Simplified Live Load Distribution Factor Equation for Steel Girder Bridges

Abstract

The "S-over" equation for load distribution factor (LDF) was first introduced in the 1930s in the AASHTO standard. Finite element studies, however, have shown it to be unsafe in some cases and too conservative in others. AASHTO LRFD 1994 introduced a new LDF equation as a result of the NCHRP12-26 project. The equation is based on parametric studies and finite element analyses. It is considered to be a good representation of bridge behavior. However, it involves a longitudinal stiffness parameter that is not initially known in design and thus requires an iterative procedure to correctly determine the LDF value. The need for an iterative design procedure is perceived by practicing engineers as the major impediment to wide acceptance of the AASHTO LRFD equation. A new, simplified equation that is based on the AASHTO LRFD formula but does not require an iterative procedure is developed. Fortythree Indiana bridges were selected and analyzed with a sophisticated finite element model. The new, simplified equation produces LDF values that are always conservative compared with those obtained from the finite element analyses and are generally greater than the LDF obtained using AASHTO LRFD specification. Therefore, the simplified equation provides a simple, yet safe specification for LDF calculation.

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