4'-0" Allowable Shearwall Offset? Not Allowed Anymore? 1

[jtwright4216]

Hey Guys,

I am working on a custom residence up in Oregon. The plans examiner won't accept the typical 4'-0" allowable shear wall offset to be considered in the same line. Additionally, he won't accept any method of transferring the shear through say transfer diaphragms. His code reference is in the SDPWS 4.3.5 Stating; Where out-of-plan offsets occur portions of the shear wall on each side of the offset shall be considered as separate wall lines.

I then referenced the ACSE7-10 14.5.2;

Where offsets occur in the wall line,
portions of the shear wall on each side of the offset
shall be considered as separate shear walls unless
provisions for force transfer around the offset are
provided.

He wrote back saying that it was amended and delete in 2013. I was really trying to emphasize the unless provisions for force transfer around the offset are
provided...but apparently I can't be an engineer anymore. The OSSC does have a section in it that for conventional light framed construction that for braced panels an allowable 4'-0" offsets are acceptable, however, since it mentions braced panels he says it only for prescriptive methodology. To me it is common practice where walls are offset 4’ or less to distribute loads as if they are in the same line because that is how the diaphragm is really going to behave.

Also, I have studied the book The Analysis of Irregular Shaped Structures by Terry Malone. Considering the same line and transferring shear around offsets is nothing new.

Anyone have any experience or problems with this lately? Or have any clarification or code references they know of.

 

[jdgengineer]

I do as you typically do as well. I sometimes include walls offset more than 4' -- it depends on the diaphragm aspect. I try and rationalize how I think the wall line will behave and detail in the spirit of Terry Malone's book. I interpret the code section referenced by the plan reviewer differently than him. I feel it is saying that you have to count them as two individual wall segments not one long wall within the offset.

In reality the building won't behave like a flexible diaphragm and would probably typically tend more towards rigid so I try and not sweat the offsets too much for calculations and instead focus on good redundant lateral systems and details.

 

[jdgengineer]

While not really applicable since you are doing an engineered design, Section R602.10.1.2 of the IRC quotes the 4'-0" offset. I believe there are also similar provisions in the conventional construction requirements of the IBC.

 

[jtwright4216]

It doesn't make sense to me to consider separate wall line either. Given a full length of diaphragm, the wall that is inset 4’ isn’t going to take all of that trib load from the main part of the diaphragm and then the outer most wall only takes the 2’ of trib from the wall offset. It’s just not going to function that way. The initial force may reach the inset wall first, but once the diaphragm starts to deflect, the force will automatically distribute into all of the walls. And I don't have a problem even proving that the force can transfer around offsets. Basically, your creating a moment/couple at the bump out area assuming they are acting in the same shear line.

Your interpretation makes sense if you are considering including the offset in say a perforated wall. But in this case the SDPWS 4.3.5.1 states that; Where out-of-plane offsets occur, portions on the wall on each side of the offset shall be considered separate shear wall lines". However, say they are in the same line essentially with an jog in at the front door, then those wall segments would take the total shear load not the shared load. To me that's what it is saying. More of a directional approach from opposites sides. They are completely disregarding the possibility of transfer around an offset.

Ya, I saw that as well in the IRC. Still prescriptive though can't be accepted...blah

Anybody else have any code references that may help?

Thanks!

 

[JAE]

I don't think you need to use a code reference. Simply engineer the diaphragm piece between the two lines and show the inspector what's what.

The section of diaphragm between these lines will be affected by the offset but I don't think it is that much. You could model/analyze the diaphragm, with its expected stiffness, along with the shear wall stiffnesses and derive the type of shear flow that occurs.....in other words: "unless provisions for force transfer around the offset are provided."

So you just have to justify the offset numerically I would think. Be an engineer (as you stated in your first post) and design it.


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[jtwright4216]

JEA: Thanks for the response.

That's exactly what I'm saying. 4'-0" has just been an industry acceptance since I was intern way back when. I'm not trying to get around proving it numerically. But if you ask almost any engineer most accept this 4'-0" as code. I just want to see if I am missing something. Because obviously there is no code reference justifying this anywhere (if there ever was). It seems to me that since it is in the prescriptive code, engineers use that as justification when engineering is done.

Additionally, like I mentioned since the clause "unless provisions for force transfer around the offset are provided." was removed from the ASCE7-10 there is nothing in the code that allows you to prove it numerically anymore. That's basically what the plans examiner told me. He said that I HAVE to consider them in separate wall lines per the SDPWS 4.3.5.1. And that no where in the code anymore is force transferred allowed for offset walls.

 

[JAE]

unless provisions for force transfer around the offset are provided." was removed from the ASCE7-10 there is nothing in the code that allows you to prove it numerically anymore.

Hey, there's nothing in the code that says I can design my steel framed tower with 6x6 tubes and cross frames at 10, 16 and 23.76 feet above the foundation but I do it.
Why do you think you need a code provision saying it is OK to design a diaphragm of some specific configuration?



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[jtwright4216]

I get it, and agree JEA. I'm not looking for a code section to justify to MYSELF. I already engineered the diaphragm.

But the jurisdiction approving the plans are the ones saying that I cannot because of the code. Does that make sense??

 

[JAE]

No it doesn't. If you were relying on prescriptive code provisions to justify your design then the inspector has a point.

If you are a licensed PE/SE and design and seal the drawings I can't see where the inspector can say you can't do that unless the code specifically denies it based on NON-PRESCRIPTIVE code provisions.

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[jtwright4216]

Hey JEA, I'm not relying on the prescriptive code. Additional engineering was provided showing valid force transfer around the offsets. I was just wondering why most engineers consider it a standard rule of thumb when it is indeed only in the prescriptive code section.

But they won't accept force transfer either because of the SDPWS 4.3.5.1. This is a plan examiner not an inspector. He is a licensed PE too. The drawings have been sealed by a registered PE as well.

I'm not trying to argue with you.

 

[JAE]

Hey no arguments from me - very passive today.

I just think it is absurd that a PE/Plans examiner can't tell the difference between prescriptive code provisions and simple engineering.

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[jdgengineer]

The language is clarified in 2015 SDPWS. You can access electronic copy for free from AWC.

"Where out of plane offsets occur, portions of the wall on each side of the offset shall be considered separate shear walls"

Keep in mind the section of the code this comes from. This is from the individual wall segment section. The SDPWS is only telling you with that approach you cannot count it was one wall (perforated and FTAO have similar verbiage). The section, in my opinion, is not intended to restrict the diaphragm force distribution. If that was the intent it would be in a different section of the code relating to general design requirements.

 

[FoxSE14]

I agree with jdgengineer's last comment. The offset item referenced has nothing to do with distribution to the wall, but everything to do with the shear forces and holdowns on the wall segments. One way to think of it: they are saying once the lateral forces are within the top plates, you cannot pass the shear around corners and through a plan jog in the wall.

Did you submit calcs with the permit? Is he reviewing those? If you are sure that you've done your job, then tell him so. Confidently and concisely state to him your case in the simplest terms possible. The very fact that you are detailing/or offering to detail a transfer diaphragm to ensure adequate load path should prove to him that you are not trying to skirt around the code. From what I've seen, detailing a proper transfer diaphragm is beyond what many engineers will do (or what most owners will be willing to pay the appropriate engineering fees to have done right).

If he resists, offer to discuss with him personally and show him line by line how you are meeting the requirements of ASCE 7-10 12.1.3 "...continuous load path..." and be prepared to drag your calculations, send snippets of the Malone book/other materials to do so. If needed, offer to meet with him personally. Do not be combative in the slightest, but very confident and motivated to discuss your methods with him.

Sometimes the plan checkers will back off once they feel a purposed, licensed engineer heading their direction armed with solid calculations.

 

[SLTA]

someone take JAE's temperature... not arguing? are you ok?

Please remember: we're not all guys!

 

[JAE]

I am currently kicked back with my feet up and taking in a 12 yr. old single malt scotch. I'm good!



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[msquared48]

I agree with FoxSE on this interpretation. It's not about the total load to the line, it's about the nailing, hold downs and detailing to the individual walls.

After all here, we do not need to be Scotch regarding the quality of the shear walls.

Mike McCann, PE, SE (WA)

 

[jtwright4216]

Thanks for comments guys.

jdgengineer/FoxSE14...I agree with the interpretation. I actually think of it as directional more than anything. One segment taking on the entire shear from the left and the other from the right. Assuming there in the same line and there is a jog in the middle. However, if detailed correctly around the jog, you can consider both of those walls to resist force from either direction. Which would reduce the shearwall specs considerably. But ya, no perforated walls over jogs.

I didn't end up taking it that far as too sit down with this guy. We agreed upon increasing the shearwall nailing. Which really didn't make sense but got it through the plan check.

Anyway, thanks again guys!