ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/297006
44 | ASPIRE , Summer 2013 S A F E T Y A N D S E R V I C E A B I L I T Y The shear connection between pr ecast composite concrete bridge decks and girders is receiving renewed attention as precast concrete deck panels have become an alternative to cast-in-place decks in accelerated bridge construction. Shear connections are commonly made in precast concrete deck panels by using steel connectors, such as studs or bars. The steel connectors extend from the top of the girder into blockouts in the panel, which are then grouted. However, the maximum connector spacing limit of 24 in., defined in the AASHTO LRFD Design Specifications, poses a dilemma for precast concrete deck design. There are two important reasons why closely spaced blockouts are undesirable in precast concrete deck panels. First, each blockout requires special labor for grouting. Second, a precast concrete deck panel manufactured with blockouts spaced at 24 in. may have a definite plane of weakness through the blockout openings. Special care must be taken to reinforce this weak section, particularly for effects of moving and handling to avoid cracking. In both cases, minimizing the number of connectors and blockouts is preferred. The original sources of the current maximum connector spacing of 24 in. are unclear. An investigation described in NCHRP Report 584, 1 on full-depth precast concrete deck panels, attributes that limit to a "rule of thumb" in design suggested in 1943 by Newmark and Siess. 2 The 24-in. limit first appeared in the 4th edition of the AASHTO Standard Specifications for Highway Bridges in 1944. These connections using studs were the focus of a push-off investigation by Issa 3 in which clusters of studs in a pocket were found to have less capacity than the sum of individual studs. The AASHTO Q n capacity value for studs in Article 22.214.171.124 of the 2012 AASHTO LRFD Specifications, however, matched Issa's measured capacity for two stud clusters in a single pocket. The test data showed a 15 to 25% capacity reduction when the number of studs was increased or the number of pockets increased. AASHTO does not specifically change capacity with the number of studs in a cluster, but taking the capacity as 85% or less of AASHTO's value when more than two studs are clustered in a pocket is recommended based on Issa's work. A series of recent research activities has re-examined the needs of composite deck connections. Testing of panels connected to 84-ft-long steel girders with both 24-in. and 48-in. connector spacing was conducted at the University of Wisconsin and found no reduction in composite action stiffness or strength with the wider spacing, even after 2 million cycles of repeated service loading. On testing to the theoretical ultimate capacity, no deck uplift was detected and the 48-in. connector spacing provided an actual ultimate load capacity higher than predicted using AASHTO procedures, even though no capacity reduction was taken for multiple studs per pocket. Additional work described in the NCHRP Report 584 resulted in suggested guidelines for designing deck panels with wider connection spacings. These research activities were followed by the construction of two bridges using 48-in. spacing for the connectors. The Live Oak Creek Bridge built by the Texas Department of Transportation (TxDOT) in 2008 used 8-ft-wide by 32-ft-long precast concrete, 8-in.-thick deck panels on precast concrete I-girders. The shear connections were spaced at 48 in. and used three 1¼-in.-diameter steel rods, in a group, extending from the girder into blockouts of the deck panels. This bridge has been reported by TxDOT as performing very well. Wisconsin also used precast concrete deck panels with 48-in. connector spacing in the widening and redecking of a heavily traveled bridge on Interstate 39/90. The panels were 8-in. thick with rectangular blockouts into which clusters of 6- to 10 7 / 8 -in.-diameter headed studs from the steel girders were embedded and grouted. No capacity reduction was used for the large stud clusters. The Wisconsin bridge was built in 2006 and averages 60,000 vehicles per day. Inspections have found that the precast concrete deck, on one of a pair of bridges, appears to be performing better than the cast-in- place concrete deck on the twin bridge. A wider spacing than 24 in. for the composite connections between precast concrete deck panels and bridge girders appears appropriate. Forty-eight-inch spacings were successfully implemented in these recent research and construction projects. Recent research has not, however, examined the behavior of composite decks with shallow beams. As the beam becomes Composite Girder Connections for Precast Concrete decks by Michael Oliva, University of Wisconsin, and Pinar Okumus, SUNY-Buffalo Three threaded-rod shear connectors extending from the girder will be grouted into the blockout in the Texas Department of Transportation deck panel. Photo: TxDOT. Installed precast concrete deck panels with pockets and beams with shear connectors. Photo: Purcell Associates. Interstate I-39/90 deck with studs in pockets at 48-in. spacing. Photo: University of Wisconsin. Book_Sum13.indb 44 7/1/13 7:02 AM