Development of Live Load Distribution Factor Equation for Girder Bridges

Abstract

Live load distribution factor (LDF) equations are among the most important bridge design calculations because they provide the distributed moment and forces, which are needed for designing new or for evaluating existing bridges. The AASHTO load and resistance factor design specification is the current standard for bridge design. It uses a power function containing parameters that are often unknown when bridges are being designed and thereby allows the design procedure to be iterative. This iterative design procedure is perceived by many practicing engineers as impractical and has raised questions regarding the desired level of simplicity in bridge design calculations. Therefore, simpler but more accurate LDF equations are needed. An experimental program to evaluate the behavior of girder bridges was launched. The program consisted of live load testing of actual girder bridges. A parametric study was carried out by using a detailed three-dimensional finite element analysis to determine how sensitive various bridge parameters are to the LDF. On the basis of the parametric study, new simplified LDF equations are proposed for various types of girder bridges, including steel, prestressed, and spread box girder bridges. The equation format is simplified as a function of two bridge parameters only (girder spacing and span length) while an equal level of accuracy is maintained. Results are compared with those from other available equations and are shown to have excellent correlation with the more exact finite element method.