New live load models for highway bridge design in Brazil are under development by assembling real traffic database, traffic simulations, analytical-numerical modeling of the dynamic interaction between vehicle and structure and statistical extrapolations. This paper presents and discusses the results obtained in the first stages of this work which includes the comparison between the static effects due to the actual traffic of heavy vehicles and those generated by the live load model given in the current national code NBR 7188. It is demonstrated that this live load model is not appropriate to represent the actual traffic effects and may be, in some cases, non-conservative. The present work deals with short span bridges for two lanes single carriageway under free flow traffic scenarios. The representative static effects in these bridges due to the actual traffic of heavy vehicles are obtained by extrapolating its probability density functions to a certain return period. To this purpose, a traffic database was constructed by gathering data from several weighing stations in Brazilian highways which was then applied to perform traffic simulations through a specially developed computational tool.
The Brazilian code NBR 7188 for highway bridge design prescribes a live load model consisting of a 3-axle vehicle plus a uniform load, which are both affected by an impact factor when considering dynamic effects. It does not play out actual traffic static effects and may be, in some cases, non-conservative. This work presents two load model configurations developed by gathering a real traffic database, traffic simulations, modeling of the dynamic interaction between vehicles and structure, and statistical extrapolations. Proposed load models comprise dynamic effects and were calibrated for two-lane single carriageway bridges with span lengths up to 40 m under free-flowing traffic. The target values of the effects were compared to those generated by the new proposed load models, as well as by the Brazilian design load model. Adequacy of the proposed models is demonstrated; results show that the effects generated by the Brazilian design load model are underestimated in many cases, particularly for negative moments in cantilevers.
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