What is dynamic analysis?
The first step in dynamic analysis is the determination of the frequencies and shapes of the natural vibration modes. In a 3-D structure there are three dynamic degrees of freedom (DDOF) for every unrestrained node with non-zero mass and there is potentially a natural vibration mode for each DDOF. Thus, there may be hundreds of potential vibration modes in a typical structure, but usually, it is only a small number of vibration modes with the lowest frequencies that are of interest. In a multi-storey building, for example, it might be only a few in each of two horizontal directions, plus one or two torsional modes that have to be considered. The vibration mode shapes are normalized. This means that the largest value in each tabulated mode shape is +1.0. Determining the natural mode shapes and frequencies does not provide any quantitative information about the response of the structure to excitation, but in some cases it may be sufficient to know what the natural frequencies are so they
