OMF Continuity Plate Design?

[AaronMcD]

I typically just provide continuity plate to match beam flange thickness. I decided to actually go through AISC 341-16 in detail to figure out minimums, and from what I can tell continuity plates are unclear.

Under OMF section, the reader is referred to the SMF section for continuity plates. The SMF section instructs to design based on concentrated load derived from "Mf" which appears to be the maximum probable moment at the column face, derived from the maximum moment at the reduced beam section/plastic hinge. There is no indication for what moment to use for OMF.

If I use 1.1RyMp, I will absolutely always require continuity plates, and maybe require plates to be thicker than the beam flange. Is this the intent of the code? I could design for R=1 and have much less moment than that.

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[JoshPlumSE]

IMO, the newer seismic codes sort of shaft you on OMF's. The requirements are pretty close to the IMF/SMF requirements without the benefits. Maybe it's okay when you have a system that is basically like the AISC 358, but doesn't quite qualify for some reason.

I can't remember how far back it goes, but you used to be able to design them for Omega level forces. But, not anymore. You've still got the caveat related to the maximum force capable of being generated though. Two interpretations on this that can help you:
a) Limited based on panel zone capacity. I think this is spelled out in the commentary or in the seismic manual.
b) Some might interpret using the unreduced moment (i.e. R=1) as being the maximum force. Higher than the Omega loads that used to be allowed, but not usually as high as the 1.1Ry*Mp loads.

 

[AaronMcD]

Josh, do you know off had how to determine maximum panel zone capacity? Would it be multiplied by 1.1Ry? So 1.1Ry*0.6Fy*tw*d? (for Pr/Pc < 0.4)

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[kwinner]

While it appears they are trying to discourage this type of design, AISC 341-18 Section 4.2(b) allows for the "overstrength seismic load" to be the limiting design moment for OMF connections. This is similar and replacing the R=1 option from AISC 341-10.

 

[AaronMcD]

kwinner,
AISC 341 has no 2018 version, nor a section 4.2(b) - are you thinking of something else?

In the OP, I was focusing on the prescriptive design in E.6b(c)

Part (a) is material overstrength, and part (b) is maximum moment that can be transferred to the connection. There is no allowance to design for seismic overstrength, or R=1 (unless you consider this to be the maximum moment that can be transferred to the connection).

Section (c) has specific weld and access hole requirements, as well as shear designed for 2(1.1Ry*Mp)/Lcf.
If one decides to go with section (b) and use the maximum transferable moment, can these requirements be ignored? Can we just use fillet welds if they develop expected panel zone capacity?

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[AaronMcD]

OK, I should have read the commentary!
In the commentary, it specifically says overstrength is acceptable for section (b), and panel zone expected shear is indeed calculated by multiplying the unreduced capacity by 1.1Ry.

http://www.hellodwell.design

 

[kwinner]

Sorry, AISC 341-16.

"unless you consider this to be the maximum moment that can be transferred to the connection)."

That is exactly the case. In Part 4 (Moment Frame Section) it gives a similar list of limiting aspects of the system, including overstrength seismic load (AISC 341-16) or R=1 (AISC 341-10).

 

[JoshPlumSE]

Sorry about that Aaron. I skimmed the commentary last night when I had insomnia (hoping that it would help me sleep!) and didn't see the Overstrength note. Despite the fact that it was right above the panel zone option that I mentioned to you.

The provision is definitely worded differently than I remember in the past where the Overstrength option was explicitly spelled out on the spec, not the commentary.