Analysis of large quantities of weigh in motion data and the implications for bridge and pavement loads

The national association of Australian state road authorities is revising its bridge design code and adopting a limit states approach. This paper describes the philosophy adopted for the definition of the design traffic loads and studies to estimate these loads from traffic data. Design methods require definition of three types of load: (i) 'average load' and frequency of occurrence for fatigue design; (ii) a serviceability load expected to occur a few times in the design life of the structure; (iii) an extreme load with a very low probability of occurrence to ensure adequate strength, stability and safety. 'Average' loads can be predicted with relatively small amounts of data. There are a number of mathematical 'tools' that can be used to estimate the more extreme loads. Whatever tools are used, the more data available, the more confident the prediction. The culway system has enabled measurement of traffic loads with recurrence intervals ranging from seconds to more than a year and a large database is becoming available for analysis (a).