Space Station Structure Modal Selection Criteria

Abstract

In many dynamic analysis procedures the size of the problem is curtailed by truncation of the modes selected for study. A preferable approach in the case of a structure such as a large space frame is to identify and include those natural modes in the order of their increasing importance. Two of the many criteria which can be used to determine importance are examined in this paper. The first involves establishment of a “completeness index” reflecting the model identities satisfied by the natural frequencies and integrals of the mode shapes. The second involves displacements at a point of excitation and a response point. Both selection methods are implemented on a simplified model of the Initial Operating Configuration Space Station and comparative transient response analyses are made. It is shown that mode selection is a function of more than one variable. The first method performs well in reducing model error yet the second is necessary to predict those modes excited by various forcing functions when control of displacements is the chief concern. Different mode selection criteria will be necessary when other variables are involved.