Validation of a modal identification procedure for linear frames under unmeasured seismic input

The paper deals with the frequency domain identification of linear structures subjected to unmeasured seismic excitations. Starting from the classical and widely acknowledged transfer-function based modal technique for structures under known excitation, one of the writers co-authored, in previous theoretical works, an approach to solve the identification problem in case of unmeasured seismic excitations. This paper reports the first results of an experimental campaign, still in progress, aimed at validating the procedure. It is found, at least for the experimental data so far obtained, that in the case of unmeasured excitation multiple solutions can be found, depending on the initial estimate of the parameters to identify (natural frequencies, damping coefficients and modal shapes). To this respect, an appropriate choice of their lower and upper boundaries can be crucial; fortunately, to some extent, such information is always available for real structures. A minimisation algorithm is therefore introduced in the paper that decompose the complex-valued transfer functions and the unknown modal parameters in their real and imaginary parts. Considered that this output-only identification procedure needs at least three known time-histories of the outputs (e.g. floor accelerations), the sample case of a three storeys shear-type frame is discussed.