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In the doctoral dissertation a live load model for long span structures was derived. The live load model is valid for spans between 600 ft and 5000 ft and it is intended to reflect current traffic patterns, quantities of trucks and their weights. The live load models available were developed for short and medium span bridges. Those models were not appropriate for long span bridges due to different types of structure and critical traffic patterns. Live load on long spans depends on traffic mix. One heavily overloaded truck does not have significant influence. Moreover, the continuous increase in the number of the trucks, their weights, and high percentage of overweight trucks led to a search for the newest traffic data. The database includes variety of sites within many different states. A numerical procedure was developed to process the database and simulate traffic jam situations. From the simulation the values of uniformly distributed load were derived. Trucks were kept in actual order, as recorded in the WIM surveys. Results of the simulations were plotted as a cumulative distribution function of uniformly distributed load for considered span lengths. For longer spans, uniformly distributed load decreases and is closer to the mean value. The bias factors were calculated for the heaviest 75-year combination of vehicles. The 75-year uniformly distributed loads were derived from extrapolated distributions. It was stated that for most of the bridges current live load HL-93 is appropriate. It was also noticed that some bridges, characterized by high ADTT and increased percentage of overloaded loaded vehicles, require special attention and application of increased design live load. The developed live load model is recommended to be taken into consideration in the bridge design code.