Live load distribution factors for a short span timber bridge under heavy agricultural vehicles

The American Association of State Highway and Transportation Officials (AASHTO) Specifications provide simplified formulas to determine Live Load Distribution Factors (LLDFs) for timber girder bridges with timber decking. The AASHTO formulas have been developed based upon the parametric study for timber bridges subjected to typical highway vehicles. However, the majority of short span timber bridges are located on secondary roadways where agricultural vehicles often operate. These vehicles have more vastly different configurations and weights relative to highway vehicles; thus, they can result in more different LLDFs than highway vehicles. Hence, the focus of this study is on the investigation of agricultural vehicle induced LLDFs for a short span timber bridge with timber decking through field tests, simulations, and AASHTO-based codified methods. The bridge was tested with four heavy agricultural vehicles and a highway truck. Commercially available finite element analysis (FEA) software was used to generate an analytical model of the bridge. The model was refined with field data to replicate field responses and analytical LLDFs of the bridge. To account for uncertainties in agricultural vehicle parameters, a large number of agricultural vehicles with different characteristics in use in the United States were used as input loads in the model. Field and analytical LLDFs were compared to those obtained from the AASHTO-based codified methods. Findings indicated that the field and analytical LLDFs for most girders occasionally exceeded the AASHTO codified values.