Live load distribution factors have been used in the design of highway bridges since the first edition of the AASHTO Standard Specifications were introduced in 1931. Revisions were made to the AASHTO Standard Specifications in 1943 based on work conducted by Newmark. These changes lead to the S/5.5 factor. In 1988, an effort was made to revise the AASHTO Standard Specification equation for live load distribution to produce less conservative results. NCHRP Report 12-26 successfully developed an equation involving girder spacing, girder span length, girder stiffness, and slab thickness; and was adopted into the AASHTO LRFD Specifications. The primary goal of this effort is to identify and assess various methods of computing live load distribution factors and to use the results of laboratory and field tests to compare these methods. It is further a goal of this work to use these methods to perform a parametric study over a wide range of typical slab on steel I-girder bridges to assess the accuracy of both the AASHTO Standard and AASHTO LRFD specifications and to propose an empirical model that correlates better with the analytical results within the range of parameters that are to be studied. These studies include: (1) a verification study into the FEA techniques used in modeling bridge geometry, (2) selection of procedure of calculating load distribution factors from FEA data, (3) a verification study of the selected procedure, (4) a parametric study to assess the influence of bridge parameters on the contribution to load distribution factors, (5) the development, using regression techniques, of a new equation for live load distribution factors, and (6) a comparison of proposed distribution factors against FEA, AASHTO LRFD, and AASHTO Standard Specifications. Results from this work that over a wide range of typical bridge parameters both the AASHTO Standard and LRFD Specifications may produce conservative results and indicate the proposed equation provides a good foundation for the development of new equations for live load distribution factors. Girder spacing and girder span length were found to have the most influence of load distribution. The proposed equation developed showed good correlation to the FEA data and also correlated well against actual DOT bridge inventories used in the development of the AASHTO LRFD equation for live load distribution factors in slab on steel I-girder bridges.
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