Development of a methodology to estimate ESALs for Widebase Steering Tires

British Columbia Ministry of Transportation and Infrastructure (TRAN), and other Canadian regulators, utilize the ESAL concept for vehicle impact evaluations and(or) pavement design. TAC's ESAL equations originally were developed in RTAC's Heavy Vehicle Weights and Dimensions Study (1986) and are widely accepted by the Canadian transportation industry. Unfortunately, TAC's ESAL equations do not account for tire size and, consequently, overestimate steering axle impacts when those axles are equipped with widebase steering tires. Most new vehicles proposed for use in B.C. (or in other Provinces) feature tridem drive tractors which, by regulation, must carry at least 25% - 27% of the drive group weight on the steering axle—these heavy loads necessitate the use of widebase steering tires. In order to optimize high efficiency truck configurations in Canada, therefore, accurate estimates of widebase steering tire ESALs are needed. This paper describes a methodology that was recently developed by FPInnovations, in consultation with TRAN, to estimate ESALs for widebase steering tires.

Using layered elastic pavement modeling, FPInnovations evaluated key strain responses to widebase steering tire traffic in the 14 RTAC-86 test pavement sections. The results were transformed to estimates of pavement life and then calibrated to RTAC-86 ESAL model to develop ESAL relations for seven popular North American steering tire sizes, including four widebase steering tires. The ESAL relations produced in this research extend the TAC single axle/single tire ESAL equation to all popular North American widebase steering tire sizes and offer regulators, researchers, and consultants a means to more accurately estimate steering tire pavement impacts.

The paper includes a practical example of the use of these widebase steering tire ESAL equations. In order that new truck configurations in B.C. evolve to create less road damage, TRAN requires that they meet certain safety and performance criteria. One performance criterion is that new truck configurations generate at least 5% less pavement damage (in terms of ESALs per tonne payload) than a specified reference vehicle. Using the widebase steering tire ESAL equations developed in this study, FPInnovations demonstrated that 9-axle tridem-drive log B-trains can have 7300 kg steering loads and still meet this performance threshold. Currently, TRAN is considering the results of this evaluation and, consequently, may increase the steering axle loads of 9-axle tridem-drive log B-trains permitted in B.C.