This paper will describe algorithms by which the dynamic reanalysis of structures can be carried out with a minimum preparation of data and at minimum computer costs. These techniques allow the designer to study, on an interactive basis, those parameters in a large structural idealisation which affect the dynamic response. The work has practical significance in monitoring structures through response to random excitation for changes in structural performance, and interactive design of structures where the influence of properties on the dyanamic response is required, i.e. dynamic sensitivity analysis.

The method is based on the Subspace Iteration Method developed by Bathe and Wilson in conjunction with work carried out by Fox and Kapoor and is a two stage process.

The first stage of the method is to carry out a complete dynamic analysis of the structure using subspace iteration. The working matrices from this first analysis are saved to be operated on later by the second stage analysis associated with changes in the original structure. These changes can be for example, changes in boundary conditions, elements properties (stiffness or mass) and/or removing or including more elements. Since only the changes to the original system need be described, data definition and solution for this reanalysis is such that it can be carried out on an interactive basis.

This method of analysis has been incorporated into the general purpose civil engineering computer analysis program FLASH. Numerical results and examples will be presented to show the limits on changes in structural properties that are possible before a complete reanalysis of the structure is required and to illustrate the practical application of the technique.

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