The development of accurate codes for the design of bridges and the evaluation of existing structures requires adequate assessment of heavy traffic loading and also the dynamic interaction that may occur as this traffic traverses the structure. Current approaches generally first calculate characteristic static load effect and then apply an amplification factor to allow for dynamics. This neglects the significantly-reduced probability of both high static loading and high dynamic amplification occurring simultaneously. This paper presents an assessment procedure whereby only critical loading events are considered to allow for an efficient and accurate determination of independent values for characteristic (lifetime-maximum) static and total (including dynamic interaction) load effects. Initially the critical static loading scenarios for a chosen bridge are determined from Monte Carlo simulation using weigh-in-motion data.The development of a database of 3-dimensional finite element bridge and truck models allows for the analysis of these various combinations of vehicular loading patterns. The identified critical loading scenarios are modelled and analysed individually to obtain the critical total load effect. It is then possible to obtain a correlation between critical static load effect and corresponding total load effect and to extrapolate to find a site-specific dynamic amplification factor.2

When formulating an approach to assess bridge traffic loading with allowance for Vehicle-Bridge Interaction (VBI), a trade-off is necessary between the limited accuracy and computational demands of numerical models and the limited time periods for which experimental data is available. Numerical modelling can simulate sufficient numbers of loading scenarios to determine characteristic total load effects, including an allowance for VBI. However, simulating VBI for years of traffic is computationally expensive, often excessively so. Furthermore, there are a great many uncertainties associated with numerical models such as the road surface profile and the model parameter values (e.g., spring stiffnesses) for the heavy vehicle fleet. On site measurement of total load effect, including the influence of VBI, overcomes many of these uncertainties as measurements are the result of actual loading scenarios as they occur on the bridge. However, it is often impractical to monitor bridges for extended periods of time which raises questions about the accuracy of calculated characteristic load effects.Soft Load Testing, as opposed to Proof Load or Diagnostic Load Testing, is the direct measurement of load effect in bridges subject to random traffic. This paper considers the influence of measurement period on the accuracy of soft load testing predictions of characteristic load effects, including VBI, for bridges with two lanes of opposing traffic. It concludes that, even for relatively short time periods, the estimates are reasonably accurate and tend to be conservative. Provided the data is representative, Soft Load Testing is shown to be a useful tool for calculating characteristic total load effect.

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Critical static bridge loading scenarios are often expressed in terms of the number of vehicles that are present on the bridge at the time of occurrence of maximum lifetime load effect. For example, 1-truck, 2-truck, 3-truck, or 4-truck events usually govern the critical static loading cases in short and medium span bridges. However, the dynamic increment of load effect associated with these maximum static events may be assessed inaccurately if it is calculated in isolation of the rest of the traffic flow. In other words, a heavy vehicle preceding a critical loading case causes the bridge initial conditions of displacement and acceleration to be nonzero when the critical combination of traffic arrives on the bridge. Failure to consider these pre-existing vibrations will result in inaccurate estimation of dynamic amplification. This paper explores these dynamic effects and, using statistical analyses, outlines the relative importance of pre-existing vibrations in the assessment of total traffic load effects.

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