Experimental Determination of Dynamic Allowance for Traffic Loading in Bridges

Bridge codes adopt values for dynamic allowance in traffic load models that are necessarily conservative to cover for an entire range of bridges with different mechanical characteristics, boundary conditions, and the large number of uncertainties associated to the vehicle-bridge interaction problem. A further level of conservatism occurs due to the independent manner in which the governing static load and the corresponding allowance for dynamics are specified. In particular, certain bridges are not susceptible to high levels of vehicle-bridge interaction when loaded by a critically heavy vehicle or a critical combination of vehicles. Recent advances in Bridge Weigh-In-Motion technology allow not only to collect information on the weights, spacings and speeds of the traffic loads traversing a bridge, but also to separate the maximum static strain from the total measured strain using a filtering procedure. In this paper, maximum static and total load effects are collected and analysed for three different sites as part of the European project ARCHES (6 RTD framework programme). Bridge measurements are used to discuss the dynamics of the most frequent truck classes and the entire traffic sample. The measurements reveal a decrease in percentage increment in dynamics and a reduction on the variability of the dynamic increment as the static load effect increases. This phenomenon can be of particular relevance in the assessment of the dynamics of extreme loading cases. INTRODUCTION When assessing the traffic load on a bridge, it is important to take into count the bridge code that was used in practice at the time the bridge was designed. An accurate assessment of the current traffic load may prevent a bridge from unnecessary strengthening or replacement, but it is necessary to account for the increases in heavy trafficking and variations in vehicle populations when assessing the capacity of such a bridge. There is also a large degree of uncertainty associated with the dynamic amplification caused by the interaction of the bridge with the imposed traffic load. It is common practice to use a dynamic allowance or a similar parameter to allow for the uncertainties associated with the structure, the material and the applied traffic load. AASHTO (1) defines a factor called Dynamic Load Allowance (DLA) that is applied to the static live load. DLA is shown in Equation 1, where Dsta is the static live load effect, and Ddyn is the increase in live load effect due to dynamics. The Eurocode (2) uses traffic load models with built-in Dynamic Amplification Factors (DAFs) that vary depending on the span length, number of lanes and load effect. DAF is presented in Equation 2. Both AASTHO and Eurocode traffic load models specify additional dynamic factors for individual components such as expansion joints. These load models are based on field tests and numerical simulations that cover a wide range of scenarios (3-5). If there was no site-specific information available to the engineer, these recommendations on dynamic allowance represent conservative values to follow.