Keywords: identification, microtremor, natural frequency, natural mode, ARMAV, model updating.

The present study deals with dynamic identification and model updating of the reinforced concrete (RC) building No.5 of the College of Industrial Technology, Nihon University, Japan, before and after applying the slight and/or the partial destructive tests and the mass reduction. The results presented in this paper are based on ARMAV (Auto Regressive Moving Average Vectors) model and IEM (Inverse Eigensensitivity Method) based Model Updating. From the results of microtremor measurement by ambient vibration, the fundamental frequencies of the RC building No.5 are estimated to be about 2.76 Hz and 3.56 Hz in the longitudinal and the span directions, respectively and 4.54 Hz in the torsional mode. The changes of the live load and the damaged areas identified by model updating correspond well to the removal work and the positions where the destructive tests are applied.

The experimental dynamic parameters such as fundamental frequencies, natural mode shapes and damping factors are identified by analysis of the microtremors time-history by means of ARMAV technique [1,2,3,4] which is briefly introduced. The natural frequencies of the RC building No.5 after applying a series of the slight and/or the partial destructive tests are larger than those before applying these tests. In general, as the stiffness of members in the structure reduces, the natural frequencies of the structure become smaller. However the adverse tendency is obtained from the microtremor measurements. It may be caused by appreciable decrease of the live load due to the removal work. It is confirmed by the results of model updating based on IEM.

Model updating aims to minimize differences between an experimental measurement and a theoretical dynamic response of a model [5]. In the paper, this procedure is briefly introduced [6,7] and it is applied to the RC building No.5. The finite element (FE) model is composed of 2-node beam and truss elements. The piers and the beams are modelled by beam elements. On the other hand, an equivalent replacement is done by means of placing the stiffness of the floor slabs and the structural walls equal to those of the equivalent truss elements. The stiffness of the penthouse is ignored and only its weight is taken into account. The boundary condition at the ground level of the RC building No.5 is assumed to be fixed.

The correction procedure uses experimental frequencies and mode shapes, two principal modes in the longitudinal and span directions and a torsional mode for each state, obtained from dynamic identification. As the initial FE model is simplified, there are a few errors up to 6.58% between the measured and the estimated frequencies obtained from the FE model. The MAC (Modal Assurance Criteria) between the measured modes and the analytical ones of the updated model is close to 1 for all modes. After updating, differences between the experimental and the analytical frequencies are less than 0.79%. Having updated the model on the basis of the experimental measurements, it is possible to determine the variations in stiffness of the structural elements. A possible dangerous variation of stiffness characteristics (reduction) is identified at the east and west sides. This result corresponds well to the portions where the slight and/or the partial destructive tests are applied.

1

J.N. Marple, "Digital Spectral Analysis with Applications", Prentice Hall, Englewood Cliffs, 1987.

2

S. Olafsson, R. Sigbjörnsson, "Application of ARMA Models to Estimate Earthquake Ground Motion and Structural Response", Earthquake Engineering and Structural Dynamics, Vol. 24, 951-966, 1995. doi:10.1002/eqe.4290240703

3

A. De Stefano, D. Sabia, L. Sabia, "Structural Identification using ARMAV Models from Noisy Dynamic Response under Unknown Random Excitation", Proc. of DAMAS International Conference, Sheffield, 419-428, 1997.

4

T. Aoki, D. Sabia, "Theoretical and Experimental Analysis of Brick Chimneys, Tokoname, Japan", Computational mechanics, Proc. of WCCM VI in conjunction with APCOM'04, Beijing, September, 1-12 (CD-ROM), 2004.

5

M.I. Friswell, J.E. Mottershead, "Finite Element Model Updating in Structural Dynamics", Dordrecht, Kluwer Academic Publishers, 1995.

6

H. Jung, D.J. Ewins, "Error Sensitivity of the Inverse Eigensensitivity Method for Model Updating", IMAC 10, 1992.

7

D. Rivella, D. Sabia, L. Sabia, "Eigensensitivity Model-Updating in Bridge Structures", Proc. of the third World Conference on Structural Control, Como, 1045-1050, 2002.

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