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Architecture Engineering QUESTION #2356
Question 1
In the Response Spectrum Method of seismic analysis for a multi-degree-of-freedom (MDOF) structure, which modal combination rule accounts for both the modal responses and the correlation between closely spaced modes?
  • SRSS (Square Root of Sum of Squares)
  • CQC (Complete Quadratic Combination)✔️
  • Absolute Sum (ABS)
  • SRSS with 10% rule
Correct Answer Explanation
The Complete Quadratic Combination (CQC) rule accounts for cross-correlation between modes, which is significant when modal frequencies are closely spaced (within 10% of each other). SRSS assumes all modes are uncorrelated (appropriate for well-separated modes). ABS is overly conservative. CQC reduces to SRSS when all modes are well-separated (correlation coefficients approach zero).