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RE: OMF Connection Design

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Charlie,

I have used this approach successfully especially in industrial structures where the brace lengths can be much longer than in commercial work where brace lengths are typically from floor to floor.  With the b/t and kl/r restrictions these long braces can get huge and trying to develop the connection for the capacity of the member becomes nearly unrealistic.  Using an R = 1 in these situations to design the connection can result in forces significantly less than the capacity of the member.

The justification (or paper trail) is rather simple.  The maximum force that can be "delivered" by the system is limited by the maximum force that can go into the structure which is a force calculated using R = 1 or as Harold mentions you can make a strong argument for R = 1.5.

As a personal comment which I have stated here before is that I think this part of the AISC Seismic Provisions (i.e. what can be delivery to/by the system and all its variation) is the most misunderstood part of the entire code and the commentary does little in providing a firm and discrete explanation.  This topic alone deserves a detailed AISC Steel Tips or some other significant publication.

Thomas Hunt, S.E.
ABS Consulting




"Carter, Charlie" <carter(--nospam--at)aisc.org>

07/08/2004 05:45 PM

Please respond to
<seaint(--nospam--at)seaint.org>

To
<seaint(--nospam--at)seaint.org>
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Subject
RE: OMF Connection Design





Harold,

I'm with you, but please draw for me the paper trail that permits this
-- or are you suggesting that the building official be asked each time
to agree to this approach?

Charlie

-----Original Message-----
From: Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com]
Sent: Thursday, July 08, 2004 6:21 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: OMF Connection Design

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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