This paper describes two methods for calibrating weigh-in-motion (WIM) systems with the aid of dynamic vehicle simulations. A modified version of the vehicle simulation model VESYM, named VESYMF, is used for predicting the dynamic axle load variation exerted by heavy vehicles at a WIM site.
The first method uses test trucks performing controlled runs over a WIM system, while the second method relies on the consistent properties of the steering axle loads of traffic stream 5-axle tractor semi-trailer trucks such as the 3-S2. For the first method, test truck runs are performed at a number of replicate speeds and the WIM measurements of individual axles are plotted versus speed. For a given axle and speed, the variation in WIM measurements is equal to the precision of the WIM system. Combining this source of variation with the dynamics-induced variation calculated through VESYMF, gives the anticipated frequency distribution in WIM measurements for the pavement roughness at a site. This is used for estimating the probability of a particular WIM measurement, which in turn allows the calculation of the weighed average of the axle load WIM measurements obtained at different speeds. Calibration is effected by fitting a zero-intercept linear regression equation between the static and the average WIM axle load measurements.
The second method is based on the consistent properties exhibited by the mean load of the steering axles of traffic-stream 3-S2 trucks. Its combined variance is calculated by adding the variance in static 3-S2 steering axle loads for traffic-stream vehicles to the other two components of variance described previously. This establishes a confidence interval for the mean of the WIM measurements of a number of 3-S2 steering axle loads. The feasibility of these two methods was demonstrated by field experimentation.
Associate professor of Civil and Environmental Engineering, Washington State University, Pullman, WA
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