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.