Mixed SFRS

[hemiv]

I'm working on a project where I've been asked to use multiple types of seismic force resisting systems. I have done this before in steel and conservatively chosen the lowest R value in ASCE 7 and still come up with pretty economical results.

The building I'm currently working on will be primarily light-frame wood with rated structural panels (R=6 1/2), but the architects are wanting to utilize 5/8" gypsum wallboard for two interior shear walls rather than including an extra layer of wood sheathing on them. Does this automatically bump the whole building into the "light-frame walls with shear panels of all other materials" category and required R=2?

This sort of thing has been a question in my mind for a bit, but this is the first time it seems like it could really effect the cost of a project.

 

[jayrod12]

I've always sold them on the wood panelling on the common area (corridor?) side of the wall as it drastically improves durability of those walls to bumps and thumps.

It also removes the requirement for blocking for many items that may be mounted to the wall like signage etc.

 

[JoshPlumSE]

IMO, there are a couple of ways to look at it:

1) Standard code way: Where the R value for the whole structure (at least at that level and above) would drop down to 2.0. Ouch!

2) Using Performance Based Design. Meaning pushover analysis or such.

The idea being that if you want to demonstrate that OVERALL ductility of the building, you have to do a more advanced analysis due to the presence of some pretty non-ductile elements in the SFRS.

 

[KootK]

The way that I've handled it in the past is this:

1) If there's a gyp shear wall on any line of resistance, that whole line of resistance gets the crap R-Value.

2) The other lines of resistance get R-Values consistent with their own composition.

This is somewhat related to whether or not we consider our diaphragms to be flexible or rigid.

 

[Deker]

ASCE 7-16 Section 12.2.3.3 permits different R values for each line of resistance as KootK described above provided the building is Risk Cateogy II, less than two stories, and is all light-frame construction or flexible diaphragms. The diaphragm design would use the least value of R in that direction.

 

[JoshPlumSE]

ASCE 7-16 Section 12.2.3.3 permits different R values for each line of resistance as KootK described above provided the the building is Risk Cateogy II, less than two stories, and is all light-frame construction or flexible diaphragms. The diaphragm design would use the least value of R in that direction.

I wasn't familiar with this section. But, that is really good news. Makes a lot of sense for light frame construction less than two stories.

 

[ChorasDen]

When designing a gyp board shear wall, how do you detail the attachments? Gypsum needs to continue down and be properly connected to the top and bottom plates, which is quite rare to see in my experience.

 

[SJBombero]

I am surprised the code still allows this for SDC D. After a single cycle, I would think the gypboard would have zero resistance.

 

[ChorasDen]

Hence a change in R factor for gypsum wall board shear walls, requiring the wall to be designed with proportionally more load.