The gamma and lognormal while some authors have used

The data base includes truck configuration (number of axles and axle spacing) and weights (axle loads and gross vehicle weight). For each truck from the measured data the maximum bending moment and shear force in simply supported spans, and maximum negative bending moment for a girder with two equal spans was determined. The calculations were carried out for span lengths from 9.1 m to 61 m.The resulting cumulative distribution functions (CDF) for positive moment, negative moment and shear, were plotted on the normal probability paper for an easier interpretation and extrapolation.Moses (2001) 13 states that the truck load adopted in the American Association of State Highway and Transportation Officials (AASHTO) specifications agreed approximately with a normal distribution, where the mean value was 300 kN and the coefficient of variation was 26.5%, which were not inconsistent with those of the LRFD.O’ Brien and Caprani (2005) 14 express the inter-vehicle gaps as the time between the front axles of successive vehicles arriving at the same point on the road. Distributions that have been used include the negative exponential, uniform, gamma and lognormal while some authors have used deterministic gaps. The negative exponential distribution assumes that the arrival of vehicles is Poisson distribution.Fu and You 15, 16 investigated the vehicle load statistics characteristics, and by using the theory of extreme value they specified the possible maximum value distribution of vehicles at the bridge service life, where they obtained the vehicle load factor by reliability index analysis. Obrien et al. (2010) 17 Depending on WIM data for vehicles at two locations?  the first in Netherlands and the other in Slovakia Analyzed the effects of various parameters on the fitted results in a truck load model and showed the high sensitivity of the fit. Zhao and Tabatabai (2012) 18 Based on truck data from three sites in Indiana, USA. Evaluated a typical common vehicle model for the region using six million vehicle by using the statistical analysis of maximum bending moment and shear at the critical location of a simply supported, 2-span and 3-span continuous beams. The evaluation indicated that the measured data may cause larger bending moment and shear than the standard truck load.

Enright and O’Brien (2013) 18 make use of WIM-data to estimate the inter-vehicle gap as the gap between the rear axle of the leading truck and the front axle of the following truck. The observed data is used to fit distributions for the gaps for each lane and different flow rates in approximately 20 increments up to the maximum observed flow ate in each lane.

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