Variation of modal parameters of a cable-stayed bridge identified from ambient vibration measurements and FE modelling

During construction, the first cantilever of the Second Severn Crossing cable-stayed bridge was monitored for dynamic effects. Modal parameters of many modes were identified from ambient vibration measurements, and mode shapes and natural frequencies were matched with modes from a Finite Element (FE) model. The accuracy of the measured natural frequencies and damping ratios was estimated, and variations in these parameters were identified from long-term records. The contributions of aerodynamic and structural damping were separated, and the magnitude of the components is discussed. Variations in natural frequencies with wind velocity and, for the finished bridge, traffic loading were identified, but no changes due to temperature were detected, although the temperature range experienced was small. The variations are compared with results from the FE analysis and the significance of the measurement accuracy and natural variations are discussed in relation to Model Updating and Structural Health Monitoring. Finally, evidence of aeroelastic coupling of vertical and torsional modes, during construction, is presented. This arose as the natural frequencies of the two modes were much closer than anticipated, highlighting the need for detailed FE modelling to determine this important frequency ratio accurately.