# Question for Kevo or anyone else: 2-story 2-stage analsysis per ASCE 12.2.3.1



## McEngr (Jul 14, 2011)

I have a 2-story building. It has a regular shape 40x54, 2-story, flexible roof diaphragm, rigid floor diaphragm. 2nd floor has R=6.5 for plywood shearwalls. 1st floor has R=3.5 for Ordinary Moment Frames of Steel.

When performing a vertical distribution from section 12.8.3, can I take the Vupper + the Vlower = Vtotal for my Fx force on each or do I take the CsW=Sds*I/R * W for the lowest R over the entire system for V?

I'll admit, I've done these before, but I'm a bit rusty as I usually don't do different systems vertically...

Thanks!


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## McEngr (Jul 14, 2011)

McEngr said:


> I have a 2-story building. It has a regular shape 40x54, 2-story, flexible roof diaphragm, rigid floor diaphragm. 2nd floor has R=6.5 for plywood shearwalls. 1st floor has R=3.5 for Ordinary Moment Frames of Steel.
> When performing a vertical distribution from section 12.8.3, can I take the Vupper + the Vlower = Vtotal for my Fx force on each or do I take the V=CS*I/R for the lowest R over the entire system for V?
> 
> I'll admit, I've done these before, but I'm a bit rusty as I usually don't do different systems vertically...
> ...


OK... sorry to bug everyone on this. I realize this is not permitted because of the 10x stiffness condition of no. 1 in the code section. Embarrassing...


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## kevo_55 (Jul 15, 2011)

Doh!

I just saw this now.


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## *Ananda* (Jul 15, 2011)

McEngr said:


> McEngr said:
> 
> 
> > I have a 2-story building. It has a regular shape 40x54, 2-story, flexible roof diaphragm, rigid floor diaphragm. 2nd floor has R=6.5 for plywood shearwalls. 1st floor has R=3.5 for Ordinary Moment Frames of Steel.
> ...


Per ASCE 7-05 Section 12.2.3.1 item a. A _two stage analysis _is what is not permitted due to the 10x stiffness. A two stage analysis permits you to design upper and lower portions as seperate structures to realize the benefit of the higher R for shear walls used on the upper floor and reduce the force demand to the lower story.

As I read 12.2.3.1 and understand the intent, you can still design the entire system for the lowest R, i.e. the total design lateral force or shear at the base of the structure would require the R = 3.5. Perhaps someone will correct me?


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## McEngr (Jul 15, 2011)

Ananda,

Moment frames tend to have a high drift. Light framed shearwalls will likely not be 10x that deflection. I missed this and should've been more familiar with this.

Good to have the experience to tackle it quickly after coming upon it...


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