LOAD EQUIVALENCY FACTORS FOR TRIAXLE LOADING

The objective of this research was to develop load equivalency factors of triaxle loading for flexible pavements. Load equivalency factors of triaxle loading were determined using the mechanistic approach, AASHO's empirical approach, and the semiempirical approach. Three evaluation criteria were developed for the mechanistic approach: fatigue cracking, rutting, and performance. These criteria were developed by correlating pavement response with the performance of the experimental pavements at the Pennsylvania Transportation Research Facility. Using the BISAR computer program for spring weather conditions, the critical responses analyzed were maximum tensile strain at the bottom of the stabilized base course and maximum compressive strain on the top of the subgrade. The AASHO empirical approach was utilized to determine the load equivalency factor only of 76-kip (338-kN) triaxle loading. The experimental pavements were subjected to approximately 55,000 repetitions of 76-kip (338-kN) triaxle loading. The load equivalency factor was approximately 2.60 for the range of structural numbers studied and for a terminal serviceability index of about 2.0. The semi-empirical approach utilized the relationship between load equivalency factors and maximum subgrade compressive strain. The relationship for triaxle loading was established by using the previously determined factor, 2.60, and the relationships for both single and tandem axle loadings. The results of this study indicated that the load equivalency factors generated by the mechanistic approach were not compatible with AASHO's factors. The semi-empirical approach is thus recommended. The results established in this study, however, are only approximate values because of the limited field data. (FHWA)