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Steel LRFD spec question

Steel LRFD spec question

Steel LRFD spec question

(OP)
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.

RE: Steel LRFD spec question

Second order effects are found in Chapter C.  The interaction equation has some good commentary in the commentary section for Chapter H.

RE: Steel LRFD spec question

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.

RE: Steel LRFD spec question

(OP)
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?

RE: Steel LRFD spec question

That is correct.  

RE: Steel LRFD spec question

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).  

RE: Steel LRFD spec question

I would agree with WillisV and Galambos

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