Absolute principal factor analyses

In the response spectrum method, the maximum response of each mode should be calculated by direct use of the design response spectrum. The maximum response in each principal direction should be determined by an appropriate combination of the modal maxima, such as the square root of the sum of the squares of each modal response, or by the complete quadratic combination procedure. For closely spaced modal frequencies, a conservative procedure should be applied by taking the sum of the absolute values of each closely spaced modal and rigid response. The missing mass as a function of the modelling detail, cut-off frequencies and modal participation factors used in the analysis should also be carefully assessed and documented. 

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