WIM-based live load for bridges

Abstract

Calibration of the AASHTO LRFD Code required the statistical parameters of load and resistance parameters. The basic load combination includes dead load, live load and dynamic load. In 1980’s, there was no reliable Weigh-in-Motion (WIM) data base and, therefore, the development of live load factors was based on a small scale truck survey. In the meantime, the WIM technology was improved and millions of vehicles were recorded in various geographical locations. The objective of this study is to review the available WIM data of about 35 million trucks and determine the statistical parameters of Gross Vehicle Weight (GVW) and live load moment. Moments were calculated for simple spans using influence lines. The span length range is from 30 to 200 ft (9 to 60 m). The Cumulative Distribution Functions (CDF) were plotted on the normal probability paper for an easier interpretation. CDF’s of GVW and moments show a considerable variation depending on WIM station location. Maximum expected values of live load depends on the considered time period and, in general, for strength limit states the time periods can be 75 or 100 years, and for service limit states they are much shorter, few days or weeks. Therefore, the statistical parameters are determined for time periods from 1 day through 100 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 of moments provides a basis for the development of national live load parameters and live load factors in the bridge design code.