# A Feasibility Study of the Drive-By Method for Damage Detection in Railway Bridges
## 4 Simulation Results
### Acceleration Level and Damage Localization  
As already mentioned, one of the more challenging issues of the drive-by method is related to the small differences in accelerations to be detected, to discern between a case of a healthy bridge from an unhealthy one. This leads to problems of a low signal-to-noise ratio, and the sensitivity of the accelerometers in the lowest frequency range. To comment on the feasibility of this method, this section shows the results of the acceleration levels of the damage component, which should be as high as possible. Figure 16 reports the influence of the train speed and of the damage entity on the damage component of the acceleration. These exemplary results refer to bridge #2 and to CSA train type, results from the TSR train being similar in order of magnitude. The damage is located at mid-span.  
As obviously expected, the magnitude of the damage component increases both with speed and damage level, with high damage levels being more speed-sensitive. The highest speed seems to be the best perspective for damage identification; however, this aspect may contrast with the need of the accuracy of damage localization, as shown in Figure 17, reporting the influence of damage level and the train speed on the accuracy of damage localization. The vertical red lines in the figure define the non-dimensional positions of the starting and ending point of the damaged finite element. It can be observed that the algorithm correctly localizes the damage within this range for lower speeds, whereas at a speed of 140 km/h, the damage is located a bit further the actual defect (non-dimensional location 0.516, with an absolute error corresponding to less than 1 m). When increasing the train speed, a delay in the damage localization appears. This trend is representative of all the other bridge models, measuring point locations on trains and damage located at a quarter of the span length.  
Figure 18 shows the relation between the magnitude of the damage component and the span length of the bridge. These results are obtained from simulations with all the bridge models and damage located at midspan, considering all damage levels. The results corresponding to CSA train type at 100 km/h are shown, with similar results being obtained for other speeds and the TSR train.  
The highest magnitudes of the damage signals are obtained for the shortest span lengths. This result is consistent with the fact that, even if the bridges with the longest span are subjected to higher displacements due to the simultaneous loading of more than one bogie, shorter bridges are crossed in a shorter time, resulting in higher accelerations on the bogie. Assuming that the acceleration difference between the damaged and healthy bridges is proportional to the peak value, the effect of a defect in the bridge is therefore more visible in shorter bridges.  
To analyze the influence of the span length on the accuracy of the damage localization, the DLE index was calculated, averaging over all of the speeds for one bridge model and one damage  
Figure 16: Acceleration magnitude of the damage component. Train CSA type, bridge #2, damage located at mid-span.  
level. Results showed that the accuracy of the damage localization technique is higher for the shorter span length, so there is no conflict in this case between the two prerequisites of accuracy and acceleration amplitude.