# A Feasibility Study of the Drive-By Method for Damage Detection in Railway Bridges
## 1 Introduction  
Health monitoring of railway bridges is gaining more and more interest in the management of railway assets, since most of the commuter networks date back many decades, and the requirement to increase speed and axle loads on existing lines poses new challenges to the maintenance of aged structures. Several kinds of instruments are proposed in the literature to measure bridge strains, accelerations, and displacements, and structural properties like natural frequencies, mode shapes, and modal parameters [1]. Some of the main ones to be mentioned are strain gauges and fiber optics, accelerometers, potentiometers, global positioning system (GPS) receivers, and vision systems for displacements. These measures are then adopted to assess the health state of the bridge, either through model-based or data-driven methods [1]. The monitoring of railway bridges through sensors installed on the structure is economically viable in the case of the main bridges or modern high-speed lines [2; 3], but it is not practicable for the large number of bridges of different ages and types that are present in the main ordinary railway lines. These bridges are therefore usually monitored through periodic visual inspections, which nevertheless require a relevant effort in terms of manpower and time. Moreover,since visual inspections rely on human assessments, their outcomes can significantly vary, depending on the inspector. Between two following checks, no information regarding the health of the structure is gained, which can lead to a late detection of the damage, and to service disruption. In this respect, a continuous follow-up would be preferable. The issue of how to assess the structural condition and integrity of many bridges of different ages and types in an affordable and efficient way is therefore a relevant topic, currently under discussion. A recent perspective to monitor the structural status of railway bridges consists of so-called drive-by damage detection [4; 5; 6], i.e., the concept of directly using the transit of the train on the bridge as an excitation source, and sensors installed on the vehicle rather than on the bridge. A restricted number of commercial trains fitted with few sensors would enable the monitoring of the status of a great number of structures, using the train response to detect the presence of damage in the bridge. The detection method basically relies on the comparison of the acceleration data obtained from instrumented trains against a reference condition corresponding to the healthy structure; accelerations measured on train are processed through proper algorithms, which should be able to detect an alteration in the behavior of the train-track-bridge system. If on the one hand, the drive-by method can successfully be implemented in the case of resonant bridges, thanks to the high level of acceleration generated when the train excites the natural frequency of the bridge [7], the application becomes more challenging when the first mode of vibration of the bridge is not relevantly excited by the train passage [5]. The main challenge in this case is the small difference in the acceleration levels to be detected, to discern a damaged structure from an undamaged structure, which may result in a low signal-to-noise ratio. Suspension stages from the bridge structure to the measuring vehicle, consisting of railway equipment, of the interface contact between wheel and rail, and finally of the train primary and secondary suspensions, represent an extra barrier that makes damage detection harder. For these reasons, a successful application of the drive-by method necessarily requires the investigation of the best position for sensor placement on-board train (i.e., axlebox, bogie frame, or carbody), and the implementation of a suitable algorithm for the analysis of the acceleration signals. This work presents the results of a plan of numerical simulations, intended to be an analysis of the different aspects which are likely to affect the applicability of the drive-by damage detection method.