Disadvantage of modal analysis for safety assessment shown by vibration examples in concrete structures

System safety assessment in Structural Health Monitoring (SHM) systems can prevent structural failure of safety-critical structures both in mechanic engineering and civil engineering. As a linear method, modal analysis has been applied extensively in SHM systems. However, it is not reliable for civil engineering application due to the unique damage mechanism in concrete structures. This paper shows major disadvantages of damaged assessment index in civil engineering based on modal parameters based on four examples including numerical, experimental examples, as well as a field test. The first two numerical examples with different crack models demonstrate the low sensitivity of frequency index. The third one shows damping ratio cannot correspond with reality during the damage evolution from a field test. The final one is an experiment that shows both low sensitivity of frequency index and scatter of damping ratio index. Therefore, it is necessary to study nonlinear methods, which are probably suitable to reflect the damage mechanism and features of structures.