Wood shear wall in Seismic Cat. D 1

[bnickeson]

I am designing a structure with wood shear walls in a Seismic Category D location. I have a question about an interpretation of part of the IBC code. If you look at the IBC 2003 (or 2006, I believe it's the same) Table 2306.4.1 footnote i it says "In Seismic Category D, E, or F, where shear wall design values exceed 490 plf factored or 350 plf service, all framing member receiving edge nailing from abutting panels shall not be less than a single 3-inch nominal member..."

My question is this: does the 350 plf design value mean the TOTAL design shear in the wall or does it mean the shear PER PANEL face (if you had sheathing both sides)? So for instance, your 350 plf limit would actually go to 700 plf total shear if you had sheathing both sides. What are your thoughts on this? We've interpreted it as meaning per panel face, since this is the load that you enter the Table with to get your design. However, if we are wrong, it is a huge increase in cost to the owner if it needs to be changed.

Thanks for your input.

 

[msquared48]

As the increase in stud width has to do with the number of nails penetrating into the stud that could cause splitting of the stud and failure of the shearwall, I have always considered it to refer to the total shear wall assembly, not just one face.

It does mention a clarifying situation if the sheathing is staggered on each side of the wall. Note "h" still requires 3X studs in this condition too.

Bottom line is that you heed the 3X minimum studs. You might consider using 4X studs as these could be more readily available.

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

 

[bnickeson]

Just to play devil's advocate...

msquared, what you are referring to seems to apply more to footnote h than to i, as h refers to the splitting of the wood during nail installation. I took note i to apply more to a "side face breakout" (if you want to loosely compare it to concrete) of the wood during a seismic event. If we sheath both sides, then the shear at the interface of the nail and stud is half that of only one face (assuming the same nailing pattern). Under this assumption, then it would seem that the 350 applies to the seismic shear on each face.

But as you said, if we have any double sheathed walls with nails 4" or closer, we fall under footnote h with staggered joints or 3x members.

If it didn't make as much of a cost difference, I would just be conservative and use 3x's where ever. However, 3x lumber costs four times as much as 2x and the 350 plf implication could affect as much as a quarter of our building.

 

[Cadair]

I would go with what the Special Design Provisions for Wind and Seismic has to say about the matter. From the 2008 SPDWS (Section 4.3.7.4 pg. 28-29)

"The width of the nailed face of framing members and blocking shall be 2" nominal or greater at adjoining panel edges except that a 3" nominal or greater width at adjoining panel edges and staggered nailing at all panel edges are required where:
...
c. Required nominal unit shear capacity on either side of the shear wall exceeds 700 plf in Seismic Design Category D, E, or F."

To change from nominal unit shear capacity to allowable shear capacities, divide by 2 (which gives you the 350 plf from the IBC)

I think that answers it.

 

[azcats]

Do note that in the 2006 code, the same note (i) allows you to use a double 2x or the 3x stud.

 

[bnickeson]

Cadair, what you posted makes it sound like the 350 plf limit is referring to each sheathing face. Am I interpreting that correctly?

azcats, I was aware of that footnote. Unfortunately the governing code is the 2003 IBC in our structure's case. I may ask the building code enforcement department if they can grant us an exception in this case though.

 

[Cadair]

I see it as saying each sheathing face as well. The SDPWS also allows for double 2x if your code enforcement dept. is more willing to go with a more modern version of that standard instead of accepting the whole IBC.