Stochastic Vehicle‐Queue‐Load Model for Large Bridges

For long‐span bridges, the traffic load can be modeled as white‐noise‐load fields along the lanes. The mean and the intensity of the white‐noise field depend on the traffic situation. Theoretical expressions in terms of the traffic parameters are available for these white‐noise‐field characteristics covering the entire range from free Poissonian traffic to dense congested traffic that in the limit of zero vehicle velocity becomes a standing queue of vehicles. This paper presents a stochastic model for the load‐effect pulse process caused by the formation of queues of stopped vehicles. The key assumptions leading to the model are the following: (1) The probabilistic structure of the succession of cars and trucks in the queue is generated on the basis of the free Poissonian traffic situation; (2) the occurrences of standing queues are Poissonian and sufficiently rare to justify the neglect of the effect of within‐lane overlap; (3) the queue durations and lengths are exponentially distributed; and (4) the central‐limit theorem is effective implying asymptotic Gaussianity of the load effect from any given lane interval fully covered by a stochastically homogeneous traffic load. Assumption 3 is not in conflict with actual observations on a German freeway, whereas assumption 4 has been justified for specific examples by computer simulations using different truck‐weight distributions of given mean and standard deviation. Finally, it is shown by use of the model that the effect of queue overlap in the case of two‐way traffic on the bridge can be decisive for the reliability analysis of the considered component of the bridge.