Perforated vs. Segmented Shear Wall Design

[cuels]

My question is about wood shear wall design (i.e. 2x6 DF studs/7/16" OSB sheathing/6"-12" 8d nailing). I have worked with segmented shear walls on single story buildings in an area that doesn't have a lot of wind or seismic lateral forces. However, I am now getting into more critical areas and had a couple questions about lateral design with shear walls. I have noticed on some new wood construction where the designer has spec'd strapping around openings in the shear walls (i.e. windows, doors). My question is when is this something that should be considered? I have looked in the codes (IBC 2006 and NDS 2005) to find examples or explanations of the codes. I haven't found anywhere that shows that those forces need to be transferred around an opening in the shear wall. I see that the perforated shear wall allows me to reduce the allowable shear panel strength based on the MOHR, but is there something that is requiring me to strap around the openings to still achieve that strength, or is that considered in the code. Or, when I analyze a segmented shear wall instead of the perforated, do I need to transfer those loadings around the openings. This doesn't seem to be the case because the double top plate acts as the drag strut and the segmented shear panels will be isolated from the rest of the wall (so far as the analysis is concerned). Any comments or discussion of this would be appreciated!

 

[jdgengineer]

You do not need to provide straps using a perforated wall. If you are using the "Force Transfer Around Openings" method then you generally do need to provide straps around the openings to develop the forces at the openings.

There are significant limitations to the use of a Perforated Shear Wall, so when these limitations are exceeded the Force Transfer Around Openings method is used.

 

[cuels]

Are there any good resources for explanation of the "Force Transfer Around Openings" method? I don't know that in my readings that I have come across that.

 

[jdgengineer]

It's described as an option in the AF&PA Special Design Provisions for Wind and Seismic. If you don't have this publication, you need to get it, as it is part of the building code if you have adopted 2009 IBC.

The code allows any method that is based on engineering principles. Very vague...

There are a couple of methods that I have used in the past. One is shown in the book "Design of Wood Structures" by Donald Breyer -- a must for wood design.

The other is in the SEA 2006 IBC Structural/Seismic Design Manual...

 

[msquared48]

FYI:

I never use perforated wood shear walls - CMU and concrete, yes, but not wood. Just my thing I guess.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[abusementpark]

I've used the perforated method when it is convenient to implement. I've never used the force transfer around opening method and would only use it as an absolute last resort. I'd rather double the sheathing, adding tension strapping, increase nailing, etc.

 

[jdgengineer]

From an engineering point of view I think the force transfer method is more "sound" than perforated as it actually accounts for the forces around the openings. The perforated method just limits your forces to a level where it "should" be ok.

A lot of the time with custom homes we don't really have an option. It's use the force transfer method or else add strong walls, cantilever columns or similar. On an upper story this gets tricky...perforated walls also cannot be used on gable walls.

 

[RFreund]

Attached is a pretty good read on the Force Transfer Method. It seems that depending on your design approach you could be ultra conservative or slightly unconservative.

For some reason I can't upload the presentation pdf but woodworks has a webinar and the pdf you can download:

http://www.woodworks.org/educationTraining/onlineTraining/OTL-Full-scale-Shear-Wall-Tests.aspx

EIT

 

[cuels]

Am I reading correctly on the perforated shear wall section in IBC 2305.3.8.2.4 that there is a minimum uplift anchorage requirement. Even though I can hold down uplift forces with dead load, I am still required to provide a minimum amount of anchrage?

 

[JoshPlumSE]

Some basic comments:
1) Perforated. There are code restrictions on why you can use this. It is generally less expensive than the FTAO (Force Transfer Around Openings) method because it does not require you to use straps around all the openings.

2) FTAO: To me, this method is merely a "rational" method. Unfortunately, it is difficult to find a "right" way of doing this. Though I have given some references below:

SEAOC 2006 IBC Structual / Seismic Design Manual Volume 2: Design Example 1A - item 6b.

Breyer's "Design of Wood Structures" 6th edition example 10.6

There are othes as well, I'm sure. However, the various references that I've found for FTAO are all a little bit different. In fact, when my company (RISA) added this in to our programs we found that a number of the assumptions made in these hand calcs were not really all that accurate. For that reason, I prefer the FEM based methods that we put into our programs for calculating the shear forces in each panel and the strap forces for each opening.

At one point, I planned on putting together a "white paper" on this method which pointed out some of these incorrect calculation assumptions. But, I never seem to have enough time to put this together.

 

[jdgengineer]

Josh Plum. Interesting post. Are the assumptions generally conservative or non conservative that are used in the various Force Transfer methods as compared to you FEA results?

 

[cuels]

After reading the linked presentation from RFreund about the three different FTAO methods, I would be interested as well to know how the FE results compare to those methods.

 

[JoshPlumSE]

It's hard to say whether they're conservative or not. The assumptions will often be something like, "assume there will be a point of inflection at the mid-height of the pier". Well when the point of inflection does not end up being at the mid-height of the pier the shear and moments calculated for that pier are different.... Hence the strap forces are different.

Honestly, any method that obeys statics should work fine. It's just that the static force and moment calculations get tricky for more complex arrangements of openings. That's why we settled on the FEM solution, because we always know that the results will obey statics.

Then we just had to come up with a design methodology based on shear forces and moments.... Take the average shear force in the pier to compare to the capacity of the sheathing/ nailing. Take the moments in the pier to design for the strap forces around the openings. Take the overall moment at the base of the wall to come up with hold-down forces. I really need to put together that paper. It'll be much easier to explain with a design example and calculations to compare to.

 

[RFreund]

@Josh - looking forward to it.

After I saw the woodworks presentation on the subject, I was surprised at how different the answers were depending on what assumptions are made.

EIT