftedandinstalledelementsoftheprecastspandrelwall.

TheuseofStoneStrongblockstoformtheabutment,andpartsofthespandrelwallsrequiredminimumfoundationsandnoformworkonsite.

The construction methods used ensure that working in waterways can beavoided and therefore the waterway is not impeded nor passage of fish orhabitataffected.ThismayhelpavoidtheneedforResourceConsentsinsuitableapplications.

PHOTOGRAPHS

Fig1.Archunitsaredeliveredflattositeandliftedintoarchshapepriortoplacement.

Fig2.Liftedarchunitinshapeonendofcranehook.Notespreaderbarsandindividualchain.

blockstocreateandholdarchshapeinposition.

Fig3.Twoarchunitsinposition.Notepreplacementofrockripraperosionprotectionunder

bridgepriortoarchinstallationandclosefittingnatureofarchunits.

Fig4.Allsixarchunitsinposition

Fig5.Allsixarchunitsinposition

Fig6.Sidepanels–endpanelunitsinplace.

Fig7.Toppanelpiecebeingliftedinplace.Panelsheldinplacewitharangeofreidbartie

fittingsandgeogridreinforcementinhardfillbackfillasrequired.

Fig8.Completedbridge

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