# MODAL PARAMETER IDENTIFICATION OF A RECTANGULAR BEAM USING EXPERIMENTAL AND OUTPUT-ONLY MODAL ANALYSES

## Palavras-chave:

Output-Only Modal Analysis (O-OMA), Multiple Reference Impact Testing (MRIT), Polyreference Least Square Complex Frequency Domain (p-LSCF)## Resumo

In order to obtain the modal parameters of a structure, different approaches can be used: (i)

Experimental Modal Analysis (EMA), which uses excitation (input) and response (output) data; (ii)

Output-Only Modal Analysis (O-OMA), which uses only the output. Both techniques are widely used

and their advantages and disadvantages are known. The EMA allows performing a full numerical

validation, because the estimated modal matrix is already mass normalized. In the O-OMA case, since

the excitation force is unmeasured, the estimated modal matrix can only be normalized to unity. This

paper aims to identify the modal parameters of a rectangular beam, simulating “free-free” boundary

condition in the excitation’s direction. The beam was instrumented with 9 accelerometers equidistantly

arranged along the length and the excitation was through a multiple reference impact testing. The

modal parameters were identified using the Polyreference Least Square Complex Frequency Domain

method, considering two approaches: (i) EMA and (ii) O-OMA methods. Results from the EMA and

O-OMA showed that the discrepancies between the natural frequencies were at most 0.5%, and all the

vibration modes presented MAC values higher than 0.9. Experimental results were also compared with

those obtained through a finite element model of the rectangular beam, showing small differences both

in the natural frequencies and vibration modes. It is therefore concluded that the two approaches lead

to good estimates of natural frequencies. As for the vibration modes, the modal matrices obtained by

the two methods present a small difference, since the mass of the beam is small.