A computationally efficient methodology is presented for· determination of the
modal parameters of a structure from its measured ambient vibrations at several
instrumented locations. The method, which is an extension of a well
established and very efficient algorithm, MODE-ID constitutes the first step
of a proposed two-step methodology for continuously monitoring the health of
a structure by utilizing its natural ambient vibrations. During this first step,
changes in the values of the modal parameters identified. The second
step involves the determination of the corresponding loss of stiffness within
the structure. The advantage in performing the two steps separately is that
the computationally intensive second step need only be performed when the
identified changes in modal parameters exceed certain thresholds indicating
that structural damage is likely to have occurred. Results of testing the proposed
methodology on simulated data and from an ambient vibration survey
of Caltech's Millikan Library are presented.