Keywords: structural identification, wind-induced loads, traffic-induced loads, blast-induced loads.

The reinforced concrete tower of the provincial administration building of Bari, Italy, is a building of the 1930s, about 60 meters tall. It is situated at the seafront; the principal facade is in the north-west direction, that is the dominant wind direction, and in the vicinity of roads with highest traffic level. For this reason, the ambient excitations due to wind and to traffic can cause non-negligible vibrations of the tower, sufficient to extract, through appropriate identification techniques, the dynamic characteristics of the structure, to be used in a health monitoring procedure.

The difficulty of characterizing and measuring the significant parameters of environmental actions, has motivated in civil engineering the study of techniques of identification under unknown input, which are based on the measurement of the structural response at some points. Such techniques have also the advantage that a limited number of operations are carried out on the building; the small number of sensors for the measurement of structural response; and the non-interruption of the normal functionality of the building itself. Moreover, they do not require the additional costs of devices for acting artificial excitations. In addition, the high precision of the measuring instruments allows accurately vibrations to be recorded due to very low actions, for which a linear structural model will be sufficient even in the presence of previous damage.

For this reason, an in-situ investigation under ambient excitations has been performed, with measurements at several levels; experimental data has been used to identify the modal parameters in a linear model of the structure, namely natural frequencies, modal shapes and damping coefficients.

For structural systems with linear behavior [1,2,3] a procedure in the frequency domain for the identification of the modal parameters (components of the modal shapes, natural frequencies and modal damping) in the case of unknown and non-stationary seismic excitation has been recently introduced. The procedure, which can be transferred to the time domain and extended to different external actions, is based on the observation that the ratios of the Fourier transforms of the responses in different points of the building, which turn out to be independent of the forcing itself, and therefore can give experimental information for the identification of modal parameters.

This paper reports the details of the experimental investigation and the preliminary analyses, performed both in the time and in the frequency domain.

1

D. Capecchi, M. De Angelis, V. Sepe, "Identification of structures subject to unknown seismic excitation", 3rd Joint Conference GIMC and AACME, Giulianova, Italy, June 2002.

2

D. Capecchi, M. De Angelis, V. Sepe, "Modal model identification with unknown nonstationary base motion", Meccanica, 39: 31-45, 2004. doi:10.1023/A:1026210723791

3

M. De Angelis, V. Sepe, D. Capecchi, "Identification of modal parameters with unknown input", Problems in Structural Identification and Diagnostics: general aspects and applications, CISM Course n.471, 2003.

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