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P-Delta Code Question

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I couldn't sleep one night and was reading the code a lullaby.
Something interesting caught my attention.  AISC and ACI both have a
very similar coefficient for estimating second-order column effects.
They both use the variable "Cm" but have a slightly different definition.

   AISC defines the variable as Cm = 0.6-0.4(M1/M2) (LRFD V3, equation
C1-3).
   ACI defines it as Cm = 0.6+0.4(M1/M2)>0.4 (ACI 318-05, chapter 10).

Please notice the sign in the equation.  Both terms are used to modify
non-sway frame column moments and go into very similar equations.  Any
thoughts as to why the signs are different?  Is is a 10 year old typo?

Jake Watson, P.E.
Salt Lake City, UT


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