SDPWS Diaphragm Aspect Limits for Partially Blocked Diaphragms 1

[TehMightyEngineer]

I've been searching for guidance on a situation where you terminate blocking in a blocked diaphragm at a point where an unblocked diaphragm is sufficient to carry the shear forces. If you do this what do you consider the aspect ratio limit to be for the diaphragm sections?

My understanding is it would be acceptable to consider the aspect ratio of the unblocked region as the length of the unblocked region. Then, for the blocked diaphragm you would consider the aspect ratio for the entire diaphragm. However, I could see arguments for conservatively considering that the diaphragm only qualifies for an unblocked diaphragm or that you could argue it could be considered a blocked diaphragm for the aspect ratio limits.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

It all comes back to what the purpose of diaphragm aspect ratio limits is. I've never seen that spelled out explicitly any place. My guess is that the idea is to ensure that the diaphragm is not too much of a flexural element and, threfore, limit the in plane tensile stresses generated at the joint between adjacent sheathing panels.

To that end, I would be inclined to examine the diaphragm as a whole but intepolate the aspect ratio limit based on the percentage of the diaphragm length that receives blocking. If 75% of the diaphragm were unblocked, the limit would be 3.25:1.

Obviously, I'm just improvising all of this.

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.

 

[TehMightyEngineer]

Hmmm, interesting idea. Talking with other engineers it seems clear to me that this hasn't been readily addressed from what we can find. We've shot an email off to the NDS and APA to hopefully get an official opinion.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[TehMightyEngineer]

A response from the AWC (and APA):

"I touched base with APA to get their opinion before I answered your email. The following is excerpted from their response:

"The below information might be used to make an engineering judgment argument for greater aspect ratio.

The 3:1 limit is arbitrary. I believe it first appeared in the IBC. (We think the philosophy of IBC for seismic design was to assume that NEHRP Provisions were the most cutting edge seismic provisions). The 1997 UBC permits 4:1 unblocked diaphragms. We are not aware of any events where 4:1 unblocked diaphragms have caused a problem.

We believe that the 3:1 aspect ratio limit originated with National Earthquake Hazards Reduction Program (NEHRP), which also requires diaphragms to be blocked in "high" seismic areas.
The code does not contain any definitive guidance on the case of mixed blocked and unblocked diaphragms. In a partially blocked diaphragm we would say that the unblocked portion must not exceed 3:1 and the whole diaphragm not exceed 4:1. "

In our Wood Frame Construction Manual, we also employ a mixed diaphragm approach in section 3.3.5, where it is required to provide blocking in the first two bays in high wind areas. This requirement does not have any impact on the aspect ratio requirements put forth in the WFCM.

If you have other questions I would be pleased to discuss them with you."

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[jayrod12]

Thanks for the update!

 

[KootK]

The update is great but I find the answer pretty unsatisfying. Basically, they have no idea.

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.

 

[jayrod12]

Are you surprised? I'm not, they are essentially telling you it's guess work and try to keep the aspect ratio somewhere south of 4:1. That's not unreasonable. I've never had an unblocked diaphragm with an aspect ratio considerable worse than 2:1.

 

[KootK]

I think that it's unreasonable if they're not able to provide a justification for it. We're engineers damn it! We're supposed to have reasons for doing stuff. They don't have to be super precise reasons, but they should be reasons none the less. To get that much of an "explanation" without ever even mentioning stiffness shocks the heck out of me.

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.

 

[jayrod12]

If the research has never been done, how can they answer with anymore than the truth? In fact, I kind of applaud them for admitting it is arbitrary instead of attempting to skirt the fact that they just don't know the impacts of exceeding their aspect ratios.

For the longest time, wood design was more of a "It worked before it'll work now" and now that more research is being done capacities always seem to be going down.

Lately with everyone wanting to push wood framing beyond the envelope we're coming into these issues where old rules of thumb are being questioned. There hasn't been enough research done yet to answer all of them in entirety. Perhaps this is the case.

Until recently, wood was reserved for smaller square buildings, now the push is towards long narrow multi-family residences that just keep increasing in height (the new NBCC is apparently allowing 5-6 storeys). How do you comfortably extrapolate currently accepted values to buildings twice the height and half the width than most structures built to date? My feeling is you can't without testing to back it up. And that is expensive, time consuming and you'd want multiple tests done by un-affiliated programs to reach a reasonable consensus.

 

[KootK]

If the research has never been done, how can they answer with anymore than the truth?

Sure, being honest about having no justification beats lying about it. But that still doesn't obviate the need for some justification in my opinion.

Suppose I made a rule that all wood roof decks had to be covered in chocolate garden gnomes at 10' o/c max. And then, when you asked about it, I'd tell you that it's arbitrary, some other documents mentioned chocolate garden gnomes at 10' o/c, and I know of know issues with roofs constructed with chocolate garden gnomes at 10' o/c. You gonna applaud my frankness?

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.

 

[TehMightyEngineer]

I'll applaud you, but only if the chocolate garden gnomes also count for uplift resistance.

In all seriousness (which is hard to do in a post-chocolate-garden-gnome world) I think that KootK has the right thought. It was the right thing to admit the limit is arbitrary, or at least admit they have no known rational beyond historic precedent. But, we aren't given a Christmas bonus simply for showing up to work roughly on time nor should gold stars be handed out for simply admitting that this isn't an engineering limit; simply a historical limit. This is a "we haven't tested it yet" limit and is entirely self-imposed. For all we know 4:1 isn't conservative but works enough that it's not been noted as causing failures. Maybe 4:1 works but only if chocolate garden gnomes are used elsewhere in the LFRS and, lacking the gnomes, it should be 3.5:1.

Short answer, this limit is archaic and should be tested. Honestly, how does the ICC justify updating the building code every 12 seconds if not to fix these kinds of issues? In the interim I shall no longer consider this a "do not exceed" limit and take it as more of the "KL/r < 200" recommended limit you find in AISC.

However, it was good to see that they more or less agreed with what we surmised regarding partially blocked diaphragms.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[jayrod12]

I applaud your creativity if nothing else Koot.

I did not intend to make it seem like I am happy about their lack of a real answer. I'm just glad they didn't try to bullshit us.

 

[MotorCity]

Could you split the diaphragms up into two separate diaphragms (1 blocked, 1 unblocked) and design accordingly?

 

[KootK]

I applaud your creativity if nothing else Koot.

Well, creative bitching comes naturally to me.

I'd be curious to see how the steel deck folks would answer the same question. It's been a while but, if I recall correctly, they specify an aspect ratio and give you some kind of out for when you've assessed deflection and determined that all is right with the world. Implicitly, that would imply that the diaphragm stiffnesses presumed for diaphragms meeting the limit would obviate the need for a deflection check. No such language in the wood standards though to my knowledge.

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.

 

[msquared48]

There are also limitations places on deep beams with stress lomitations based on the aspect ratio.

I think this is the ultimate reason for the restriction as a diaphrag is just another form of a beam. And this gets back to the no research argument.

Mike McCann, PE, SE (WA)