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I'm assuming you are performing a multi-mode spectral analysis.  As such, the
Square Root of the
Sum of the Squares method (SRSS) is the simplest, and provides adequate results
when the modal
periods are well spaced. However, when the modal periods are close, coupling
between modal
response could occur.The Complete Quadratic Combination method (CQC) takes into
account the
coupling between modes, maintains the algebraic signs of the cross-modal terms,
and is founded
on the fundementals of random vibrations.

Most analysis software (STRUDL, SAP90, etc.) have the CQC method, and I would
suggest using it
since doing so would eliminate the judgement and time needed to determine just
what comprises
closely spaced modal periods.

Hope this helps.

Tom Cummings, PE

---------------------- Forwarded by Tom Cummings/NewYork on 05/27/99 10:21 AM

Francis.Ang(--nospam--at) on 05/27/99 05:05:09 AM

Please respond to seaint(--nospam--at)

To:   seaint(--nospam--at)
cc:    (bcc: Tom Cummings/NewYork)

Subject:  CQC and SRSS

I would like to ask the difference with CQC and SRSS. Does anyone know how
they would differ in the results and when to use the CQC or SRSS
computation. Just a question. Thanks !

A. Yango