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RE: Dynamic Analysis Question

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Ben,
In my opinion, if you decide to analyse each part seperately, scaling and dealing with periods should be performed based upon seperate analysis. However combining the forces to get overall results needs engineering judgment based upon combined model behavior. If I convert this for a time history analysis, it means that you should consider the time that your ultimate forces occure. So it's important, and sometimes too complicated, that combine the forces in right way. My feeling about this precedure makes me to use combined model with some modifications to consider the effects that I mentioned in my previous email.
Good luck,
Fariborz Tehrani,                      Phone: (310)509-8826
Civil Engineer                           Fax: (530)481-9532
BSCE, MSCE, PE                    Voice Mail: (877)743-6206
email: fmtehrani(--nospam--at)hotmail.com



From: "Yousefi, Ben" <Ben.Yousefi(--nospam--at)ci.sj.ca.us>
Reply-To: seaint(--nospam--at)seaint.org
To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: Dynamic Analysis Question
Date: Thu, 13 Jan 2000 17:14:23 -0800

Thank you everyone for your input.

	Now let me provide you some additional relevant information, or
maybe the second part of the question. The dynamic analysis had revealed
that the dominant modes for upper tower and lower podium were dramatically
different. Assume the decision is made to use the distribution of forces per
the model of building as a whole. When it comes time to scale the forces
back to the code level forces what do you scale it to? And would you scale
the forces for each section separately? According to which period?"

Thanks again

Ben Yousefi, SE
San Jose, CA

	-----Original Message-----
	From:	Walter Sawruk [SMTP:sawruk(--nospam--at)ix.netcom.com]
	Sent:	Thursday, January 13, 2000 3:17 PM
	To:	seaint(--nospam--at)seaint.org
	Subject:	Re: Dynamic Analysis Question

	Ben,

	In general, I think the coupled analysis would be the more
realistic.
	However, there are many potentail pitfalls which could have
introduced
	extra conservatism into the combined model (e.g., eccentricities)
and the
	solution details (e.g., modal combination method).  Without knowing
the
	specific details, no one can say for sure which one of your results
is the
	most realistic.

	Walter Sawruk
	EQE International, Inc.
	Shillington, PA
	email: ws(--nospam--at)eqe.com

	At 02:40 PM 13-01-2000 -0800, you wrote:
	>For those of you who are dynamic analysis experts, or have had
substantial
	>experience in this field, here is a question:
	>
	>We have a structure that is comprised of an irregular midrise tower
(say 15
	>stories) utilizing moment frames located on top of a low rise,
substantially
	>rigid base (concrete shear walls). Originally the building was
dynamically
	>analyzed as two separate sections. In the first run the tower was
assumed to
	>be a separate building located at grade and in the second run the
forces
	>from the tower were applied at the top of the base and then
analyzed. This
	>appeared to be crude approximation of the building behavior. So
another run
	>was performed modeling the building as a whole.
	>
	>As one might expect when the building is analyzed as whole, a
substantial
	>portion of the forces induced by the base is getting dumped into
the tower
	>and penalizing the tower frames.
	>
	>The question is which one is more realistic? Is there a middle
ground or
	>better alternative?
	>
	>Thanks in advance
	>
	>Ben Yousefi, SE
	>San Jose, CA
	>






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