Deflection of Wood Shear Walls
Deflection of Wood Shear Walls
(OP)
You would think after reading Breyer's book backwards and forwards a few times I would have no more questions. Here are the latest questions that I can't seem to resolve with my various texts:
1. The drift limits and criteria given in ASCE 7-10 seem to deal primarily with seismic forces. In the case of wind loads governing (by a rather large margin) do the same drift limits apply using the lateral wind forces to calculate the story drift? I noticed that with calculating the drift using seismic loads Breyer's book uses the strength-level forces and not the ASD value (.7E). For wind would the strength level be used as well for drift calculations?
2. I have a shearwall composed of two separate panels that differ in length. To calculate the shearwall deflection do I simply calculate the deflection of each panel and then average the two? or is there a simpler method than this?
3. Now let's assume that I've sheathed both sides of the shearwall. My thinking is that the 2nd and 3rd term of the four term deflection equation will be affected. The unit shear in term 1 will remain the same? and the anchorage slip (term 4) will also remain the same?
1. The drift limits and criteria given in ASCE 7-10 seem to deal primarily with seismic forces. In the case of wind loads governing (by a rather large margin) do the same drift limits apply using the lateral wind forces to calculate the story drift? I noticed that with calculating the drift using seismic loads Breyer's book uses the strength-level forces and not the ASD value (.7E). For wind would the strength level be used as well for drift calculations?
2. I have a shearwall composed of two separate panels that differ in length. To calculate the shearwall deflection do I simply calculate the deflection of each panel and then average the two? or is there a simpler method than this?
3. Now let's assume that I've sheathed both sides of the shearwall. My thinking is that the 2nd and 3rd term of the four term deflection equation will be affected. The unit shear in term 1 will remain the same? and the anchorage slip (term 4) will also remain the same?
A confused student is a good student.






RE: Deflection of Wood Shear Walls
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Deflection of Wood Shear Walls
Basically, the jist of my conversations have alway concluded that the IRC braced wall provisions are too complicated and too restrictive, you end up compromising the architectural design in order to "get it to work".
If you hop onto my website you will notice that my 3 car garage design (18 months ago) attempts to comply with this method and as a result I have an interior shearwall that effectively cuts up the interior garage space. Not only is this detrimental to the design but the shearwall panels due to their size and aspect ratio would probably be mostly ineffective. The only reason to place them there was to make the design work using the IRC method.
I haven't yet found the time to re-analyze the garage design using standard methods of structural engineering but once I do I will probably find I can eliminate that annoying interior shearwall.
A confused student is a good student.
RE: Deflection of Wood Shear Walls
Shearwall Spreadsheet
I've done my best to add in deflection analysis based on the example problems in Breyer's book and the SDPWS-2008 Commentary.
The only thing missing is an analysis of the compression of the shearwall chord and bearing on the sill plate, but I couldn't fit it all in on one sheet.
A confused student is a good student.
RE: Deflection of Wood Shear Walls
Wind drift limits are generally much more restrictive than seismic drift limits. I've been out of the US a while but I believe that seismic is H/40 under strength level forces and wind is voluntarily H/500 under service level forces.
That sounds reasonable. You could also do a weighted average using the amount of shear attributed to each wall as the weighting factor. Ideally, the forces would be distributed such that the deflections would be compatible. I realize that can be a tall order using wood shear wall methods however.
I agree with the exception of the 3rd term (nail slip). Chords and anchorages are what they are and require no modification. The shear deflection component should be modified to use 2t instead of t (t being the sheathing thickness on one side). I believe that the nail slip term should remain the same as it would be with a one sided wall. The nail slip is a prediction of the wall displacement that would accompany a "taking up of the slack" in the fasteners. The magnitude of that slack ought to be the same for a one sided wall as for a two sided wall.
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 that I want to either change it or adopt it.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Deflection of Wood Shear Walls
RE: Deflection of Wood Shear Walls
I've basically updated my spreadsheet so that it requires the seismic as well as the wind loads (lateral) so that the deflection can be calculated per seismic loads and then compared with the limits set in the ASCE 7-10 Sec. 12. The allowable for wood structures appears to be 0.02H which would be H/50, however this is after you apply the amplification factor of 4 to the story drift, so its confusing IMHO.
With the H/300 I'm assuming that is looking at the deflection or drift directly without any amplification factor and also at an ASD level and not strength level?
A confused student is a good student.
RE: Deflection of Wood Shear Walls
I don't think you will find one definite answer but more and range of acceptable limits that will vary between engineer's and engineering consultancies.
The drift limits that I have put above are for service level wind forces.
RE: Deflection of Wood Shear Walls
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.
RE: Deflection of Wood Shear Walls
A confused student is a good student.
RE: Deflection of Wood Shear Walls
I do very little work with wood. For concrete and masonry shear walls, the forces are typically distributed by the relative rigidity of the panels (piers, segments). So, all of the segments on a line deflect the same. I do not know what is typically done with wood shear walls.
RE: Deflection of Wood Shear Walls
2) I agree with KootK; seems reasonable but compatibility would be the gold standard.
3) If you are calculating the nail slip via the actual shear on the nail then the component would change in that you are changing the nail shears.
RE: Deflection of Wood Shear Walls
"What a Hospital Engineer May Want to Know About Structural Engineering"
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.
RE: Deflection of Wood Shear Walls
There are no hard code limits for wind deflections like there is for seismic design.