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Light-framed structure above rigid structure: location of seismic base

Light-framed structure above rigid structure: location of seismic base

Light-framed structure above rigid structure: location of seismic base

(OP)
If I have a multi-story light-framed structure constructed above a rigid structure (i.e. an elevated concrete slab with very stiff lateral resistance system below), could I justify considering the "base" to be the top of the elevated concrete slab for measuring the seismic height of the light-framed structure?  In my opinion, the seismic response differential between these two systems makes such an assumption a more accurate analysis, but it is clearly not the conservative approach.

ASCE 7-05 section 11.2 definition of "Base":
The level at which the horizontal seismic ground motions are considered to be imparted to the structure.
 

RE: Light-framed structure above rigid structure: location of seismic base

Has to do with relative deflections.  Depending on the jurisdiction and how they read the code, you may or may not have to justify it thru calculations.  I have done it both ways.

The top of a podium slab is frequently used as the seismic base in multi-family construction - wood or steel stud framing above concrete.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Light-framed structure above rigid structure: location of seismic base

See 12.2.3.1 of ASCE 7-05.  For structures like these you are permitted to perform a two-stage equivalent lateral force procedure where the structure on top if designed independently of the structure on the base if certain stiffness limits are met.   

RE: Light-framed structure above rigid structure: location of seismic base

We are looking at similar designs for a project.
We found a design example at www.woodworks.org (link publications, case studies/design examples)
for a four-story wood frame structure over podium slab.
They consider the seismic base at the top of the ridgid floor.

RE: Light-framed structure above rigid structure: location of seismic base

(OP)
Thank you all for your input!  Your valuable responses have helped me to feel secure in my design approach, and via the "two-stage equivalent lateral force procedure" I can ensure it is code supported.

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