The Highway Subcommittee on Bridges and Structures (HSCOBS) of the American Association of State Highway and Transportation Officials (AASHTO) maintains the Load and Resistance Factor Design (LRFD) Specifications for bridges as part of its responsibility to design and manage the nation’s highway infrastructure.

 

The LRFD Specifications contain requirements for design of interface shear applications. In the fourth edition with 2008 interim revisions, a stress limit of reinforcement was added to keep the rebar stress from exceeding 60 ksi in calculating interface shear capacity, even if rebar with a strength greater than 60 ksi is used. The stress limit was based on a small number of laboratory tests. This could result in overly conservative estimates of interface shear capacity.

 

Moreover, it would be wise to revisit the current interface shear design provisions. Investigation is necessary for contemporary concrete strength, surface preparation methods, shear key shape, interface locations and orientations (horizontal and vertical), mechanics of failure along the joint, and reinforcement distribution across the shear interface. Different equations may be appropriate for different interface locations: joints between top of precast girder flanges and cast-in-place bridge deck; cold joints between cast-in-place girders and cast-in-place decks; vertical cast-in-place connections between spliced precast girders; and connections of flanges and webs of box girder bridges in both orientations. The distribution of reinforcement across the shear interface may currently be over- or under-designed.

 

Revising the current design equations and construction details has the potential to provide a number of benefits: less congestion of reinforcement cages resulting in better concrete quality, reduced shipping costs, reduced construction costs, faster construction, and improved structural performance of interface shear connections.

The objective of this project was to propose revisions to the AASHTO LRFD Bridge Design Specifications related to the interface shear design capacity equations and construction details including the stress limit to be applicable for currently available rebar grades up to 100 ksi. This revision will improve interface shear design and construction details for bridges and fully utilize any additional capacity of high strength bars.