MS19.3 - Traffic Induced Vibrations

The limit values commonly used in Sweden and Norway for structure borne noise from railroad are strict when seen in an international context. At the same time, tunnels with low coverage are regularly built through densely populated areas, which can cause complaints about high structural borne noise levels in nearby dwellings. Traditionally, ballasted tracks are used in most railway tunnels in Norway and Sweden and under ballast mats (UBM) are the usual structure borne noise mitigation measure. Since UBMs that meet technical regulations for rail in many cases do not provide sufficient effect, dispensations have been granted for the use of softer ballast mats in some projects. However, there is limited information about the effect of softer UBMs on maintenance and about whether soft UBMs lead to excessive rail deflections, which could affect the track adversely. There is therefore a need for more knowledge about how UBMs affect the track and especially at higher speeds. When assessing the effect of UBMs on the track, both information about the track displacement amplitude and the shape of the deflection curve is important. As a first step, a pilot study has been carried out to find a suitable method to measure rail deflection, which can be used for long-term monitoring in tunnels with UBM to gather more information. Three measurement methods were compared in a laboratory test rig: Linear Variable Differential (LVDT) displacement sensors, Fiber Bragg Grating (FBG) optical strain sensors, and high sensitivity low frequency accelerometers. The results indicate that by using a high-sensitivity, low-frequency (DC) accelerometer in combination FBG strain sensors, both an accurate determination of the displacement amplitude and information about the shape of the deflection curve can be obtained.

Railway ground-borne vibration emission is an ongoing issue, which shows a need for vibration emission characterization and prediction where the approach used should ensure transferability from one site to another. This transfer is often calculated using heavy frequency-based (narrow band) numerical models. This paper suggests a quicker approach, where the rolling stock/tracks/ground system is considered as a source-receiver mechanical system and the three parts of the system represented by their mechanical mobilities. In such a mobility approach, the passing train acts as a primary vibration source (represented by a line of uncorrelated forces applied to the tracks), and the tracks act as a vibration transfer system between passing train and ground. The couplings between passing train and tracks and between track and ground are both represented by a multi-contact line link with the same spatial resolution. The output quantity is the line of uncorrelated contact forces (contact force density) applied to the ground and the quantity used to evaluate the railway vibration emission is the vibration power transmitted to ground. An application of this mobility approach to IC trains rolling on ballasted tracks lying on typified homogeneous soils (soft, medium and stiff) is presented.

Vibration induced by the train passing causes the stress fluctuation of structural element, which makes it essential to investigate the train-induced fatigue damage of the railway bridge structure. This paper proposes a method for fatigue damage assessment of the rail-way steel truss bridge based on a finite element model, and evaluate the fatigue damage dis-tribution in the bridge structure. This method adopts the rain-flow counting algorithm, the Palmgren-Miner rule, and recommended S-N relationship for steel from the EuroCode to estimate the linear accumulation of fatigue damage. The fatigue damage assessment of the railway steel truss bridge is investigated through a case study of Åby Bridge with a main span of 33m, which is over Åby River in northern Sweden. In order to conduct the dynamic analysis of this bridge structure subjected to moving load, a finite element model which is verified by in-situ measurements is established. Stress time-histories of main structural members obtained from the train-induced dynamic analysis are used to evaluate the fatigue damage distribution and damage degree. Furthermore, the influence of train speed on fa-tigue damage distribution and damage degree of this steel truss bridge is investigated. The results show that the train speed has a significant effect on the fatigue damage distribution and damage degree of this steel truss bridge.