Procedures for Calibration of Eurocode Traffic Load Model 1 for National Conditions

Since April 2010 Eurocode Load Model 1 (LM1) is the prescribed traffic load model to be employed in the design of highway bridges in the European Union (EU). Uniquely, the code permits member states to calibrate the load model, through the application of 'α-factors' to allow for national or regional conditions. Some countries with high volumes of very heavy traffic may find that they require α-factors in excess of unity whilst other less heavily trafficked road networks may require much lesser values. The importance of accurate calibration of the α-factors is clear from a safety and economic point of view. This paper describes procedures for calibration of α-factors using Weigh in Motion (WIM) data. WIM data allows classification of the traffic loads in individual countries, enabling the specific Gross Vehicle Weights (GVWs), axle loads and frequencies of heavy trucks to be taken into account. Simulations calibrated using this data, for a wide range of structural forms (i.e., influence lines, spans and numbers of lanes) and scenario types (i.e., free flowing, congested and mixed traffic conditions); allow comparison of the load effects generated by the site-specific traffic to those obtained when employing LM1. Statistical Extreme Value Distributions (EVDs) are fitted to simulated results to determine characteristic load effect values using the same methodology as was employed in the calibration of LM1 itself. Appropriate α adjustment factors are then determined to cater for variation in predicted characteristic extreme load effects on a network by network basis. Where α<1.0, the prescribed approach delivers significant savings by preventing unnecessary overdesign of bridges. On the other hand, for cases where α>1.0 it allows bridge designers to design bridges with adequate levels of safety.