# Re: Dynamic Analysis Question

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: Dynamic Analysis Question
• From: "Haris Karamaneas" <hkaraman(--nospam--at)civil.upatras.gr>
• Date: Fri, 14 Jan 2000 14:07:34 +0200
```Ben:

I would scale and evaluate the forces just as you said, according to each
buildings period. But isn't that just as if you had two diferent buildings
at the end? I also have to mention to you that analyzing the two buildings
seperatly you gain a better evaluation of the dynamic charactiristics of
each one since you analyze more modes per building and you skip numerical
mistakes that a programm could make on a complex structure.

Are you interested in preforming only a modal analysis for your building or
you plan to also use accelerograms later? If accelerogram are on the plan,
analysing the structure as a holle will raise questions about the Rayleigh
damping factors you would have to consider. Witch mode? The first two? One
for every Building. That can change the results sagnificantly.

At the end i belive that the main issue isn't if you should analyze the
buildings together or seperatly but if the structures will behave as two
seperate buildings. And maybe there is the point where you should focus your
effords.

Hope i helped...

Haris Karamaneas S.E.

-----Original Message-----
From: Yousefi, Ben <Ben.Yousefi(--nospam--at)ci.sj.ca.us>
To: 'seaint(--nospam--at)seaint.org' <seaint(--nospam--at)seaint.org>
Date: ÐáñáóêåõÞ 14 de Éáíïõáñßïõ de 2000 3:40 AM
Subject: RE: Dynamic Analysis Question

>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?
> >