WIM-Based Live Load Model for Bridges

Abstract

The knowledge of the actual live load is essential for a rational management of highway structures. Overloaded vehicles can cause damage including concrete cracking, potholes, excessive vibration and deflection, and even a catastrophic collapse. The design live load HL-93 in AASHTO LRFD Code was developed in the 1990’s based on the truck survey from Ontario (Canada) as the US data was not available. In the meantime, an extensive weigh-in-motion (WIM) database has been collected and provided a rational basis for verification of the live load model. The analyzed WIM database includes over 200 million records, about 1-2 million per year for each location. The economic life of a bridge in the AASHTO code is 75 years. Therefore, the maximum 75-years live load effects, i.e., moments and shear forces are determined using extrapolation and Monte Carlo simulation. A design live load representative for the whole country is proposed. Prediction of live load involves consideration of three groups of vehicles: legal, permit and illegally overloaded vehicles. Therefore, it is essential to identify these three groups in the WIM data. Vehicles that require permits in the WIM database are sorted out using the parameters of issued permits to identify which vehicles have permits. The remaining vehicles can be considered as illegal traffic. The procedure of separation of permit traffic and illegally overloaded vehicles is tested and demonstrated on the example of recorded WIM data and issued-permit data in Alabama. There has been considerable progress in the reliability-based code development procedures. The load and resistance factors in the AASHTO bridge design code were determined using the statistical parameters from the 1970’s and early 1980’s. Therefore, the original calibration was revisited and the load and resistance factors as coordinates of the so-called “design point” were calculated. The analysis is performed for the same types of girder bridges, i.e., reinforced concrete T-beams, prestressed concrete girders and steel girders as in the original calibration presented in NCHRP Report 368. The recommended new load and resistance factors provide consistent reliability and a reasonable safety margin.