Introduction

Abstract

Dynamic analysis in structural engineering falls into two different classes, one involving low frequency loading and the other involving high frequency loading. Low frequency problems are categorized as Structural Dynamics problems while those involving high frequency loading fall into the category of Wave Propagation problems. In structural dynamics problems, the frequency content of the dynamic load is of the order of a few hundred hertz (Hz) and the designer will be mostly interested in the long-term (or steady-state) effects of the dynamic load on the structures. Hence, the first few normal modes and natural frequencies are sufficient to assess the performance of the structure. The phase information of the response is not critical here. Most of the dynamic problems in structures will fall into this category. On the other hand, for wave propagation problems, the frequency content of the input loading is very high (of the order of kilohertz (kHz) or higher) and hence, short-term effects (transient response) become very critical. Further, many higher order modes will participate in amplifying the dynamic response. Impact and blast-type of loading fall into this category. The multi-modal nature of wave propagation makes one parameter very important, and that is the phase information.