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Re: Dynamic Analysis Question

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>when a specific structure needs dynamic 
>analysis for irregularities, the interaction effects become important.
Generally this is right: provided your model is accurate, the combined 
analysis is the better approach. I couldn't make a whole lot out of the 
first post, but it sounded as if your frequency response was considerably 
different with the combined model. That tells me that each part 
contributes both stiffness and mass (including rotary inertia) to the 
response of the other. If this were not the case, you'd expect the 
results of the combined model to show only the individual responses.

As an example, imagine a tee structure formed by attaching a vertical 
cantilever member at the midpoint of a simple beam. If the vertical 
member is stiff and massive, it only contributes mass to the system and 
the combined system acts as simple beam with a lumped mass in the middle. 
If the vertical beam is soft and light the combined response is that of a 
cantilever mounted rigidly. In both of these extremes  a combined model 
will show that the response of one member has no effect on the other and 
that the system frequencies are those of each part considered separately. 
If the two beams are nearly alike in stiffness and mass, the responses 
will be coupled, with the cantilever member contributing rotary inertia 
and the simple beam offering only elastic support to the cantilever beam. 
Only a combined analysis will reflect the interactions.

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)