Optimization of Post-Tensioned Box Girder Bridges with Special Reference to Use of High-Strength Concrete Using AASHTO LRFD Method

With the Federal Highway Administration-mandated implementation of the LRFD specifications, many state departments of transportation (DOTs) have already started implementing LRFD specifications as developed by the AASHTO. Many aspects of the LRFD specifications are being investigated by DOTs and researchers in order for seamless implementation for design and analysis purposes. This paper presents the investigation on several design aspects of post-tensioned box girder bridges designed by LRFD Specifications using conventional or High-Strength Concrete (HSC). A computer spreadsheet application was specifically developed for this investigation. It is capable of analysis, design, and cost evaluation of the superstructure for a cast-in-place post-tensioned box girder bridge. Optimal design of a post-tensioned box girder is achievable by correct selection of design variables. Cost evaluation of superstructures with different geometrical and material configurations has led to the development of optimum design charts for these types of superstructures. Variables used to develop these charts include, among others, span length, section depth, web spacing, tendon profile, and concrete strength. It was observed that HSC enables the achievement of significantly longer span lengths and/or longer web spacing that is not achievable when using normal strength concrete. 1. Introduction American Association of State Highway and Transportation Officials (AASHTO) standard specification [1] has been the main bridge design specification in the United States since the 1940s. During the last two decades, there have been significant developments in concrete bridge design methods and utilization of new concrete materials. The implementation of load and resistance factor design (LRFD) and the use of High Strength/High Performance Concrete (HSC/HPC) are important subjects of investigation. The state DOTs have increased the use of HPC/HSC concrete and implementation of the AASHTO LRFD specification [2]. LRFD is based on the latest developments in structural analysis and materials to assure desired serviceability and ultimate behavior, safety, aesthetics, and economy. It benefits the valuable experiences of AASHTO allowable stress design (ASD) and load factor design (LFD) methods, which have been in use since the 1940s and comprise the Standard specification. This new specification resulted in design procedures significantly different compared to the earlier methods. The new LRFD specification is based on a probability-based approach in which load and resistance factors are based on