Wood Shear Wall detailing questions

[MILRAD]

Two questions for detailing construction of shear walls:

The edge nail spacing is tight (2 or 3 inches) so at a panel joint 2 studs are used with each panel attaching to its own stud. Are the studs connected? It seems like this would be slip plane if not. What force is used to design the connection?

Are top plate splices just designed to transfer the shear or is there something more to do? Same approach for bottom plates?

Thanks

 

[ChorasDen]

Check section 4.3.6.1.1 of the 2015 SDPWS for your first question. Top plate may be detailed as the boundary member for the diaphragm, and if so, would need to be designed to transfer the design moment at splices that is induced by the bending force within the diaphragm.

 

[MILRAD]

Thanks for the response.

 

[lexpatrie]

If possible avoid 2" and 3" nail spacing, if there are any installation errors in the field there's not very much you can do. With 4" spacing you can at least double the nails to 2" spacing, although you have to consider nailing this tight generally requires a 3x member or a (2) 2x that is nailed together "as required", and the panel nailing must sometimes be adjusted to prevent splitting of the framing wood.

3x framing tends to be unpopular but there's nothing that particularly requires you to provide a (2) 2x at the vertical panel edges.

If you can endure a two sided shear wall the panels can be offset between the two sides and the nailing will then not need to be as tightly spaced, reducing concerns with splitting from nailing too close into the studs through the panels.

 

[ChorasDen]

If you can endure a two sided shear wall the panels can be offset between the two sides and the nailing will then not need to be as tightly spaced

I'd suggest avoiding combining materials for this, however. I would not personally rely on a wall that has wood structural sheathing on one side, gypsum sheathing on the inside, and try and use both material for lateral load. Wood structural panel shear walls have higher stiffness than gypsum shear walls, so more load will track to the OSB side, and using a combined material will significantly alter your base shear, if the design is seismic and you are using the Equivalent Lateral Force method.

 

[lexpatrie]

Those stiffness comments are not accurate, for one. Look at some of the seismic research on the subject, as well as the damage assessment reports.

For two, the intent of my suggestion was plywood/OSB on both sides. In seismic you're not allowed to combine different panel types, in wind, you are, but it is rarely of much use. When the nailing for what we both presume is plywood/OSB is already 2"/3" it's unlikely a combo wall would work.

Milrad - the (2)2x studs at the panel edges need to be fastened to each other for the shear. Top plate splices are not designed to transfer the shear from the (2) 2x vertical studs. Top plate splices, as Chorasden mentions, are designed for a tension/compression load the originates from the diaphragm force in the perpendicular direction (so it's a "flange" in the I-beam and the diaphragm is the web, and the beam is loaded along the span. The Special Design Provisions for Wind and Seismic should have some figures to help you out.

 

[ChorasDen]

Those stiffness comments are not accurate, for one. Look at some of the seismic research on the subject, as well as the damage assessment reports.

What are you saying here, can you provide more info to this claim? If I am understanding your comment correctly, that assumption would deviate directly from the reference standard.

 

[lexpatrie]

Happily.

Wood structural panel shear walls have higher stiffness than gypsum shear walls,

The forbidding of mixed type shear wall on panels in the same wall is seismic, wind you can use both. If they had dramatically different stiffness, de facto, one or the other takes the majority of the load, as you mentioned earlier.

Unless they changed SDPWS while I wasn't looking two different materials on the opposite sides of the same shear wall is not typically done, but is "allowed" for wind design.

The gypsum board panels get destroyed faster in seismic as they are less drift tolerant, They have reasonably similar stiffness. Anyway, so eventually it's just the plywood/OSB holding the entire load, plus as you mentioned for combined you are looking at what R=2 or 2.5, so it severely penalizes the use of the plywood with gypsum together, versus a "normal" seismic design ignoring the gypsum board panel (which is required anyway), using (as I recall) R=6.5, 7, or something like that. (The plywood only is designed for about a third of the not allowed combined system)

Refer to SDPWS as well as the various Woodworks publications on 4 story wood frame buildings (Five Story wood frame structure over podium slab, December 2017, Thompson, page 60).

Woodworks has a pretty decent rubber hits the road discussion of it. Just beware it's a wall of text.

It's not an assumption, it's reality, it's just not useful in a seismic design, even if you are in lower seismic design categories it can still dominate the design because the R=2 will create gypsum (only) shear walls that don't satisfy code required forces.

Seismic one wouldn't use different shear wall panel types on each side of the wall because it's simply not allowed, the reason has nothing to do with the relative stiffness of the gypsum or plywood/OSB panels. I expect the G[sub]a[/sub] values in the SDPWS are fairly similar as well. You'd normally not use a few plywood/OSB shear walls and a few gypsum board shear walls elsewhere because it penalizes the plywood/OSB walls by forcing a R=2 or 2.5 on all the panels.

 

[lexpatrie]

I should amend here, persusal of the SDPWS doesn't indicate that gypsum board shear walls or plywood/OSB shear walls have all that comparable G[sub]a[/sub] values.

For the more residential grades of plywood/OSB the G[sub]a[/sub] values are still above gypsum board, though not dramatically so, but from a pure mechanics standpoint combining the gypsum board and plywood/OSB wall strengths purely based on capacity isn't extremely precise. The plywood/OSB wall will become progressively stiffer as the nailing tightens and the panel thickness goes up.

It's permitted, so I suppose a design professional could consider that "based on generally accepted principles of mechanics" but it's not rigorously accurate (based on G[sub]a[/sub] values). Load sharing between the two has been documented, even on opposite sides of the wall, and the SDPWS has those odd (California?) walls with plywood/OSB on the inside against the studs and gypsum boards mounted to the outside of the plywood/OSB. Those values might be more representative that the "add naively together" approach permitted for wind loads.

If one wished, checking the deflection of the wall as if one panel type was missing and comparing the two may show that the algebraic combination of the two is reasonably close, probably more true on the low end of the nailing schedule for plywood/OSB.

Could be a nice little Plan B Master's project. Or one of those University of Penn State Senior projects.