Calibration of the AASHTO LRFD Code required the statistical parameters of load andresistance parameters. The basic load combination includes dead load, live load and dynamicload. In 1980’s, there was no reliable Weigh-in-Motion (WIM) data base and, therefore, thedevelopment of live load factors was based on a small scale truck survey. In the meantime, theWIM technology was improved and millions of vehicles were recorded in various geographicallocations. The objective of this study is to review the available WIM data of about 35 milliontrucks and determine the statistical parameters of gross vehicle weight (GVW) and live loadmoment. Moments were calculated for simple spans using influence lines. The span lengthrange is from 30 to 200 ft (9 to 60m). The cumulative distribution functions (CDF) were plottedon the normal probability paper for an easier interpretation. CDF’s of GVW and moments showa considerable variation depending on WIM station location. Maximum expected values of liveload depends on the considered time period and, in general, for strength limit states the timeperiods cane be 75 or 100 years, and for service limit states they are much shorter, few days orweeks. Therefore, the statistical parameters are determined for time periods from 1 day through100 years and for traffic volumes with ADTT from 100 through 10,000. For longer time periods,the results were obtained by extrapolation of the available WIM data. The statistical analysis ofmoments provide a basis for the development of national live load parameters and live loadfactors in the bridge design code.
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