Response mofication coefficient for mixed lateral system

[agnoyes]

I am designing a five story building. The top four stories are load bearing wall system laterally supported by a combination of steel braced frames and a concrete shear wall. The bottom story is a flat plate post tensioned concrete slab supported laterally by concrete shear walls. What should my response modication coefficient be? I am designing to the IBC 2000.

 

[twinnell]

Are the top 4 stories wood or masonry? I am assuming wood. Is the steel bracing in the load bearing walls or are the load bearing walls shear walls and you have steel braces in the interior of the building? Need more description of the construction.

I have done 4 and 5 story table tops where 3 or 4 stories of wood framing on top of flat plate with concrete columns and concrete shear walls. We would assume the plate was the base for the wood structure and design the lateral for the wood using the appropriate R value for wood shear walls. Then we would apply the wood loads to the flat plate and design the concrete portion with it's appropriate R value.

 

[agnoyes]

The top floors are supported by load bearing light gage metal studs. The lateral bracing will be a combination of structural steel chevron bracing and concrete shear wall (around the elevator/stair). The steel braced frames will stack over a concrete shear wall below the flat plate concrete PT slab. Should I still design the upper portion (3-5) for a different R (4 for bearing wall systems) and an R of 5 for the second, concrete floor and shear walls? Thank you for your help.

 

[twinnell]

That is the way that we would do it.