Wood Shearwall Aspect Ratio
Wood Shearwall Aspect Ratio
(OP)
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?
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?






RE: Wood Shearwall Aspect Ratio
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.
RE: Wood Shearwall Aspect Ratio
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.
RE: Wood Shearwall Aspect Ratio
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.
RE: Wood Shearwall Aspect Ratio
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Wood Shearwall Aspect Ratio
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.
RE: Wood Shearwall Aspect Ratio
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.
RE: Wood Shearwall 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
RE: Wood Shearwall Aspect Ratio
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Wood Shearwall Aspect Ratio
RE: Wood Shearwall Aspect Ratio
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com