ASPIRE Spring 11 - Aspire - The Concrete Bridge Magazine

Asymmetric EquilibriumSupporting the bridge deck with cablesattached to only one side of the deckwas an extremely challenging designfeature. Looking at a typical section ofthe bridge, the unbalanced nature of thedeck becomes very apparent. It seemsas if the weight of the deck wants toroll the section about the suspendersupport. The torsion generated by theunbalanced support location must becompensated in some way. The bestsolution would be to get the line ofaction of the suspender supporting forceto pass through the center of gravity ofthe bridge section. An unsymmetricalcross-section was developed to try tomove the center of gravity of the sectionas far toward the suspenders as possible.Unfortunately, the center of gravitycould not be shifted far enough towardthe suspenders to achieve a balanceddesign. The support point was thenmoved to the top of the railing in aneffort to shift the line of action farthertoward the center of gravity of thesection and thereby achieve a balancebetween the dead load of the bridgeand the suspender force.It turns out, it wasn’t quite that simple.The dead load torsional force was stillgreater than the resisting horizontalcomponent of the suspenders; thebridge wasn’t balanced for torsionaldead loads. In order to achieveequilibrium, an additional horizontalresisting force was needed. To providethis necessary horizontal balancingforce, an additional longitudinal posttensionedcable was added along thetop of the railing post. Due to the curveof the bridge in plan, this created anadditional radial inward force at the topof the railing to provide the balancingtorsion needed.Main Cable andHangersThe main cable is 36strands of 0.6-in.-diameterwaxed and sheathedstrands inside a groutedstainless steel guide pipe.The cables from eachside of the main span areanchored at the top ofthe pylon. The bundle of36 individual strands waslaid out adjacent to thebridge and taped togetherin a compact bundle. Theentire bundle was pulledthrough the stainless steel guide pipewhich was suspended in position usinga temporary support wire. The hangerswere cut to a predetermined length.They were attached to the main cableguide pipe while the pipe was in a slackposition. After all the hangers wereattached, the main cable was stressed,which removed the slack and put thefinal load in the hangers and lifting thebridge off the falsework. The pipe wasthen grouted.It’s in the DetailsFor a pedestrian bridge, the user is muchcloser to the details of the bridge and istravelling at walking speeds. The detailsof the connections and hardware on thebridge take on a much more importantrole in these types of bridges. The guidepipe eliminates the need for cable bandsand provides a smooth line along thelength of the main cable without anydiscontinuities. The suspenders are in turnattached to gusset plates on the piperather than directly fastened to the maincable via a cable band, also providing asmooth, clean look and further adding tothe pedestrian experience.An additional longitudinal tendon wasstressed at the top of the railing. Coupledwith the curvature of the bridge, thistendon creates an inward radial force toprovide the restoring torsion needed forequilibrium.ConclusionThe Harbor Drive Pedestrian Bridgecompletes the final piece of the Cityof San Diego’s Park-to-Bay Link, andprovides the city with a structuralicon for the community. The new selfanchoredsuspension bridge, withits 130-ft-tall pylon inclined over thehorizontally curved bridge deck, andsuspenders only attached to one side ofthe bridge deck, truly is a bridge fittingfor one of America’s Finest Cities.__________Joe Tognoli is vice president and principalbridge engineer and Dan Fitzwilliam issenior bridge engineer, both with T. Y. LinInternational in San Diego, Calif.For additional photographs or informationon this or other projects, visit www.aspirebridge.org and open Current Issue.The stainless steel cable stays attachedto only one edge of the concrete deckprovide a sense of openness and create adramatic cantilever effect.