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Re: FW: Bracing connection design forces

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This question has come up in the past and here is a slightly modified response I provided previously.  Hope it helps.

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?!  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 several 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.  (Some would argue for R=1.5)

4.  Floor/Roof Diaphragms

The overall lateral resisting SYSTEM could also be limited by the capacity of the floor or roof diaphragms.

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.

Thomas Hunt, S.E.
ABS Consulting

"Vasu, Ravi" <Ravi.Vasu(--nospam--at)>

10/15/2002 08:57 AM
Please respond to seaint

        To:        "seaint(--nospam--at)" <'seaint(--nospam--at)'>
        Subject:        FW: Bracing connection design forces



But as you have mentioned, it is confusing.  I have gone through the UBC,
ch. 22 & the'omega factor discussion' few times.  It is not clearly defined
and still one could interpret(from the words)any way.  
I am sort of saying YES to you.

One more question.

Pl. look at UBC section 2213.8.3.1(pg. 2-258),
In UBC section 2213.8.3.1(pg. 2-258), the forces for bracing
connection has 'The maximum force that can be tranferred to the
brace by the system'(3 rd item).  Does it mean, the load
combinations to use to find the force is from section 1612.3(pg.
2-4) ONLY?  or any other load combinations required?    If the load
combinations are only from section 1630.2, then forces from items 1
& 2 in section 2213.8.3.1 may not GOVERN at all.? (since we have to take LEAST force only?)

I appreciate your response & thanks.