Abstract
It is difficult to estimate actual stiffness and mass of a structure, although the ratio of them can be easily determined from resonant frequencies. Since we have almost no direct observation of actual stiffness or mass, we try to estimate both the actual mass and stiffness of a reinforced concrete building through an innovative approach. The method of this study is based on the resonant frequency shift. First, we identify modal parameters (resonant frequency and damping factor) by the curve fitting in frequency domain for both original condition and shifted condition due to an added mass with known amount. We estimate three-dimensional vibration characteristics by mode separation based on a rigid body assumption. Then we estimate actual stiffness and mass as a lumped mass vibration model to fit the resonant frequencies for both conditions using eigenvalue analysis. We obtain the mass to be 1.44 times larger than the design value and the stiffness to be 1.36 or 2.60 times of the frame estimate. These estimated stiffnesses are within the range of standard initial base-shear coefficient determined by damage statistics in Kobe.
