Wood Shearwall Aspect Ratio

[dcarr82775]

The NDS limits a wood panel shear wall to a maximum aspect ratio of 3.5:1. It goes on to say that for seismic forces, if the aspect ratio exceeds 2:1 the shear capacity shall be reduced by (2*b)/h.

I have a wood framed building where I am doing a rigid diaphragm analysis (lots and lots of small walls few long walls) and some segments have an aspect ratio between 2:1 and 3.5:1. Since the NDS doesn't state that you are to also reduce the stiffness for this type of wall I find these panels are overloaded in shear, where as panels with an aspect ration of 2:1 or less are not. If I add thicker sheathing or more nails the problem walls becomes stiffer so suck up more load and the problem repeats.

In my way of thinking, if I am reducing the shear strength I should also be able to similarly reduce the stiffness. This will solve my problem. Does this sound rational, or is anyone aware that this is prohibited?

 

[jdgengineer]

I don't think you should be able to reduce the stiffness. The stiffness is calculated based on the combination of flexural bending, shear bending, nail slip, holdown slip - therefore the stiffness associated with tall narrow wall is fairly accurately accounted for. Reducing the stiffness, I don't think would give you a realistic distribution of forces to the member.

I think, but could be wrong, that the reason the code makes you reduce the forces for aspect ratios >2 is because the wall starts to behave more and more like a flexural member (and not a shear member) as the aspect ratios get higher. I think the reduction is to try and account for this phenomenon as the wall strength is based on a shear behavior and not a flexural one.

 

[dcarr82775]

JD,

Since it only applies to seismic loads, I don't think that is it. For wind you can use the full value, the reduction only applies to seismic.

For that reason I think the reduction has to do with the wall strength degrading during the back and forth and I would expect an similar degrading of stiffness.

 

[woodman88]

According to the ANSI/AF&PA SDPWS-2008 Commentary Section C4.3.4 Shear Wall Aspect Ratios "... is intended to account for reduce stiffness of high aspect ratio wall segments relative to lower aspect ratio wall segments (such as 1:1 aspect ratio) in the same wall line..."

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

Interesting, since all shearwalls in the same line deflect the same amount.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[jdgengineer]

I found similar reasoning as Woodman from my google searches and the NEHRP commentary.

It seems to me this is a very round about way of accounting for the reduced stiffness. My logic is this. The code reduces the strength of the Shearwall based on the aspect ratio. Therefore, in order to make up this reduction you either increase the nailing of the wall, make the wall longer, or increase the nailing of other walls or make them longer. By increasing the nailing or length of the wall in question you are also making it stiffer. By increasing nailing of the other walls you are making them stiffer. Either way the code is essentially requiring you to compensate for the reduced stiffness of the large aspect ratio walls. It seems to me that this is an odd way to handle it if the intent is to account for a reduced stiffness, but it likely makes for a more simple implementation.

Regardless, I don't think it's the codes intent to reduce the stiffness of your wall in question by the 2b/h ratio. At least that is my interpretation.

 

[woodman88]

By using the rational analysis in accordance with well-established principles of mechanics (2009 IBC section 1604.4) you could calculation the deflections of the shear walls in a wall line and adjust the load per shear wall so the deflections are the same to justify the wall line shear capability.
I keep my shear walls that line up of similar lengths rather than doing this calculation. I have done this for Simpson shear walls (looking at the C-SW09.pdf, page 40) of difference lengths that line up. That is, I have a SW18x8 that resists 1100# with 0.33” deflection and a SW48x8 that resists 3390# with 0.27” deflection. As the SW18x8 can only deflect the 0.27” without causing a failure of the SW48x8, I have the SW18x8 only being able to resist 1100#(0.27/0.33) = 900# in this wall line.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[RFreund]

I think I agree with jdg - the code wants you to compensate for the fact that you will lose stiffness in a SW with a large aspect ratio.
Therefore in your situation instead of decreasing the failing walls stiffness to attract less load you should increase the stiffness and strength (if possible) of the other walls in the same line.
Not sure how that will work out, but it sounds rightish.

Could you possibly use a perforated shear wall?

EIT

 

[msquared48]

What it is sayong to me is, given the same deflection for shearwallos of equal deflection in the same SW line, the higher aspect shearwalls will take less load (the same as a CMU shearwall). Therefore, for the walls to take the same load with similar deflections, the higher aspect walls need to have more nailing, thicker studs, more bolts, thicker ply, etc.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dcarr82775]

The shearwalls in a line all have to deflect the same and take loads proportional to their stiffness. This is accounted for in the calculation of stiffness. If the Code is going to derate the performance of a certain type of wall because of its lower stiffness, it seems to me more appropriate to reduce the stiffness rather than the strength. IMO the Code is it back asswards on this one.

 

[msquared48]

I agree...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com