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RE: 2002 aisc seismic provisions

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Title: 2002 aisc seismic provisions

Search the archives for “Seismic” and “Charlie Carter”.  There was a thread on (began with questions from me) this very issue 12 to 18 months ago.  You should read the entire thread.  It is both enlightening and confusing.  Based on that thread and other information, here is what I try to do:

 

Whenever possible, I design for the expected axial tension strength.  I am a firm believer in the “fuse” concept.  You, as the engineer, need to determine how your system should behave.  Design your system to go ductile where you want it.  Don’t simply expect the system to behave ductile, design that into the system from the beginning.  This means if you want the connections to remain elastic, the connections must be stronger than the brace than your yielding element.

 

Secondly, don’t pretend we fully understand dynamic behavior.  We have educated guesses.  Because of that, I also don’t like using the foundation as a limiting factor.  There are many studies available where buildings should have failed in overturning, but didn’t.  Also, if you assume the foundation moves before the brace yields, then you are assuming the brace is rocking.  If it’s rocking, where is the ductile energy dissipation & how far is your building moving?  There is also the other extreme in this opinion.  The Steel Tips document “Seismic Design of Gusset Plates” (I hope that’s the correct name) opines that foundations should be made strong enough to yield the brace.  As I said, we don’t understand dynamic loading half as well as we pretend.

 

Last, but not least, Em does not equal the maximum possible force.  It is theoretically the force required to keep the system in the elastic range.

 

A little judgment can go a long ways.  If you KNOW the system will remain elastic, fine.  Design it for Em or greater.  If there is any doubt in your mind (don’t think code forces, think reality), you are far better off designing the system to yield the brace.  This opinion is nowhere in the AISC provisions.  Use at your own risk.

 

Jake Watson, P.E.

Salt Lake City, UT

 

P.S. I have had many discussions about this matter and freely admit my opinions are in the minority.

 


From: David Adie [mailto:DavidA(--nospam--at)cplinc.com]
Sent: Thursday, March 10, 2005 12:14 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: 2002 aisc seismic provisions

 

the commentary for part 1 - special concentrically braced frames - bracing connections - required strength (c13.3a) pg. 118 states "the minimum of two criteria, (i.e. the nominal expected axial tension strength of the bracing member and the maximum force that COULD be generated by the overall system) determines the required strength of both the bracing connection and the beam to column connection if it is part of the bracing system".

question #1 - how many of you design the connection per the nominal expected axial tension strength?

question #2 - how many of you are using the maximum force that COULD be generated?  that's also know as "Em", right?

tia
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