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RE: P-Delta Code Question

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Jake,

Look in both codes for the definition of "M1/M2".  AISC says 'positive
for reverse curvature bending' and ACI says 'positive for single
curvature bending'.  Gets you to the same value ultimately.  I guess the
concrete guys wanted to be different (since they followed the steel
guys).

Eric

Eric R. Ober, PE, SE
Associate
Holbert Apple Associates
Olney, Maryland
 

-----Original Message-----
From: Jake Watson [mailto:jwatson(--nospam--at)utahisp.com] 
Sent: Monday, April 10, 2006 9:37 PM
To: seaint(--nospam--at)seaint.org
Subject: P-Delta Code Question

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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