A suggested approach using frequency analysis to detect the damages in reinforced concrete structures

Abstract

A damaged structure has reduced stiffness due to material and/or some weakened cross-section, and degradation in strength. The motivation for this study is to find out what the distinguishing characteristics in the spectral response of such a structure are during the process of supporting the external load. By applying some tests in both numerical and experimental physical models, which are carefully designed to experience the most representative working conditions concerning controllable and uncontrollable factors, the response of the structure is analyzed extensively for figuring out the typical modes of damages or failure. If the amount of data of the response of a real structure are obtained sufficiently in terms of signals in the time domain and then analyzing them in the frequency domain, the kind of damage could be determined. The structure to be investigated in this study is a simple beam with 1% reinforcement which is deliberately damaged. A numerical model as the pilot test is developed first associated with scenarios of crack positions, symmetric and unsymmetric located damages, low-density. A two-stage strategy including static and vibration tests is suggested. For ensuring the most real condition, some kinds of restraints are altered and assigned into the model for bringing the structure closer to the reality. Results indicate that the variation in the natural frequency of the simply supported beam modeled by SAP2000 is more complicated in the unsymmetric pattens of cracks than that of the symmetric ones; and if the cracks are alternatingly located, the structure still have higher stiffness than that of continuous cracks. Besides, low-density results in a decrease in responsive frequencies from 5 % to 7.2% those in good quality RC structures.