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Steel LRFD spec question

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Lion06

Lion06

Structural
Nov 17, 2006
4,238
My office is currently switching from ASD to LRFD and I am trying to familiarize myself with the LRFD spec. The question I have is regarding combined stresses and the P-delta effect. In the ASD spec the moment was magnified by a factor to account for this effect. From what I am reading of the LRFD spec, the moment is actually being reduced by 11%? Am I reading that correctly or am I missing something drastic. The combined stress equation (for Pu>0.2phiPn - which is the case where Pdelta must be considered), is Pu/(phiPn) + 8/9*(Mux/phiMnx + Muy/phiMny). At first, I thought this may have something to do with Cm, but then that would be applying a constant value to it instead of allowing you to specify Cm.
 
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Second order effects are found in Chapter C. The interaction equation has some good commentary in the commentary section for Chapter H.

 
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to further JAE's response, the magnification has been removed from the interaction equations in ASD and it is found in Chapter C (?) as the B1 and B2 factors. B1 accounts for combined compression and flexure (p-small delta) at the member level, while B2 accounts for frame action (p-big delta).

Multiply your moments by the appropriate B1 and B2 factors. if you look at B1 closely, you'll see a similarity to ASD.
 
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JAE and Galambos-
Thanks for the clarification there. I assume then that the Mux and Muy in the interaction equation reflect the second order effects, i.e. that B1 and B2 must be used in computing them?
 
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Of course B1 and B2 do not have to be used in computing them - they just have to reflect the second order effects (a second order analysis could be used as well).
 
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I would agree with WillisV and Galambos

 
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