Summing Shear Wall Lines 1

[medeek]

To be honest it seems like everytime I open the AWC SDPWS-2008 I find another tidbit I seemed to have overlooked or plain out missed.

My latest gem is section 4.3.3.4 (Summing Shear Wall Lines)

It reads: 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, d[sub]sw[/sub], in each shear wall.

Typically when I look at a segmented shearwall with multiple panels/segments with similar construction I assume that all segments have the same unit shear which is the method I took from D. Breyer's book "Design of Wood Structures". Section 4.3.3.4 appears to run contrary to this method. Am I interpreting this incorrectly?


A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[sandman21]

I am not sure why the change is so confusing to some people.

Link

 

[medeek]

Thank-you for posting that link, it was informative. However, I am still confused about how there are now two methods, the 2bs/h method and equal deflection method. From my reading of the 2015 SDPWS I don't see where they have kept the 2bs/h method, or maybe that is implied somehow but at least not clearly stated in my opinion. I will have to spend some more time digesting the article referenced above and the 2015 SDPWS to fully understand all of the implications of these changes.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[RWW0002]

I have just downloaded the 2015 SDPWS for the first time, and therefore have not dug through it with much detail. However, assuming non-perforated shear wall segments, my understanding is the following:

Without considering the "Exception" to 4.3.3.1, shear is distributed based on stiffness, and shear wall capacities for walls with high aspect ratios are reduced based on the new reduction eq (1.25 - 0.125h/bs).

The other option is outlined in the "Exception" to 4.3.3.1. In this case the shear capacity of all of the segments with h/bs>2:1 are multiplied by the 2bs/h factor. You would then just have to make sure that the sum of the reduced capacities along a line is less than the total force along that line.

Therefore, considering the "Exception" method for the previous example, (9' segment, 7' segment and 4' segment with 10' plate height and 6k load), the 9' and 7' segment would each resist 313 plf and the 4' segment would resist 250 plf (resulting in 2.8k @ 9', 2.2k @ 7' and 1k @ 4')

 

[medeek]

Summary of a recent question to the AWC technical support:

Question:

In table 4.3.4 of the 2012 SDPWS if your shearwall ratio was between 2:1 to 3.5:1 for seismic then you took a reduction of 2bs/h.

Now it appears that according to section 4.3.4.2 the reduction factor is 1.25 - 0.125h/bs, and this applies for both seismic and wind (segmented and FTAO shearwalls), is this interpretation correct?


Then according to the next section 4.3.4.3 perforated shear walls are reduced with the factor 2bs/h and they are exempt from section 4.3.4.2 and exception 4.3.3.4.1.

Which then begs the question: When would exception 1 of section 4.3.3.4.1 actually apply?

I've searched the document (2015 SDPWS) thoroughly and the 2bs/h reduction factor is only used in section 4.3.4.3 (Aspect Ratio of Perforated Shear Wall Segments).
I think I am then correct in saying that Exception 1 of 4.3.3.4.1 will never apply unless you were to somehow engineer a shearwall using the 2012 and 2015 provisions simultaneously, perhaps this is the reason for the exception?


Answer:

There are two basic approaches in the 2015 SDPWS. The first is to reduce the capacity by 1.25 – 0.125h/bs for high aspect shear walls, and then distribute the shear based upon stiffness. Note that distribution of shear based on stiffness (e.g. the equal deflection requirements of 4.3.3.4.1) can produce greater strength reductions than the 2bs/h adjustment noted in Exception 1 of 4.3.3.4.1. The second option is to apply a 2bs/h reduction for high aspect shear walls, and then distribute the shear based upon capacities.


Exception 1 to 4.3.3.4.1 is the allowance for reducing wall capacities to avoid more calculation intensive distribution by stiffness. You are correct that 1.25 – 0.125h/bs isn’t applicable to perforated shear walls and that 1.25 – 0.125h/bs is applicable to both segmented and FTAO methods.


A web blog on this topic was recently posted that seems to cover many of the questions you have raised. Please see:

http://seblog.strongtie.com/author/rshackelford/

for discussion on SDPWS 4.3.3.4.1 and 4.3.4.2.


I hope this helps,

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[manstrom]

I've spent a few hours studying this. This is a pretty significant change. Or "clarifichange" as they call it in the racing world.

High aspect shear walls are all reduced by up to 0.81. This is a killer. I don't know where this came from.

Shear walls in a line can either use the 2b/h reduction or do the worst case of a deflection compatibility calculation with a 1.25-.125h/b reduction. OK, I get it.

The clarifichange is that the 2b/h reduction now applies for BOTH wind and seismic, while the before it was only for seismic and is now used in terms of deflection compatibility where before it seemed to refer to the loss of strength in high aspect shear walls.

I get it now, but it is a PITA.

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

 

[medeek]

I am still confused with the whole thing and reverted back the tried and true methods of Breyer's book and the 2008 SDPWS for now. Once the AWC comes out with a printed version of the 2015 standard with a commentary and some helpful worked problems then I will approach it again and reconfigure my shearwall calculator. The 2b/h reduction for wind is huge for me in my locale so that will changes things and make a lot of contractors I work with upset with even more stringent requirements.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[manstrom]

It is incredibly complicated for what it is.

I have to give a presentation on this tomorrow. I can hear the groans now.

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

 

[medeek]

Too bad I can't attend your presentation, I'm sure it would be enlightening.

I just watched a webinar by the AWC on the changes to the SDPWS 2015 and the coverage of the shearwall changes was way too brief. I couldn't get anything from it.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[manstrom]

I'll email you my info, but I'd rather not post it online. Is your email on your website?

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

 

[medeek]

I would appreciate that. My direct email is nathan at medeek.com.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

Okay, let me see if I have this straight. If you have a shear wall line with a couple of shear wall segments and they both have aspect ratios less than or equal to 2 then there are no reductions and there is no requirement to check that the shear distribution to each segment imparts the same calculated deflection in each segment?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[manstrom]

Yes. When the aspect ratio is 2:1 or better, the walls essentially move together regardless of their length. The vh/1000Ga factor in the wall deflection equation makes up the majority of the deflection.

So unit shear essentially becomes equally distributed in all wall segments.





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

 

[medeek]

I think one can get this from the SDPWS 2015, if interpreted carefully, but it is not clearly spelled out this way so I probably would have missed this unless I had looked at your presentation and the flow chart you provided.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com