3x Required at Shear Wall Panel Joints? 1

[dhtrible]

In the IBC 2003, Table 2306.4.1 (p.469), footnote i. states that "In Seismic Design Category D, E, or F, where shear design values exceed...[350 plf] (ASD) all framing members receiving edge nailing from abutting panels shall not be less than a single 3-inch nominal member. ..."

My question is this: if wind loading requires a wall with capacity higher than 350 plf (ASD) (and seismic loading does not require over 350 plf capacity shear walls, even though the structure is in SDC D/E/F) is a 3x at the panel joint still required?

Has anyone run into this - how did you handle it?

Thank you!

 

[GNight]

Similar provision in UBC is based on the capacity of the wall, not design value, so the controlling load case is irrelevant. Interesting if the IBC interpretation is different.

 

[dhtrible]

Yes, GNight, the IBC does base it on the capacity of the wall; but does the UBC make any distinction between wind and seismic?

Often the provisions for seismic are more stringent than for wind.

It's not clear to me in this case.

In my office, we have a table with typical shear walls we use listed with their construction and capacities... If the above IBC 2003 table's footnote applies to both wind and seismic controlled designs, then our table is in error, and we ought to be specifying 3x's at panel joints in several situations that we aren't currently... The reason I was given by a colleague why this wasn't so was that this would only apply if seismic is the controlling lateral force for the direction (wall) in consideration. If wind controlled, a 3x at the panel joint wouldn't be required at this "350plf(ASD)" cutoff point.

Any comments on which is true/used are most appreciated!

 

[structuralaggie]

Most of my wood projects are in Texas and near the coast, so wind always controls. To answer your question, I see any reason why you would have to use 3x. The old UBC didn't differentiate between wind and seismic. The 3x requirement was due the wood splitting when installed for close nail spacing, not to what kind of loading was being applied. I don't know what the reasoning is for seismic 3x requirement, other than it is more conservative. If someone knows, please let me know. Also worth looking at is the new portion of the NDS on shear walls and diaphragms. There is a lot more info there.

I usually don't spec any shear walls that require 3x because it is not available locally, so the contractors will use 2-2x's. That opens a whole different can of worms, so I just avoid it. Not always, but 10d @ 4" is what I normally spec for shear walls. I will add walls until this works, or I will use steel frames.

 

[dhtrible]

Thanks, structural aggie. Yes, with footnote e. for this IBC table, 3x's at panel joints are always required for edge nailing spacings of 2" o.c. (and footnote f. requires 3x's at panel joints for some shear wall constructions with edge nailings at 3" o.c.). So, it does make sense that close edge nailing spacing should be the reason for having to have a 3x at the panel joint.

It was just pointed out to me that IBC 2003 Section 2306.4.1 says that the IBC table values may be increased by 40% for wind...so that would make the cutoff for a 3x at the panel joint for wind controlling be at 1.4*350= 490 plf which is a little better for my current design - doh! However, for wind or seismic, we haven't been putting a 3x at the panel joint in several of the situations that table footnote implies we should. But apparently you can ignore the table and it's footnotes completely if you "calculate by principles of mechanics...by using values for nail strength given in the NDS and wood structural panel design properties given in the APA..." as 2306.4.1 states...

 

[dhtrible]

For anyone who was interested in this thread, if you use the tabulated values for your wood shear panel walls, I haven't found any way to get out of the provision of using a 3x when in SDC D/E/F for applied seismic &/or adjusted wind loads over 350 plf.

However, in the 2006 IBC, the 2-2x in lieu of 3x at a panel joint has been adopted from the APA Techical Topic on the same subject, which gets rid of some 3x's. Watch the 3x sill requirements, though...you can get away from a 3x sill for applied loads over 350 plf but less than 600 plf (in your sill, not your wall) if you provide enough anchor bolts.

Of course, if your project is not in these higher Seismic Design Categories, you don't have to worry about all this! ) I'm still not sure why the seismic provisions seem to be stricter than the wind ones...do we have any more certainty either way?...but maybe when you consider you've decreased your seismic loads at the beginning it all evens out - give a little, take a little, keep the code game fun!

 

[jdstructure]

All the information I have read implies that the 3x nailing requirement was a result of the '94 Northridge earthquake. And because seismic design incorporates an overstrength and ductility factor, R, it seems to make sense that the 3x (at 350plf) requirement would not necessarily be applicable for similar wind loads. A 1.4 factor would make sense.

 

[Joe52]

Even if wind governs desigh as it does for several projects in high seismic areas you detail for seismic loads. They are cyclic and have a greater demand on the structure and last far longer than a wind event. As far as the double 2x ..do you feel confindent that the framer will nail the boards together correctly, the plywood dege and field nailing will be at least 1/2" away from the edge of either 2x member then use double 2x's but if not use 3x. The reason for 3x members was because so many walls were torn appart at sill plates along the line of edge nailing durning Northridge Equake.

 

[msquared48]

If 3X's or (2)2X's are a problem, then use a 4X. No worries.

Mike McCann
McCann Engineering