01 May 2020 Result Combination Methods
Results Combination methods are used in dynamic analyses, such as Response Spectrum Analysis (RSA) (https://www.fea-solutions.co.uk/response-spectrum-analysis/), in order to combine results from multiple analyses of a structure to find the overall response of the system. These multiple analyses are from the excitations of the various modes (https://www.fea-solutions.co.uk/modal-analysis/) of a modal superposition (https://www.fea-solutions.co.uk/modal-superposition/), or from the structural response in the X,Y, and Z directions, or both.
A key feature of all combination methods is that the results produced are always positive due to the formulae used. Therefore, displacements and stresses in any direction will always be positive, as all negative results (e.g. compressive stresses) are reversed into positive. That is why it is misleading to show deformed geometries of results obtained by combination, as shown in the image.
Three common types of combinations are: Square-Root-of-Sum-of-Squares (SRSS), Complete Quadratic Combination (CQC) and Rosenblueth’s Double Sum Combination (ROSE or DSC).
SRSS is the most common method used in FEA and is most accurate when analysing structures with natural frequencies that are spread apart. It is the most conservative of the three combination methods.
Both CQC and DSC are seen as an improvement on SRSS in models where some of the natural frequencies are very close together, typically within 10% or less. Those frequencies will be considered being the same. That means for analyses where all natural frequencies are more than 10% apart, CQC and DSC produce very similar results to SRSS.
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