From: "Dr. Ravi Sinha" <rsinha(--nospam--at)civil.iitb.ernet.in>
Date: Mon, 7 Jun 1999 23:44:25 +0530 (IST)
I am joining this discussion late, since I was not following the
mails earlier, and the main points have not been brought out.
SRSS is the traditional method that is best suited for widely-spaced
frequencies. When the frequencies are closely spaced, their
maxima tend to get close to each other, and the cross-modal terms
also become very important.
If the frequencies are widely-spaced, the CQC and SRSS methods will give
almost identical results.
If the frequencies are closely-spaced, the CQC results are generally
MUCH more accurate than the SRSS results. It should be kept in mind
that CQC results can be HIGHER or LOWER than the SRSS results; however,
the CQC results will be more accurate.
In general, for any complicated structure, closely-spaced frequencies
will definitely be found. SRSS method should not be used at all.
In some very rare situations, some response quantities calculated using
SRSS may be more accurate than those calculated using CQC. So, CQC does
not always *guarantee* better results. Those interested can refer to
the book "Response Spectrum Methods" by Dr. Ajaya K. Gupta of North
Carolina State University, in addition to the references cited below.
Hope this clarifies this issue better.
Dr. Ravi Sinha
On Fri, 28 May 1999, Ali Bakhshi wrote:
> Dear colleagues,
> With full respect to all who correctly pointed out some useful comments,
> you may have a look at the followin papers for further details, of course
> there are many other else. Just keep in mind that the CQC appears much
> superior when the torsional modes are among the dominated modes, i.e. 1st
> 3-5 modes. This may be caused by closedly-spaced modes and/or specific plan
> 1. Eidinger, J. M.; Kok, S. B.; Murray, T. "Practical application of CQC
> modal combination technique for a fuel reprocessing plant", Transactions of
> the 8th International Conference on Structural Mechanics in Reactor
> Technology, North-Holland Physics Publishing, Amsterdam, 1985, pages
> 515-522, Vol. K(a), Paper K 11/1
> 2. Cheng, F. Y.; Ger, Jeng-Fuh "Maximum response of buildings to
> multi-seismic input", Dynamics of Structures: Proceedings of the Sessions
> at Structures Congress '87 Related to Dynamics of Structures, American
> Society of Civil Engineers, New York, 1987, pages 397-410
> 3. Amini, A.; Trifunac, M. D. "A statistical basis for spectrum
> superposition in response to earthquake excitation", Proceedings of the
> Eighth World Conference on Earthquake Engineering, Prentice-Hall, Inc.,
> Englewood Cliffs, New Jersey, 1984, pages 179-186, Vol. IV,
> 4. Sinha, R.; Igusa, T. "CQC and SRSS methods for non-classically damped
> structure", Earthquake Engineering & Structural Dynamics, 24, 4, Apr. 1995,
> pages 615-619.
> Hope this helps.
> Ali Bakhshi, Dr. of Eng.
> Earthquake Engineering Lab
> Dept of Structural Eng.
> Hiroshima University
> TEL: +81-824-24-7862
> FAX: +81-824-24-7796
> E-mail: bakhshi(--nospam--at)ipc.hiroshima-u.ac.jp
Ravi Sinha, Ph.D. email: rsinha(--nospam--at)gemini.civil.iitb.ernet.in
Associate Professor rsinha(--nospam--at)civil.iitb.ernet.in
Department of Civil Engineering Phone: (91-22)-576-7336, 576-7301
IIT, Powai, Mumbai - 400 076, India Fax: (91-22)-578-3480, 578-3557