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Cm-- Reduction Coefficient

Cm-- Reduction Coefficient

Cm-- Reduction Coefficient

(OP)
Has three values per AISC 9th edition, Chapter H, page 5-55.  Could someone focus more light on when to use which value.  Will appreciate your reply.

RE: Cm-- Reduction Coefficient

I would respectfully suggest that you move on up to the 13th edition steel code (blasphemy!) and stop using the 9th edition. The 13th edition allows you to use some more advanced analysis (using a program like RISA) to avoid having to deal with some awkward questions (like how to calculate Cm or K).  

That being said, you may want to clarify your question.  You say three values.  I assume you are talking about items a, b and c on page 5-55.  If so, then I would interpret their language as follows:

a) This one is primarily for columns in moment frames.

b) This one is primarily for columns in braced frames.

c) This one is primarily for compression chords of a truss.  But, it also applies to column in braced frames if they receive any lateral loading between floor levels.  

That's probably an over simplification.  
 

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