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RE: OMF Connection Design

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I was just trying to illustrate a mechanism that limits the amount of force that a system can transfer. The OMF's, OCBF's in steel do have crosshairs on them in the "buildings" community, but the nonbuilding structures people have retained them. If one drops the response factor low enough even nonductile systems can work. The "nonbuilding structures" community was forced to retain them because: 1) Good past performance; 2) There are no other options for many types of nonbuilding structures.

Developing systems to limit the transfer of seismic forces is a bit tricky, and some reviewers may not allow them. Reviewers like well established conservative systems.

Harold Sprague

From: "Bill Allen, S.E." <T.W.Allen(--nospam--at)>
Reply-To: <seaint(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: RE: OMF Connection Design
Date: Tue, 6 Jul 2004 10:05:57 -0700

Harold -

I understand that concept for a special frame, but I don't think that's true (having to design the connection for Mp, particularly the panel zone) for an
OMF (at least in the flavors of AISC provisions I've read). Of course,
there's no such thing as an OMF in concrete, and it's a fading concept in
steel. Sigh...

But, then again, I could be wrong. It's happened before, just not today

I agree with Bill Caine in that any "limiting" load would be by the
collector or diaphragm. The rub is, of course, determining what that
limiting load is at strength level.

T. William (Bill) Allen, S.E. (CA #2607)
Allen Designs
Consulting Structural Engineers

-----Original Message-----
From: Harold Sprague [mailto:spraguehope(--nospam--at)]
Sent: Tuesday, July 06, 2004 9:15 AM
To: seaint(--nospam--at)
Subject: Re: OMF Connection Design

I think that the best example of the logic of this provision is for a
reinforced concrete SMRF.  After you have computed the minimum reinforcing
steel in the connections, you then compute the plastic moment Mp for the
frame based on the actual rebar selected.  You then multiply the calculated
moment capacity times a 1.25 multiplier to account for material overstrength

to get the Mp.  Then you use the plastic moment to calculate the shears
induced into the system, and you design the stirrups/hoops to resist the
shears due to the frame's plastic moment.  There can be no more shear than
what the plastic moment can deliver.  That is the concept intended by the
authors of the steel provisions.  Perhaps it could have been written more

The next generation ASCE 7-05 of seismic code should help.  As usual, the
ASCE 7 seismic section will be largely based on the preceding NEHRP
Provisions.  In this case the 2003 NEHRP Provisions.
The 2003 NEHRP is now available on line at

I like to read the NEHRP Provisions to my kids when I tuck them in at night.

  When they ask why, I read them the Commentary.  They look up at me and
say, "...Dad, get a life!"

Harold Sprague

>From: BCainse(--nospam--at)
>Reply-To: <seaint(--nospam--at)>
>To: seaint(--nospam--at)
>Subject: Re: OMF Connection Design
>Date: Sat, 3 Jul 2004 00:24:01 EDT
>Maybe I'm missing something but it seems that "the frame" is the "lateral
>resisting element."   "The frame" consists of columns, one or more beams
>and the
>joints connecting them.  Now the code limits the design of the "lateral
>resisting element" to the maximum force that can be delivered "to" it not
>it.  It would seem, then, that the force used for design of "the frame"
>be limited by a collector attached to the frame but not something integral
>the frame.
>Bill Cain, S.E.
>Berkeley  CA

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