combining different R values

[cf2]

Hi,

Basic question:

I have a two story wood duplex with shearwalls. Below the main floor, for various reasons, I would like to use a cantilivered wood column system. I'm not quite sure how to combine the loads to get the correct base shear. At the moment, I am using:
V = Cs*W(upper flrs) + Cs * W(main flr)
I am using R=6.5 for the upper floors to calculate Cs and R=1.5 at the lowest floor to calc it's Cs. This method puts more seismic load into my shearwalls. I had thought I would only be penalized at the lowest level. What is the best way to calculate this?

 

[msquared48]

Man that wood column first floor scenario scares me - has soft story written all over it.

I would seriously consider ordinary moment frames (w shape) at the lower story if you have to have the structure more open. These frames can be integrated into the wood scenario without too many structural problems.

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

 

[woodman88]

You might want to read ASCE 7-05 section 12.2.3.1. The only times I have use two R values in the same direction is for a three/four story wood frame over a concrete parking garage. The lower section must be at least ten times the stiffness of the upper portion, etc.

Garth Dreger PE
AZ Phoenix area

 

[cf2]

I should clarify. I definitely hear you about the soft story issue. The cantilivered columns are at the crawl space level and will be at most 2-3' tall. (I've also considered using HSS posts but I think that would be out of the budget). There will be about 10 posts each wall line, so a good bit of redundancy.

Any more thoughts about how to calc base shear with different values of R above and below?

Thanks!

 

[msquared48]

Is the intent of the posts to be part of a break-away wall for a 100 year flood scenario?

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

 

[cf2]

It's for a low-impact development. So we are using diamond piers as the foundation system. The architect would like to show off the minimal impact of the foundation. Thus keeping it open as opposed to shearwall below.