RELATIVE RIGIDITY IN WOODEN SHEAR WALLS 1

[ARJ1]

I know with masonry shear walls you take the height divided by the depth plug the answer into a chart and get the R value. I also know that rigidity is the reciprocal of deflection. Is the rigidity of a wooden shearwall the reciprocal of the deflection? When designing wooden shear walls, how would you calculate relative rigidity?

 

[woodengineer]

Here is a quick answer:

http://www.curee.org/projects/woodframe/element5/modules/shearwall/deflection.htm

woodengineer

 

[ARJ1]

How do you convert the shear deflection to relative rigidity?

 

[msquared48]

1 / Deflection.

Mike McCann
McCann Engineering

 

[ARJ1]

I have noticed on some structural calculations that while the relative rigidity seems to be proportionate to the reciprocal of the deflection, the values seem to be multiplied times some factor. For instance, lets say the deflection is .5", then the relative ridgity would be 2. But on some structural calc's I've notice that the value is a lot higher, like 6.5. I asked a structural engineer and he said that his company has their own "R" value (not code based) that they apply. It seems to be proportionate to the reciprocal of the deflection. What would be the point of doing this instead of using the reciprocal of the deflection as your relative rigidity?

 

[msquared48]

Off hand, I do not know. I would have to see what they are doing to answer. Maybe you should ask him why...

Mike McCann
McCann Engineering

 

[whyun]

Actual rigidity is a reciprocal of actual deflection subject to a certain design lateral coefficient. For relative rigidity, the absolute values are meaningless. You are trying to determine the "relative" stiffnesses for the purpose of distributing the shear to the LFRS elements.

The "ratio" between the actual rigidity and relative rigidity for the respective LRFS should be the same.

 

[ARJ1]

Thanks everyone,

Mike,
I did ask him why. He didn't know.

Whyun,

Yes, you are correct. And I guess ultimately it really doesn't matter what factor they used. The relative "stiffness" to be applied to the shearwalls can be determined from the reciprocal of the deflections of the shearwalls. I saw their calcs and was just curious.

Thanks to everyone again.

 

[Lion06]

well, what whyun is saying is that the relative stiffness is when you compare the stiffness of the shearwalls to some known value.
It is not the reciprocal of the deflection of the shearwalls. It is the reciprocal of the deflection of yoru shearwall divided by the reciprocal of the deflection of the shearwall that you arbitrarily set as having a relative rigidity of 1.

 

[spats]

If you have wood shear walls, you probably have a wood diaphragm that distributes the shear to the various walls. A wood diaphragm is a flexible diaphragm, and thus there would be a simple tributary load distribution, not a distribution based on relative wall stiffnesses... therefore, relative wall stiffness does not matter.

 

[whyun]

Wood diaphragms can be proven to be rigid or semi-rigid per the code definition especially if the structure has a small footprint.

 

[gbucko]

Spats, I've always had a problem with this assumption...if I have two walls in the same line, of different lengths, shouldn't the force be distributed to each wall, as a percentage of their rigidity? (even for the flexible diaphragm case) By deformation compatibility (neglecting axial deformations in the drags), the walls should have equal displacement.

Am I missing something? Even the SEAOC design manual, in my opinion, seems to incorrectly distribute the loads based only on the wall lengths. Any thoughts?

 

[buster777]

gbucko,

while it is technically correct to distribute the shear load to walls in the same line based on rigidity, it becomes a moot point with wood shearwalls as the height to width is generally limited by the 2:1 ratio. With this configuration the "bending" portion of the stiffness contributes less than 8% of the total wall stiffness,so when using identical panel designs in the same wall line, the rigidity is almost directly proportional to length anyway.

If you design a shearwall that has a smaller ratio then you would need to take the rigidity into account, but the 07 CBC "kind of" does this for you by requiring what would be a stiffer panel, via strength reductions, at locations with smaller ratios.