Agricultural Vehicle Load Distribution for Timber Bridges

Agricultural or farm vehicles with various configurations and weights are frequently driven over timber bridges on secondary roadways in the United States. Lateral live-load distribution factors (LLDFs) for these bridges loaded with farm vehicles are not well known. Further, the effects associated with farm vehicles have not been considered in the current AASHTO specifications that solely provide codified formulas to calculate LLDFs of timber girders. To estimate the timber-girder LLDFs more explicitly, three multispan timber-girder bridges in Iowa were selected, and LLDFs for each of girder were determined on the basis of various methods, including codified processing, field testing, simulation, and statistical analysis. For field LLDFs, the bridges subjected to four farm vehicles were tested and a roadway semitruck was used as a reference for determining roadway-vehicle load characteristics. As part of analytical LLDF investigation, analytical models of the bridges were generated and calibrated with field data using finite-element analysis (FEA) software. To consider the effects of farm vehicles with vastly different features, configurations of approximately 121 different farm vehicles were gathered and used as applied loads in the FEA models to determine analytical LLDFs per bridge. The analytical LLDFs were used to create LLDF limits for each bridge in a statistical manner. The resulting LLDFs were compared to those determined from the codified formulas, showing that the AASHTO LLDFs were, in specific cases, insufficiently conservative for the bridges studied.