Identification of localized frame parameters using higher natural modes

The use of sensitive higher modes in physical structural parameter identification of local members in a frame system is discussed in this paper. Analytical studies are supported by low-level vibration tests on a third-scale four-story reinforced concrete frame structure embedded in soil to represent a realistic foundation system. Preliminary numerical modal analysis are carried out to establish the sensitivity of higher modes to localized damage in the frame. Damage to one of the elements on the first story of the frame was simulated by adding a mass to the column and examining the vibration modes before and after the addition of the mass. Through forced vibration tests and selective placement of strain transducers, the Poly-reference least-squares complex frequency domain method (PolyMAX) is used to determine the existence of ‘highly sensitive higher modes’ (HSHMs). An evaluation of the identified higher modes further enables the identification of physical parameters of the target column element using a simple minimization scheme. Findings from the present study indicate that physical parameters of a local element in a frame structure can be identified effectively using HSHMs and that the higher modes of vibration are more sensitive to changes in local physical parameters than lower global modes. The identification of physical parameters as outlined in this study can be applied in structural damage detection.