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

[designrider]

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.

 

[msquared48]

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

 

[WillisV]

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.

 

[port125]

We are looking at similar designs for a project.
We found a design example at

http://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.

 

[designrider]

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.