Maximizing the Quality of Shape Extractions from Base Shake Modal Tests

Abstract

The promise of extracting fixed base modes from structures mounted on shake tables is enticing since doing so allows a testing organization to save a considerable amount of schedule and money by reducing two traditionally separate tests into one. Oftentimes, however, the modal analysis results are not of high quality because the test planning and conduct of base shake environmental tests are not conducive to performing a high-quality modal survey.

This paper will discuss test planning and test conduct methods that can be used to maximize chances for successfully extracting high-quality mode shapes from structures mounted on shake tables. These methods are the same for any modal survey test; sensors should be placed to adequately observe and differentiate modes of a structure, and the excitation must be long enough in duration to adequately define high-quality frequency response functions (FRFs). Finally, methods to separate closely spaced modes by using multiple references, either with multiple-degree-of-freedom shake tables or a single-axis base shake test supplemented with modal shakers, will be discussed.