Shear Behavior and Capacity of Large-Scale Prestressed High-Strength Concrete Bulb-Tee Girders

摘要

The current shear design provisions of the AASHTO LRFD Bridge Design Specificationslimit the concrete compressive strength to 10 ksi due to a lack of experimental evidence for theirextension to high strength concrete. To overcome this limitation, the National Academy ofSciences funded National Cooperative Highway Research Program (NCHRP) Project 12-56“Application of the LRFD Bridge Design Specifications to High-Strength Structural Concrete:Shear Provisions”. This report presents an analysis of project 12-56 experimental test data fromwhich an in-depth understanding of the shear response of prestressed girder was obtained andnew models were developed.The experimental work comprised a total of 20 tests on ten 52-foot long and 6-foot deepbulb-tee girders. All girders were designed to satisfy the requirements of the LRFD BridgeDesign Specifications and then subjected to a uniformly distributed load until failure occurred inshear. The primary test variables were concrete compressive strength (ranging from 10 to 18 ksi),the maximum shear design stress (0.7 to 2.5 ksi), strand anchorage details (straight, unbonded,and draped), and end reinforcement detailing (bar size, spacing, and level of confinement). Alarge number of both traditional and advanced instrumentation systems were used to measureresponse. A new data visualization tool was developed to provide a detailed analysis of the denseexperimental test data.It was concluded that the AASHTO LRFD Sectional Design Method, as well as theAASHTO Standard Specifications and the Canadian Standard Association A23.3-04 DesignMethod, could be extended to up to 18 ksi concrete. It is also recommended that the maximumshear design stress be reduced from 0.25 fc’ to 0.18fc’. Both the angle and the strength ofdiagonal cracking could be accurately predicted using Mohr’s circle of stress. The web shearbehavior could be characterized as a tri-linear relationship separated by web cracking, stirrupyielding, and failure and the inelastic tangent stiffness before stirrup yielding could be modeledas a polynomial function of shear reinforcement ratio. Based on the development of 350 crackbasedfree-body diagrams, the components of the concrete contribution to resistance (two flangesand web) over the loading history was characterized as a function of the geometric and materialproperties of the girders. A general expression, which adopted the calculated crack angle for thecomputation of shear reinforcement contribution and provided clear physical explanation forevery part of concrete contribution, was suggested for the future shear design practice.From the measured test results, an analytical model, Crack Displacement Field Theory(CDFT), was developed for predicting the shear response of prestressed/reinforced concretemembers. Compared to other existing models, it could capture the discrete displacement due tocrack opening and crack slip along crack surface and can take account the variation of stresses inreinforcement due to bond. Based on this model, expressions were derived for shear stiffness andshear resistance at stirrup yielding, and the derived equations produced good agreement with testresults.