Simple “S-over” live load distribution factors for shear and moment have been used for bridge design since the 1930s. The traditional factors, which are included in the AASHTO Standard Specifications, are easy to apply, but can be overly conservative and even unconservative in some parameter ranges.
New, more accurate, and more complex, live load distribution factor equations were developed under NCHRP Project 12-26 and were included in the AASHTO LRFD Bridge Design Specifications (LRFD specifications). The “S-over” factors are not included in the LRFD specifications. The new distribution factor equations have limited ranges of applicability. When the ranges of applicability are exceeded the LRFD specifications mandate that refined analysis is required.
Designers find the complexity of the current equations troubling. Simpler live load distribution factor equations would be welcomed by the design community. The objective of this research was to develop new LRFD live load distribution factor design equations for shear and moment that are simpler to apply and have a wider range of applicability than those in the current LRFD specifications and reduce the need for refined analysis.
This research and the companion NCHRP Project 12-62A was performed by BridgeTech, Inc., with contributions from Tennessee Technological University, HDR, Inc., and Dennis Mertz. The report fully documents the research leading to the recommended live load distribution factors.