Modal model identification of structures under unmeasured seismic excitations

Abstract

Techniques developed for structural identification of a structural model are typically based on information regarding the response and the forcing actions. However, in some situations it can be necessary, or simply useful, to refer only to the measured responses. In this paper we describe a technique suitable for identifying the modal model of a spatial frame in the frequency domain when the seismic input is unknown both in time contents and direction. In some previous theoretical works we established that this identification problem has a unique solution when at least three time-history responses are known. Here numerical techniques are developed which allow the evaluation of the modal quantities in practice. Numerical applications are carried out on plane and spatial framed structures by using a modal model which may be complete, including all the structure's modes, or incomplete, including only the lowest modes. In most cases the obtained results are satisfactory. Copyright © 2005 John Wiley & Sons, Ltd

Modal Analysis of a Frame Model Under Unmeasured Seismic Input

“…The identification problem can be therefore solved, by minimizing the error between the theoretical expression of Rij() and its measured counterpart, that only depends on the measured response. As already demonstrated in [1], the problem thus defined admits only one solution if the measures of the structural response in at least three different dofs are available.…”

“…In this paper the attention is focused on the dynamic identification of linear structural systems submitted to non-stationary input, by means of a numerical formulation developed in the frequency domain by one of the writers [1] in case of unknown input. It is based on the independence by the input measure of the ratio between the Fourier transforms (FT) of the responses measured at different dofs of the structure.…”

“…It is based on the independence by the input measure of the ratio between the Fourier transforms (FT) of the responses measured at different dofs of the structure. The procedure has been numerically validated in [1] and checked by the writers in a preliminary experimental campaign [3].…”

“…seismic input simulated by a unidirectional base motion, as considered here), the time-history of the external action can be difficult to measure. In this case, however, the identification problem can be solved [1] by evaluating the ratio between the FT of the output measured at two degrees of freedom i and j of the structure, that does not depend on the input:…”

“…A possible solution to these limitations has been already given, among the others, in a previous paper of one of the writers [1], where an extension of the classical structural identification procedure in frequency domain (e.g. Goyder [2]) has been provided in the case of spatial frames subjected to unknown (time contents and direction) seismic input.…”