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RE: OMF Connection Design

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Charlie,
You may recall that this debate occured when we were doing the AISC Seismic Design Guide. What precludes one from going to an R of 1 and using that as the maximum force that can be transferred ... In theory this should be an elastic response to the design basis earthquake. We could even be more conservative and multiply by 1.5 to get an elastic response to the MCE. For the vast number of pre-engineered metal buildings, wind will still govern, but the calculations are easier.

Regards,
Harold Sprague


From: "Carter, Charlie" <carter(--nospam--at)aisc.org>
Reply-To: <seaint(--nospam--at)seaint.org>
To: "SEAINT List Server" <seaint(--nospam--at)seaint.org>
Subject: RE: OMF Connection Design
Date: Wed, 7 Jul 2004 20:51:27 -0500

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





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