Impact of AASHTO LRFD bridge design specifications on the design of Type C and AASHTO Type IV girder bridges

摘要

This research study is aimed at assisting the Texas Department of Transportation(TxDOT) in making a transition from the use of the AASHTO Standard Specificationsfor Highway Bridges to the AASHTO LRFD Bridge Design Specifications for the designof prestressed concrete bridges. It was identified that Type C and AASHTO Type IV areamong 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 tailorTxDOT's design practices to meet the requirements of the LRFD Specifications.Detailed design examples for an AASHTO Type IV girder using both theAASHTO Standard Specifications and AASHTO LRFD Specifications are developedand compared. These examples will serve as a reference for TxDOT bridge designengineers. A parametric study for AASHTO Type IV and Type C girders is conductedusing span length, girder spacing, and strand diameter as the major parameters that arevaried. Based on the results obtained from the parametric study, two critical areas areidentified where significant changes in design results are observed when comparingStandard and LRFD designs. The critical areas are the transverse shear requirements andinterface shear requirements, and these are further investigated.The interface shear reinforcement requirements are observed to increasesignificantly when the LRFD Specifications are used for design. New provisions forinterface shear design that have been proposed to be included in the LRFDSpecifications in 2007 were evaluated. It was observed that the proposed interface shearprovisions will significantly reduce the difference between the interface shearreinforcement requirements for corresponding Standard and LRFD designs.The transverse shear reinforcement requirements are found to be varyingmarginally in some cases and significantly in most of the cases when comparing LRFDdesigns to Standard designs. The variation in the transverse shear reinforcementrequirement is attributed to differences in the shear models used in the twospecifications. The LRFD Specifications use a variable truss analogy based on theModified Compression Field Theory (MCFT). The Standard Specifications use aconstant 45-degree truss analogy method for its shear design provisions. The twomethodologies are compared and major differences are noted.