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RE: OMF Connection Design - Force limit

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I asked this question about four or five  months with regard to braced frames, and I am still on the fence about the footing (or anything else) really limiting the force in the system.  We tend to forget that these forces are transient, they are not static like we model.  More importantly, they are inertial impulses.  Can you general more uplift force from an inertial impulse than would normally be statically resisted by a footing in uplift (yes)?  If the acceleration is high enough, even small masses can generate huge forces over short periods.  Are the elements stronger because the impulses are so short (yes)?  Is there a strength increase to force increase ratio that must be considered?  Or has that already been indirectly dealt with by "R"?
 
The idea of forcing inelastic deformation to occur in a predetermined location has great benefits.  Using the footing as a limiting factor may be expedient, but is it an answer that will work when the big one hits? (That last sentence really is a question, not a rhetorical one)
 
Inquiring minds want to know.....
 
Jake Watson, P.E.
Salt Lake City, UT
-----Original Message-----
From: Carter, Charlie [mailto:carter(--nospam--at)aisc.org]
Sent: Wednesday, July 07, 2004 7:51 PM
To: SEAINT List Server
Subject: RE: OMF Connection Design

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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.
 
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Charlie