Seismic criteria for vertical SFR system combination

[TS59401]

I am working on an industrial building retrofit project. The building system is an ordinary reinforced concrete bearing wall structure that is overly heavy due to two separate "cooling tower" like structures on the second level. These "cooling towers" are being removed during this retrofit. The portion of the building between the two cooling towers is a structural steel roof structure that is supported on tube steel legs about 5ft above the top of the concrete bearing walls. This roof looks like an after thought to me.

My question: ASCE7-10 Section 12.2.3.1 states that when the R of the upper system (ordinary concentric braced frame - 3.25) is less then the lower system (ordinary reinforced concrete bearing/shear walls) than the R of the upper system shall be used for both systems. This has an exception regarding structures of less than 10% of the total weight - which I easily meet. Since I meet the exception does that mean I use the R of the upper system for the upper system and use the R of the lower system for the lower system?

Thanks
Tom

 

[JAE]

I would use the R of the upper system to design the upper system as though it were a separate entity on a base.
Then I would use the R of the lower system applied to the entire structure to design the lower system.

I would not carry down the higher seismic loads of the upper structure, based on the smaller R, into the lower structure and combine them with higher-R seismic forces in the lower structure.



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