Experimental Methods Used in System Identification of Civil Engineering Structures

To experimentally identify the dynamic characteristics of a structure, also referred to as system identification, two methods are available: Forced Vibration Testing (FVT) and Ambient Vibration Testing (AVT). The basic ideas of these methods are shortly presented. The main part of the paper deals with practical problems which are to be overcome when performing such a system identification test. To achieve reliable, good quality results, a number of parameters have to be optimized when planning an experimental system identification investigation. Among others, such parameters may be the means and location of excitation, the density of the measurement point grid, the sampling rate and the length of the time window. As an illustration, several examples of good and worse tests on bridges and other civil engineering structures are discussed. 1. FORCED VIBRATION TESTING 1.1. Basics With Forced Vibration Testing (FVT) the structure to be identified is artificially excited with a forcing function in a point i and its response yk(t) to this excitation is measured together with the forcing signal xi(t) (Fig. 1). Transformation of these time signals into the frequency domain and calculation of all Frequency Response Functions (FRF’s) Hik between the response and the forcing function time signals yields the Frequency Response Matrix, also referred to as Transfer Matrix, H(iω) (Figs. 2 and 3). Fig. 1: Forced Vibration Testing Scheme. Fig. 2: Calculation of the Frequency Response Function Hik.