Automatic in-Operation Modal Analysis for the continuous monitoring of high-speed railway bridges

Steel-concrete composite bridges have been widely utilising in high-speed railways due to the reduced time and overall cost of the designing and construction phases. Of course a continuous improvement of the design procedures is requested to enhance safety and durability of these civil structures. To achieve these goals, the main point is a correct description of the dynamic behaviour of the structure during operation and in particular during train passages. Experimental test campaigns on existing structures and long-term vibration-based structural health monitoring are hence needed to have a more detailed information on the actual loading and fatigue resistance of this bridge typology, to be utilised for the improvement of numerical structural models. In-Operation Modal Analysis (OMA) has been developed because aimed at the experimental identification of structural dynamics models (i) based on output-only data and (ii) in those that are the real operating conditions. In the area of civil engineering where the possibility of performing a classical modal test of a large structure is complicated by how to obtain and measure the needed excitation inputs, OMA becomes the natural answer. With regards to the employment of permanent monitoring systems in this field, one has to recall that structural identification becomes closely related to the detection of changes in some of the system dynamics parameters - e. g. the time evolution of the natural frequencies - to single out the presence or the growth of a structural damage. In this paper, the possibility of performing an automatic OMA, acting in real time on the data of such a monitoring system has been analysed. The results coming by manually and automatically performing the Operational Modal Analysis of the experimental data have been successfully compared. In particular, two fundamental cases have been considered for the identification of the modal parameters of the bridge, (i) the pure ambient excitation in absence of rail vehicles and (ii) the free vibration of the bridge structure following a train passage.