Customised active monitoring system for structural control and maintenance optimisation

The paper illustrates the development and the application of an active monitoring system, and analyzes the investigated dynamic behaviour of the structure where the system is applied. The system is installed on a 250 m suspended arch steel bridge that has been instrumented with sensors of different type. This work focuses on the employed methodologies and obtained results related to the dynamic monitoring of the bridge. The use of the Operational Modal Analysis techniques with data-driven stochastic subspace identification algorithms allows the extraction of the structural dynamic characteristics: natural frequencies, damping ratios, and mode shapes. These data are in overall accordance with those calculated through the computational model utilized by the active monitoring system. Besides, the definition of a range of scattering of modal parameters, under specific conditions, has been obtained. The bridge has very low vibration frequencies, below 1 Hz for all significant modes. The outcomes indicate that the estimation of dynamic characteristics and the corresponding accuracy is influenced by several factors, including the length of the accelerometric acquisitions and the followed specific procedures. The impact of environmental factors, with particular reference to the temperature, is examined. Compared to the damping ratio, the natural frequency shows higher estimation accuracy and marked sensitivity to the environmental factor. Consequently, in the selection of benchmark for damage detection, it should be taken into account that the two modal parameters have specific criticalities. Once that the modal parameter has been chosen, values outside the estimated uncertainty range can be considered as alarm triggers.