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Title: Evaluation of Existing Prestressed Concrete Bridges and Live Load Distribution

Author(s): He Zhang

Publication: Web Session



Appears on pages(s):



Date: 10/17/2021

The live load girder distribution factors (GDF) were derived to simplify the three-dimensional analysis of the bridge system. This simplification led to using the “S-over” distribution equations in the AASHTO Standard Design Specification since 1930 for more than 70 years until the AASHTO-LRFD was published. While the GDF in the Standard specifications were considered to be conservative but simple, the LRFD are more accurate but are based on regression of multiple variables. Both GDF’s are developed to ensure the design outcome is satisfactory. Given the number of prestressed concrete bridges in the NBI database, it is critical to investigate the performance of various existing bridges with regard to their load rating performance. New Jersey owns around 1,500 prestressed concrete bridges, and 66 percent of them were built before 1990. Depending on the design year and specifications, the required load effects from LRFD and LFD of these existing prestressed concrete bridges may have been overestimated. To evaluate the actual load distribution, a load test is performed on a sixty-year-old prestressed concrete I-girder bridge. The field-testing live load distribution factors are determined for both interior and exterior girders using strain sensors. The three-dimensional finite element model (FEM) is developed for the comparison of equations for girder distribution factors. Moreover, the bridge database containing various prestressed concrete bridges from different design years is analytically assessed using the FEM model in terms of the distribution factors. Based on the comparison between the AASHTO GDF’s and other equations with more rigorous methods and field measurements, the extra load effect resulting from these equations provides additional load carrying capacity that would enhance the load rating of these bridges.