Impact of AASHTO LRFD bridge design specifications on the design of Type C and AASHTO Type IV girder bridges
Mohammed, Safiuddin Adil
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This research study is aimed at assisting the Texas Department of Transportation (TxDOT) in making a transition from the use of the AASHTO Standard Specifications for Highway Bridges to the AASHTO LRFD Bridge Design Specifications for the design of prestressed concrete bridges. It was identified that Type C and AASHTO Type IV are among the most common girder types used by TxDOT for prestressed concrete bridges. This study is specific to these two types of bridges. Guidelines are provided to tailor TxDOT's design practices to meet the requirements of the LRFD Specifications. Detailed design examples for an AASHTO Type IV girder using both the AASHTO Standard Specifications and AASHTO LRFD Specifications are developed and compared. These examples will serve as a reference for TxDOT bridge design engineers. A parametric study for AASHTO Type IV and Type C girders is conducted using span length, girder spacing, and strand diameter as the major parameters that are varied. Based on the results obtained from the parametric study, two critical areas are identified where significant changes in design results are observed when comparing Standard and LRFD designs. The critical areas are the transverse shear requirements and interface shear requirements, and these are further investigated. The interface shear reinforcement requirements are observed to increase significantly when the LRFD Specifications are used for design. New provisions for interface shear design that have been proposed to be included in the LRFD Specifications in 2007 were evaluated. It was observed that the proposed interface shear provisions will significantly reduce the difference between the interface shear reinforcement requirements for corresponding Standard and LRFD designs.The transverse shear reinforcement requirements are found to be varying marginally in some cases and significantly in most of the cases when comparing LRFD designs to Standard designs. The variation in the transverse shear reinforcement requirement is attributed to differences in the shear models used in the two specifications. The LRFD Specifications use a variable truss analogy based on the Modified Compression Field Theory (MCFT). The Standard Specifications use a constant 45-degree truss analogy method for its shear design provisions. The two methodologies are compared and major differences are noted.