Optimized Load and Resistance Factor Design of Prestressed I-Girder Bridge Superstructures with High-Strength Concrete

The use of the AASHTO LRFD Bridge Design Specifications will be mandatory for all departments of transportation (DOTs) in the United States for federally funded projects beginning in October 2007. Several states, including Arizona, have already started developing their bridge design specifications, defining pilot projects to be designed according to load and resistance factor (LRFD) specifications. In addition to implementing the new specifications, most DOTs will make wide use of high-performance–high-strength concrete. This paper presents an investigation of girder–slab bridges that were designed in accordance with both LRFD and standard AASHTO bridge design specifications and that used conventional and high-strength concrete (HSC). Major changes in LRFD design methodologies are discussed, and a general comparison between the standard AASHTO specifications and the LRFD specifications is made. A computer program was specifically developed for the analysis, design, and cost evaluation of these superstructure types. This program is capable of performing calculations for both LRFD and standard methods. Detailed observations were made of these two design methods to compare the relative value of their analysis and design features and their cost. Cost evaluations of these bridge superstructures with different geometrical configurations (girder type, spacing, etc.) made it possible to obtain practical design charts for the optimum girder type or girder spacing. Several aspects of HSC were studied to determine the effect of concrete strength on design variables. The effect of concrete strength in flexural members is more pronounced in prestressed concrete than in reinforced concrete. The use of HSC enabled significantly longer span lengths, girder spacing, or both, which is not practical with normal strength concrete. A detailed study and comparison was done on different AASHTO methods for concrete deck slab design.