The AASHTO LRFD Bridge Design Specifications (BDS) provides approximate live load distribution factors (LLDFs) for line girder analysis of steel I-girder bridges (and other beam-slab bridge types) in Art. 4.6.2.2.2. In Art. 4.6.2.2.2d, in particular, the provisions require the following: “In steel beam-slab bridge cross-sections with diaphragms or cross-frames, the distribution factor for the exterior beam shall not be taken to be less than that which would be obtained by assuming that the cross-section deflects and rotates as a rigid cross-section.” This provision is based on limited research, and various designers and owners have questioned its accuracy. The LLDFs in Tables 4.6.2.2.2b-1 and 4.6.2.2.2d-1 were developed, assuming the presence of only end cross-frames, with no cross-frames within the spans. Meanwhile, recent innovations in steel I-girder bridge design are facilitating the use of “lean-on bracing” framing systems on straight bridges with or without skew, which feature far fewer diaphragms or cross-frames than were traditionally used. There is a need to investigate appropriate LLDFs for straight steel I-girder bridges using either traditional framing or lean-on bracing framing to help maintain and enhance the AASHTO specifications.

The objective of this research is to update and/or confirm the accuracy of the moment and shear LLDFs presented in the AASHTO LRFD BDS for both interior and exterior girders in straight steel I-girder bridges with little or no skew to facilitate an accurate analysis of those types of bridges using simplified line girder analysis methods. The effects of both conventional framing and lean-on bracing framing configurations will be considered and addressed. As a value-added feature, the analytical models proposed for this research can also be used to evaluate the fatigue stress ranges in cross-frame members to confirm and augment the findings and recommendations made in NCHRP Research Report 962: Proposed Modification to AASHTO Cross-Frame Analysis and Design about the need to perform fatigue evaluations of cross-frame members in these types of "routine” steel I-girder bridges. It is anticipated that the research results would include:

1.     Proposed revisions to the AASHTO LRFD BDS LLDFs for steel I-girder bridges, and

2.     Proposed revisions to the AASHTO LRFD BDS provisions regarding the need to evaluate fatigue in cross-frame members in straight steel I-girder bridges with little or no skew.