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Delivered by the System

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I will give this a crack as we ran into this on a recent project in regards
to bracing connections.  This phrase about "maximum force/moment that can
be transferred/delivered ... by/to the system" appears in different
sections of the AISC Seismic Provisions.  Unfortunately there is very
little good commentary in the Seismic Provisions or in the SEAOC Blue Book.
This is definitely not the moments/forces from your standard analysis using
the code loads and load combinations.  When would the maximum capacity of
the member every be less than the design capacity?!  I will explain what we
ran across in our bracing study and let you figure how to apply it to your
moment frame case.  Let me also say that what follows is my own opinion
since we did not find a lot of written backup however we did do a lot of
research and consulted with such notables as Ron Hamburger and Bob Bachman.
I welcome other thoughts and comments.

This "maximum force/moment delivered by the system" is in general the
maximum capacity of the Lateral Force Resisting SYSTEM which may be less
than the maximum capacity of a single element within the system.  For
bracing we came up with four cases where this may occur (I am sure there
may be more):

1.  System Configuration

Lets say you have a 6 story single bay braced frame building and the upper
story is higher than the rest.  Your bracing members will likely be
governed by kl/r so the upper bracing could be larger than the bottom
bracing( and it's capacity greater).  In a case where you are using member
capacity loads obviously the lower/smaller bracing may fail first.
Therefore the maximum force for the design of the upper bracing could be
limited by the lower bracing SYSTEM.

2.  Overall Stability

I don't like this one and I am not sure how it would be done but your
system could be limited by the stability of the overall system.  Taking the
example above and adding some simple spread footings under the two columns
the force to overturn the entire system may be less than the capacity of
the individual members.  If the entire structure completely overturns and
fails then there is no reason to design the members/connections for more
than this.  Issues like factor of safety of overturning, what is
foundation/soil failure, etc. would come into play.

3.  Maximum Earthquake Load

This is my favorite and was used on our project and accepted by our
building department.  The maximum force delivered by the system could be
limited by the maximum force going into the system.  The maximum seismic
force going into the system could be assumed by using an R=1.0.

4.  System Capacity

According to Ron Hamburger this is what the code writers had in mind in
that it was allowed that the engineer could do a sophisticated failure
analysis such as a push over analysis, progressive failure analysis, or
some type of plastic analysis to determine the maximum forces in each
member at the point of complete instability.

I hope this gives you some ideas to kick around.

Thomas Hunt, S.E.
Duke/Fluor Daniel

----- Forwarded by Tom Hunt/DFD on 11/15/01 09:40 AM -----
                    "Joseph R. Grill,                                                                                    
                    PE"                        To:     "SEAINT" <seaint(--nospam--at)>                                      
                    <jgrill@nelsonengine       cc:                                                                       
          >                 Subject:                                                                  
                    11/14/01 09:47 AM                                                                                    
                    Please respond to                                                                                    

I have a question concerning OMF Section 11.2a.1.  "FR moment connections
that are part of the Seismic Force Resisting System shall be designed for a
required flexural strength Mu that is at least equal to 1.1RyMp of the beam
or girder or the maximum moment that can be delivered by the system,
whichever is less."   Is the part "?Maximum moment that can be delivered by
the system?"  the moment delivered to the connection due to applied
factored loads from analysis and appropriate load combinations?


 Joseph R. Grill, PE

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