Nontraditional Limitations on the Shear Capacity of Prestressed Concrete Girders

摘要

Code based shear design provisions principally use a sectional force design procedurein which it is assumed that plane sections remain plane. However, the shear capacity of amember may be limited by other shear related phenomena that are not captured in codes ofpractice. These nontraditional limitations on the shear capacity of a member can result fromincorrectly evaluating the shear stress that needs to be transmitted across a crack, shearcompressionfailure along a web-flange interface, or insufficient capacity of longitudinaltension reinforcement at the support.A series of 20 shear tests were completed on ten 52-foot long and 63-inch deepprestressed bulb-tee bridge girders cast with high-strength concrete. An extensive amount ofexperimental data was gathered and advanced data analysis tools were utilized to evaluatethese nontraditional limitations on shear capacity.It was determined that interface shear transfer resistance in high-strength concrete ispredicted well by relationships developed from tests on normal-strength concrete specimens.It was further observed that the angle of web-shear cracking was generally steeper than theangle of principal compressive stress as given by the AASHTO LRFD Bridge DesignSpecifications. This difference in angles creates a significant shear demand on a crack that isnot accounted for in the LRFD Specifications.A method is presented for determining if a shear-compression failure along a webflangeinterface is a potential mode of failure for a prestressed bridge girder. This approachprovides a means of calculating the shear stress that must be transmitted across the interfaceas a function of the geometry and loading on the member as well as a means of calculating theshear resistance along the web-flange interface. This method can be used to guard against ashear-compression failure by placing a limitation on the maximum shear capacity a member.It was also determined that the requirement for longitudinal tension reinforcement nearthe support included in the 4th edition of the AASHTO LRFD Bridge Design Specificationsmay underestimate the demand on longitudinal tension reinforcement. Alternatively anequilibrium based approach is presented for determining demand on longitudinal tensionreinforcement near the support.