CE 180: Experimental Methods in Earthquake Engineering

This experiment consists of determining the modal properties of a model structure. Many buildings in the practical world can be reasonably approximated by shear structures in which only one translational degree of freedom is taken into account for each floor level. Due to the simple characteristics of a shear structure, it is possible to determine its modal properties, such as the natural frequencies, damping, and mode shapes of all major modes from a well-performed modal test. A modal test is usually conducted to determine the modal properties of a structure and, if necessary, the analytical model of the structure can be modified accordingly for further simulation studies. Modal testing is especially useful for those structures with complex configurations for which it is difficult to get accurate results from analytical modeling. In our treatment of this linear system, we explore the concept of the transfer function which describes how the structure responds to prescribed excitation. Since we will measure the input force (force of an impulse hammer blow, white noise from a shake table) and output (displacement recorded at specific locations), the transfer function, which characterizes the dynamic properties of the model, can be computed through simple time series or spectral analysis. Note that if the input function is more complex than a simple impulse, it can be separated into infinitesimal impulses. The total output (response) can be obtained by superposition of all the individual impulse responses. Once we know the transfer functions of a system, we can predict the response caused by any simple or complex source function. The general procedure for performing a modal test is to select test points on the structure and collect transfer function data for each of these points. The model can be conveniently approximated as a lumped mass system at discrete points. These data points are then curve-fit to extract the modal properties of the test structure. A detailed discussion of application of modal tests and analysis to solve vibration problems can be found in Schmidtberg and Pal (1986) and the texts referenced below. The technical objectives are to learn how to use the Dytran piezoelectric accelerometers, and to be able to record accurate results from impact hammer excitation. Another main objective is to become more familiar with the data acquisition system, its real-time data viewing and recording software, and Matlab for subsequent analysis. During the experiment, the results will be displayed in real time and also written to files for analysis after the experiment is over.