Multi-story wood shearwalls - how to model deflection/drift?

[COEngr]

All,
My firm is getting into the delightful realm of multi-story wood construction and we are trying to get an understanding for how to accurately calculate drift in the wall so we can demonstrate compliance with ASCE 7-10 12.12. Most examples I've seen only address single story structures. We initially relied on an example published by WoodWorks for a 5-over-1 podium. There seem to be two fundamental issues with this example that we would love some guidance on.

1) When calculating the deflection of a multi-story wood shearwall, do you treat it as a single cantilevered wall with forces delivered at each level (as you would a masonry or concrete shearwall with flexible diaphragms)? Or do you consider the wall as (5) stacked cantilevered walls "fixed" at each levels and only as tall as the story (say 10ft)? The example seems to take the second approach but this doesn't not seem to be justified by statics.

2) When considering the elongation of the tension-rod anchorage system (rod, device, shrinkage residual, and crushing of the bearing plate/compression chords) to include in shearwall deflection calculations, do you sum the elongations up the height of building or only consider the elongation between stories? Keep in mind, the code specifies story level drift limits, not absolute deflection limits. The example seems to sum the elongations for systems without shrinkage devices but then doesn't sum them when shrinkage compensating devices are used (Tables 16/16A). How is this justifiable?

Any insight you can provide would be greatly appreciated!

 

[manstrom]

Welcome to the shit.

For shear wall analysis, the most common design methodology is to consider the walls as 5 rectangles on top of each other with pins at each end. I am pretty sure this is how the woodworks paper does it.

Using the same analogy, one would consider the same method for deflection analysis - a tall beam fixed a the base. I'm fairly sure that the long walls would work ok. Narrow walls (exterior walls) would probably not work. These closer to a moment frame condition where there is some fixity from the floor resisting rotation, but not enough to call it a fixed fixed cantilever. Take DL into account as well to resist the rotation. It works, but your interior walls will take much more load due to less deflection.

Regarding your second questions. You should sum the elongations. But this is another case where if you think too hard, your head will explode. There are certain expectations of what the end product should be. If you over design it, you'll hear about it from the client. Some calculations require some creative thinking to make it work.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[mike20793]

1. I had the same concerns when I first read the WoodWorks example. I believe that approach number 2 is valid because you are creating fixity at each level with the holddowns.

2. Look at drift/deflection for each floor, so you would not need to sum the elongation unless you are summing drifts to size an expansion joint. I do not include shrinkage in my drift cacluations, just the different terms for bending, shear, and nail slip.

Woodworks is extremely helpful and I've found that they are more than willing to answer any design related questions, especially when it relates to white papers that they published. They have helped me on several projects.

 

[KootK]

This is a good white paper on the issue: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[COEngr]

KootK: Thank you thank you! That white paper is exactly the summary of the situation I was looking for. It offers the most concise and holistic analysis I've seen so far. I hadn't considered the effects of rotation from the floors below, but its seems clear that you should, especially for any structures above 3 stories. It also addresses the key problem I had with the WoodWorks example wherein it didn't sum the anchorage slip at the floors below the level of interest. Leave it to the Canadians to off a rational, straight-forward response to this problem!

Thanks again!

 

[KootK]

You're most welcome COEngr. I'm glad that Canada and I were able to be of service.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.