Truck Equivalency Factors, Load Spectra Modeling and Effects on Pavement Design

The objectives of this study were to develop truck factors for pavement design in Alabama and axle load distribution models for mechanistic-empirical (M-E) pavement design. In addition, the effects of variations in axle load spectra obtained from different sites on pavement design requirements using both the 1993 American Association of State Highway and Transportation Officials (AASHTO) pavement design guide and a M-E design approach were evaluated. Information from thirteen weigh-in-motion (WIM) sites on rural principal arterials was provided by the Alabama Department of Transportation for this study. Statistical and practical tests were used to determine the daily, monthly, directional, and site variations in truck traffic relating to the development of truck factors. A sensitivity analysis was performed to determine the effect the variation in truck factors would have on the final pavement design thickness. It was determined that using a statewide average truck factor would be sufficient for pavement design of rural principal arterials in Alabama. The data from the WIM sites were also used to create an innovative statistical model of axle load distributions. Separate models were developed for single and tandem axles at each site and for the statewide average. A mixture of either a lognormal, normal, and normal distribution or a lognormal and normal distribution was used for the single axles. For all tandem axle loads a mixture of a lognormal and normal distribution was found to be the best fit. All of the developed single axle models were found to explain at least 98.6% of the total variation in the data and all the developed tandem axle models were found to explain at least 96.2% of the total variation.