Keywords: structural health monitoring, damage detection, bridges, vibration, modal analysis.

In recent times, structural health monitoring (SHM) based on variations in vibration characteristics have emerged as an efficient technique. Health monitoring techniques based on processing vibration measurements basically handle two types of characteristics: the structural parameters (mass, stiffness, flexibility, damping) and the modal parameters (modal frequencies, mode shapes and damping ratios). As the dynamic characteristics of a structure, namely natural frequencies and mode shapes, are known to be functions of its stiffness and mass distribution, variations in modal frequencies and mode shapes can be an effective indication of bridge deterioration. Deterioration of a structure results in a reduction of its stiffness which causes the change in its dynamics characteristics. Thus, monitoring the change in these dynamic characteristics enables us to infer structural deterioration. The objective of this paper is to evaluate the effectiveness of using the changes of natural frequencies, modal flexibility matrix and modal strain energy to non-destructively evaluate single and multiple damages in beam type of bridges.

Two damage localization parameters (1) modal flexibility matrix and (2) modal strain energy based damage index are evaluated from the modal parameters to assess the state of health in beam structures. The changes in these two parameters between the undamaged and damaged structure provide a basis for identification of localized damage.

Finite element techniques are used to carry out modal analysis of the beam structures. The results are used to calculate the modal flexibility matrix and the modal strain energy based damage index and thereby assess the damage in the beam structures. Initially a finite element model of simply supported beam is calibrated against the results from experimental testing. Further finite element analysis (FEA) is performed to determine the modal parameters of two-span and three-span continuous beams before and after damage. To simulate damage, the beams are cut to cause flaws on the tension face of the beam. Nine damage cases are investigated in this study.

By applying the modal flexibility method to the beams, it is observed that single or multiple damage can be confidently located with no localization error. Damage indices using strain energy changes successfully locate the single and multiple damage in most damage cases. The implementation of the second approach is more time consuming than the first approach for damage localization. It is concluded that the modal flexibility method appears to be more sensitive, precise and convenient to determine than the modal strain energy method for damage assessment of two dimensional beam structures. Changes in natural frequencies can be used to detect the presence of a state of damage, since this can be done from a single point measurement. Once the presence of damage is detected, modal flexibility method or modal strain energy method can be used to locate the damage. Finally, damage detection model based on change in natural frequency, modal flexibility matrix and damage index will be developed to provide accurate damage assessment.

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