  Operational modal analysis (OMA) aims at identifying the modal properties of
a structure based on response data of the structure excited by ambient sources.
Modal parameters of the ambient vibration structures consist of natural
frequencies, mode shapes, and modal damping ratios. In this paper, a typical
frame with arbitrary loading has been modeled in finite element software,
ANSYS, and the responses (accelerations of nodes) have been achieved. By using
these data and the codes written in MATLAB, the modal parameters (natural
frequencies, mode shapes) of the beams are identified with FDD (Frequency
Domain Decomposition) and PP (Peak Picking) methods and then justified with the
results of input-output method which was determined by modal analysis. The
results indicate that there is a good agreement between three methods for
determining the dynamic characteristics of frames. Then, the minimum number of
required accelerometer has been determined that recording the acceleration in
that location leads to identifying all the natural frequencies of the frame.
