Christopher Wright has made a good point in his recent post.
...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.
The reason for a two part analysis is to simplify analysis of structures
such as wood frame housing over concrete pedestals. If I strictly read
UBC-97 section 16220.127.116.11 (" The following two-stage STATIC analysis
procedures may be used..." [my CAPS]), I question why there was a two-stage
dynamic analysis performed at all.
It is my interpretation that the intent of the code is to allow a two-part,
simplified (analytically) analysis for qualified structures and expects a
full structure model for a dynamic analysis.
Greg Varney, P.E.
KPFF Consulting Engineers
gregv(--nospam--at)kpff.com (206)-622-5822 voice (206)-6228-130 fax