Dynamic monitoring applied to the detection of structural modifications: a high‐speed railway bridge study

Measurements of vibration on bridges is not a recent practice and many studies have been carried out in the past. What is different nowadays is the increase in the number of dynamic tests carried out on bridges for a better assessment or monitoring of their structural behaviour. The prime objective in a dynamic assessment is the determination, or identification, of the modal characteristics. This includes the natural frequencies, the damping ratios and the mode shapes. For this purpose, dynamic testing is based on three levels: data acquisition (metrology), data treatment (signal processing) and data analysis (modal analysis, assessment). The choice of an appropriate excitation is equally essential. Among its various uses, dynamic assessment as a monitoring and diagnosis technique deserves detailed attention. For many years, the detection of damage has been only by the monitoring of the natural frequencies. The inefficiency of such an approach has led to the development of further techniques. Very recently, various methods have been introduced: the damage index method, the flexibility method and updating methods. Within these methods, the damage index method presents an easy and interesting implementation. However, as to any non-destructive technique (NDT), these methods are driven by many factors that can influence whether or not the inspection will result in the correct decision as to the absence or presence of structural modifications. Their reliability is therefore an essential point. After pointing out the objectives of dynamic bridge testing, this paper highlights metrology, methodology, computational and practical aspects. All these concepts are highlighted on the monitoring of structural modifications performed on a high-speed railway bridge under ambient excitation.