OMF - Connection with Overstrength? 1

[X4vier]

Hi, I'm designing an Ordinary Moment Frame.

Must I design the connections with Overstrength?
Is there any part of codes (ASCE 7 or AISC 341) that says specifically that I must design connection of moment frames with Overstrength?

Thanks.

 

[DrZoidberWoop]

I've been familiarizing myself w/ the AISC Seismic Manual these days, and here is an extract from my notes on OMF's. Being a conx. engineer for a fabricator, I'm not an expert in general system-level stuff, but these notes may help you. I've been trying to make the material more digestible for a dummy like me. The seismic design manual is super dense and dry.

 

[YoungGunner]

The main theology of an OMF is that it is designed to have strong connections that don't yield or fail before the members yield, so already you are looking at a moment frame that has to be designed for loads higher than what the members were designed for. AISC 341 goes in detail in the commentary about this. There are several optional loading requirements associated with a FR moment connection and most of them involve increasing the loads above that which was achieved through standard load combinations. One of them involves using overstrength. So I don't think the answer to your question is a simple yes or no, it just depends on which route of the code you want to go with and how much math you want to go through. I've often used part (b) of AISC 341-16 of section 6b to design my ordinary moment frame connections.

I may be young in the industry, but I'm not willing to accept the standard as the standard without solid argument.

 

[JoshPlumSE]

AISC 341-2016: Chapter E1 Section 6b(a):
The moment connection connection is design for a moment of 1.1RyMp
The shear force for the connection is designed as 2(1.1RyMP)/Lcf (i.e. assuming the probably plastic hinge moment is formed at both sides of the beam).

This is pretty much the same shear force required for IMF and SMF.... The maximum shear force based on hinges forming on both sides of the beam.

So, this is likely a force much greater than the Omega level forces.

The only out you have is in 6b(b) which gives the "maximum moment that can be transmitted by the system". This "out" is described in the commentary as covering the following:
[ul]
[li]Column Yielding[/li]
[li]Panel Zone Yielding[/li]
[li]Foundation Uplift[/li]
[li]Overstrength Load[/li]
[/ul]

I believe there is a published example (in the seismic design manual?) using the panel zone yielding option.