Segmented Wood Shear Wall Relative Rigidity

[wannabeSE]

Is the relative rigidity of wood shear wall segments on the same line usually considered in the design?

In Design of Wood Structures, 6th edition, Breyer states "For a wood structural panel segmented shearwall with an aspect ratio (h/b) not greater than 2, the unit shear is generally assumed to be uniform throughout."

Yet the 2005 SDPWS section 4.3.3.3 states "The nominal shear capacity for shear walls in a line, utilizing shear walls sheathed with the same material and construction, shall be permitted to be combined if the induced shear load is distributed so as to provide the same deflection, δ_sw, in each wall."

I calculated a couple scenarios with different aspect ratios. The reduction in capacity is not negligible when one wall has an aspect ratio of 2 and the force is distributed so the deflection of the segments is equal.

 

[RFreund]

I'm a little confused when you say the "reduction in capacity is not negligible". What reduction are you speaking about?

EIT

http://www.HowToEngineer.com

 

[jdgengineer]

Our typical calculation process is to use relative rigidity to determine the distribution of shear forces along a line with multiple shearwalls. This requires an iterative process as changing the nailing of the walls therefore changes the stiffness of the walls so it's best to have this in excel / or other software. Typically, we keep all nailing along a wall line the same but I see no reason why you have to do that.

The new NDS equation contains three parts : shear deflection, flexural deflection, and holdown slip. In general, the flexural deflection and holdown slip are small (unless you have very slender walls <2), and therefore you could distribute loads based on only shear deflection and be reasonably accurate. If all walls have the same nailing you could then distribute them based on length alone. (Breyer's statement) I would think that this would be acceptable in most situations, but since we already have the calculations that use the full deflection equations we typically don't use this method.

Your last sentence is confusing. You are distributing load based on stiffness not strength. The reduction in capacity would effect the strength of the wall not the amount of load that is directed to the wall.

 

[wannabeSE]

RFreud,
Assuming all segments are constructed the same except the length, the longer segment will deflect more with the same load per foot.

jdengineer,
Thanks for the information. When I did a quick calculation (see attached), the flexural deflection and the hold down slip were significant. If I remember correctly, the scenario assumes 2 segments with 3/8 plywood structural panel and edge nailing at 4"OC. The wall is 8' high. One segment is 8' wide and the other is 4'. A typical Simpson hold down was selected for the anchor slip (Δ_a = 0.108"). In one case, I scaled the anchor slip based on the load, in the other, I assumed the hold down would slip 0.108" no matter what the is.

 

[RFreund]

Maybe I am still missing something. I agree with what JD is saying. Let me try to think out loud here:

When SDPWS says "if shear load is distributed so as to provide the same deflection, δsw, in each wall." They are really justing saying to distribute the force along that wall line based on the stiffness of the individual walls.
So in normal practice you choose your structural panel and nailing pattern (and I'm praying they are the same for the same wall line). You then can determine your relative stiffness of each wall. Then you determine the force going to each wall and check your design.

Now what it looks like you are doing in your spreadsheet is:
Select a structural panel, nailing pattern and holddown which are the same for both walls.
Find the deflection of the larger shear wall based on its capacity (using the capacity shear in the deflection equations).
Find the correlating shear force in the shorter wall by iterating the shear value so that deflections match.
Now we can compare this value to the capacity of the shearwall, which you (/Breyer) are saying should be the same for both walls?

I wouldn't think the values would be the same. Where does Breyer say this? I mean in actuality he may be correct but you wouldn't be able to show it on paper using those deflection equations. Due to the fact that the bending term is directly related to the length as well as the anchor slip term (and if you start changing height well that is ^3 so you would notice that too). You can see this in your spreadsheet as the anchor term is 2x the longer wall.
Maybe what he is saying is based on experience or testing.

or maybe I'm still missing something....



EIT

http://www.HowToEngineer.com

 

[msquared48]

In effect, the aspect ratio of the walls in the same line is considered when if falls below 2:1 by increasing the nailing required for shorter walls to resist the same, but amplified, load per foot to the wall.

Mike McCann
MMC Engineering