ASPIRE Summer 08 - Aspire - The Concrete Bridge Magazine

Precast, prestressed concrete beams were used because of a lack of clearancefor falsework and to speed construction.Limited Vertical ClearanceOne of the significant logistical concernswas the inadequate vertical clearanceavailable for using falsework. Using castin-placebox girders to replicate the lookof the existing bridge and surroundingstructures would have required falseworkspanning three traffic lanes in eachdirection, he explains. “The availablevertical clearance was not adequate toaccommodate falsework deep enoughto span three lanes of traffic. This wasthe key factor in choosing precastconcrete.”The girders and deck panels also offereda shortened construction time, as theycould be fabricated off-site and erectedquickly upon arrival. “The precast girderssignificantly reduced the timeframe forimpacting on-going traffic,” he says.The project was designed in six months,with construction taking about 1 year.But the actual road closure to erect theprecast girders amounted to only 2 daysduring this period.Although cast-in-place girders could notbe used due to falsework requirements,the concern was that the use of precast,prestressed concrete bulb-tee girderswould not blend with the appearanceof the rest of the bridge, or with otherbridges in the area. “We usually seecast-in-place box girders being used inCalifornia, and the agencies involvedin this project wanted to match theirdesign,” Kazmi explains.To replicate that look, precast, prestressedconcrete rectangular box girders wereused as the exterior girder on each side ofthe bridge. The girders were deeper thanthey were wide and featured a roundedoutside fascia corner to replicate the lookof cast-in-place box girders. The designprovided the box shape that all the bridgesoffered to drivers nearing the structures,he says. Inside girders, which are viewedonly when cars are directly beneath them,are bulb-tee sections. “Motorists see thelook of a concrete box-girder bridge asthey approach, and the structure blendswell with the adjacent structures.” Thiscombination of girders kept the cost ofthe project in line by using less expensivebulb-tee girders in the less visible area, henotes.AestheticsThe existing structure and several structures along the corridor arecast-in-place box girder bridges with vertical webs for the exteriorgirders. The use of more typical precast, prestressed concrete girdersat this location would not have matched the aesthetic characterof the bridges. The problem was addressed by the use of precast,prestressed concrete rectangular box girders with a rounded outsidecorner for the exterior girders. This gives the motorist the look of aconcrete box girder bridge and the structure blends well with theadjacent structures.Speed of Construction and Public SafetyAnother challenge on this project was the need for a structuretype that could be built quickly and would significantly reduce theduration of the inconvenience to the on-going traffic. This was largelyachieved by the efficient use of precast, prestressed concrete girdersas well as permanent precast concrete deck panels.32 | ASPIRE, Summer 2008