# Re: Dynamic Analysis Question

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Dynamic Analysis Question
• Date: Thu, 13 Jan 2000 17:42:01 -0800
```Ben:

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As you know we at BQE do a lot of dynamic analysis on different kind of structures.
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Depending on the height of the lower rigid portion, the approach to analyze it in parts may not be too far from the reality as far as the engineer models it correctly.
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It would appear that the complete combined model would be more accurate. However, you will be surprised when you see the results. Some of the 3-D analysis software will not lump the masses accurately (along Z axis). and therefore will dump very little force to the lesser stiff frames.
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In summary, doing it in parts is better considering most of today's software capabilities. If you are using SAP or RISA and your design engineer is experienced in finite element modeling, it would be safe to model the entire structure as one unit. Also, if the height of the lower rigid portion of the structure is greater than 15% the height of structure, then you should model it as a single unit.
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Hope that helps.

Shafat Qazi, P.E

At 01/13/00 02:40 PM, you wrote:
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```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?

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