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When to analyze for 2nd order effects

When to analyze for 2nd order effects

When to analyze for 2nd order effects

As my title sugggest I'm confused as to when we are required to account for 2nd order effects in our member analysis.  A few examples that I could use some explaining on are:

1. Examples 3.1-3.3 from the Seismic Design Manual - From reading Chapter C in the Steel Manual it seemed to me that ALL members of a lateral force resisting braced frame would be designed for 2nd order effects, but that's not the case.  Ex. 3.1 (brace) and 3.2 (column) have no 2nd order effects, while Ex, 3.3 (beam) does slip them in there for some reason.

2. Example 5.1 from the same manual.  Here I figured that a collector wouldn't have to account for 2nd order effects since it is not an integral member (beam, column or brace) of a braced frame, moment frame, or shearwall but was proven wrong.  What gives?

RE: When to analyze for 2nd order effects

2nd order effects should be accounted for in all member design.  You are correct in your initial interpretation of Chapter C.   

RE: When to analyze for 2nd order effects

The AISC code tends to be very strict in requiring the consideration of 2nd order effects.... Maybe a bit too strict.  

Some thoughts about when it may be appropriate to ignore 2nd order effects:

ASCE7-2005 section:
This section allows you to ignore 2nd order effects when the stability coefficient (which is essentially a ratio of secondary shears to original story shears) is less than 0.10.  

ACI-2002 Section
You are allowed to assume that a structure is Non-Sway (which greatly reduces the amount of 2nd order effects you have to consider) if the stability ratio is less than 0.05.  

Technically speaking those limitations are both referring to the "P-Big Delta" effect due to story drift.  That's usually the main cause of 2nd order effects.  But, when you have relatively slender members (KL/r > 40?) then you might want to include the effects of member curvature as well.  This is usually done with the old Cm / (1-fa/Fe) amplification.   

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