Keywords: track irregularities, railway bridges, ballast, high speed railway.

The computation of the response of railway bridges to moving trains is a complex task involving dynamic interaction between the vehicles and the bridge. The train crossing the bridge at high speed induces dynamic impact on the structure; the vibration of the bridge affects the stability and comfort of the train. The dynamic response of the bridge is conditioned by several factors: the characteristics of the bridge, first frequency and damping; the characteristics of the vehicles: speed, number of axles, axle spacing, axle loads and the characteristics of the ballast track, stiffness, damping and roughness profile. Several of these factors are random or dominated by statistical uncertainties, for example the track irregularities, and an appropriate assessment of the dynamic response of the bridge requires the development of probabilistic models. Several researchers and railway institutions have studied the influence of the irregularities on the dynamic behaviour of the system vehicle-ballast track-bridge.

The purpose of this paper is to present the dynamic response of a flexible bridge with a medium length span when different profiles for superelevation irregularities are considered. Two wavelength (lambda) ranges are used: D1, 3 <= lambda <= 25 m, and D2, 25 <= lambda <= 70 m. The amplitude was defined as the maximum value that triggers the alarm limits given in EN13848, for maintenance operations. The maximum response of the bridge, displacements and accelerations, at the mid span, and the acceleration on a sprung mass are analysed. Since the track is included in the model the response of the ballast is also analysed.

The analysis of the bridge response allows the conclusion that track irregularities affect more deck accelerations than defections, especially for speeds under 220 km/h. For the critical speed both responses are similar to the perfect railway. This conclusion applies also to the response of the ballast mass. However the irregularities D2 are more adverse than the irregularities D1.

The normal contact forces between the wheel and the rail are also analysed and it can be concluded that the consideration of irregularities establishes maximum and minimum envelopes of the contact forces between the rail and the wheel. Although for the perfect railway, the normal contact forces increase as the speed of the train increases. For the railway with random irregularities this conclusion does not apply. For the case study the maximum values happens for speeds between 180 km/h and 257.5 km/h, about the critical speed and mainly for the irregularities type D2.

Concerning the comfort level, the consideration of irregularities decreases the comfort level obtained on the perfect railway. For the considered case study, the irregularities D2 induce very high vertical accelerations on the fourth sprung mass of the train, leading to a level of comfort lower than the acceptable level. The irregularities D1 determine values for vertical accelerations which the acceptable level is dominated in all dynamic analysis. Therefore, we can conclude that the presence of irregularities on the railway track mainly influence the behaviour of the ballast, the contact forces between the wheel and the rail and the vertical acceleration of the sprung mass. For the bridge analysed the traffic safety and the passenger comfort are influenced by the presence of irregularities, even for irregularities defined according to the alert limit.

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