Deflection of Wood Shear Walls

[medeek]

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?

A confused student is a good student.

 

[KootK]

I'll noodle on your question later. For now, are you aware of this: Link? When it showed up in my email inbox today, my first thought was... Medeek must see this!! If you could parse every line of code behind it and verify its accuracy, that would be swell.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[medeek]

I was actually playing with the new app a couple hours ago. I haven't completely verified its output against my own methods for complying with the IRC braced wall provisions. However, based on different conversations I've had with other structural engineers and bldg. officials it is highly unlikely that I will ever try to design a structure again using this method from the IRC.

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.

 

[medeek]

What prompted this was my working on a new shearwall calculator/spreadsheet this afternoon. I've posted it here it anyone wants to give it a whirl:

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.

 

[KootK]

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?

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.

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?

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.

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?

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.

 

[asixth]

For low-rise residential buildings I use H/300 for service level winds. H/500 for multi-storey residential buildings.

 

[medeek]

Where does the H/300 and H/500 come from. This is the first I have heard of these numbers.

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.

 

[asixth]

AISC Steel Design Guide Series 3: Servicability Design Considerations for Low Rise Buildings.

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.

 

[KootK]

This thread, while not specific to wood, will give you some helpful background: Link. In particular, see the PDF uploaded by JAE.

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.

 

[medeek]

I am a little concerned that the usual wood texts and governing bodies don't seem to say anything about shearwall deflection for wind loads. I'm going to go through the NDS and commentary tonight to see if I'm missing something.

A confused student is a good student.

 

[wannabeSE]

ASCE 7 commentary to Appendix C, Serviceability Considerations, briefly discusses wind drift limits and loads for wind drift.

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.

 

[RobertHale]

1) The wind drifts should be checked by forces less than the ASD design forces. Appendix C of ASCE 7 comments on this but is rather vague. I use a 10 year MRI wind with an H/500 allowable drift as a starting point. As the importance of the building goes up I will increase the recurrence interval to 25 years. When I think it warrants it I will talk to the client and explain the tradeoffs for better performance vs. higher cost of construction. I use the ATC wind calculation to the the design and serviceability wind speed and I scale the design pressures and forces by the ratio of square velocities to get the actual load factor. ASCE 7 suggests a rounded load factor based on this methodology but they hide the math and just present an answer. It really matters when you are dealing with elevated importance buildings and 7-10. ICC has produced a book on the subject of serviceability that deals building drift here (full disclosure I work for the author). I will check my copy to see if it explicitly deals with wood shear walls when I get in the office tomorrow. The author indicated to me that the H/500 rule is a historic provision more or less haphazardly selected and we stick with it because it works. Griffis produced an article entitled "Serviceability Limit States Under Wind Load" that discusses the MRI and the allowable drifts. The article is "for" steel but applies universally.
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.

 

[KootK]

I've ordered the Serviceability book Robert. It sounds like my kind of read. Do you know if it's possible to procure this document somehow, also by your employer:

"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.

 

[LorenAWC]

This

Ideally, the forces would be distributed such that the deflections would be compatible.

Summing Shear Wall Lines: The nominal shear capacity for shear walls in a line, utilizing shear walls sheathed with the same materials and construction, shall be permitted to be combined if the induced shear load is distributed so as to provide the same deflection... in each shear wall.

There are no hard code limits for wind deflections like there is for seismic design.