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RE: OMF Connection Design

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Charlie:

Thanks. I understand the examples of appropriate ways to determine the maximum force that can be delivered to the system, but....... do the AISC Seismic Provisions 1997 or 2002, allow the maximum force that can be delivered to the system to be approximated by the special load combinations for ordinary moment connections?  Special concentrically braced frame connections?  It seems like the answer is no from your explanation.
 
Respectfully,
Scott Haan
-----Original Message-----
From: Carter, Charlie [mailto:carter(--nospam--at)aisc.org]
Sent: Wednesday, July 07, 2004 5:51 PM
To: SEAINT List Server
Subject: RE: OMF Connection Design

The original question centered upon the 1997 version of the AISC Seismic Provisions, so I will answer based upon those provisions.
 
The goal of an OMF connection in these provisions is to provide for 1 percent plastic rotation through controlled inelastic deformations. A prescribed detail is provided that can be used, but that detail is not generally applicable to the gable-frame knee joint at hand. The other option given is for the use of a tested connection, which means tested in accordance with the requirements in Appendix S.
 
Let's assume the detail proposed is so qualified as a tested connection. If the testing upon which it is based shows that the required inelastic rotation is reliably achieved through a mechanism such as shear yielding of the panel zone, it seems to me the connection proposed is acceptable. I also consider the panel zone to be a part of this connection, since it is the source of inelastic deformations.
 
The exception for the maximum force that can be transferred ... is something entirely different to me. This approach is generally not supposed to be used by looking at elements that are a part of the framing that is a part of the seismic force resisting system. Rather, the maximum force exception provides a way to examine parts of the properly designed system other than those in the seismic force resisting system and find maximums that can't credibly be exceeded without first failing them. Theoretically speaking, that is something that cannot happen if they are properly designed unless the ground motion exceeds the design value.
 
The most common example I've given to illustrate an appropriate maximum force exception is a building on spread footings having zero capacity to resist uplift but properly designed to resist the effects of overturning. The forces and moment in the seismic force resisting system would never see a force greater than that corresponding to the overturning of the building (foundation uplift) unless the design ground motion were exceeded. The same building on a pile system with tension capacity would not benefit from such a system limit.
 
Perhaps the forces and moments corrresponding to the shearing of the diaphragms off the lateral system would also be another example of an appropriate way to use the maximum force exception. There are likely many others.
 
I hope this answers the questions raised. If more is needed, please let me know. I only skimmed the many responses on this topic in catching up in order to create this posting.

Charlie