Second Order Effects in Steel Frame Design

[OR-EIT]

I am going through the examples in AISC Seismic Design Manual as prep for the Washington Structural III Seismic Specific Exam. My question has to do with the need to account for second order effects in frame element design.

say for example when looking at Example Problem 4.2 OMF COLUMN DESIGN- pg 4-7

I realize that for an exact solution you would have to check the second order effects on the frame elements. But this part of the calculations are pretty cumbersome and time consuming. I'm wondering if it would be acceptable practice (IE for use on the exam) to neglect second order effects (and state so in your problem solution) to come up with approximate first order member sizes (using Table 6-1 of the manual). If time allows you could check for second order effects, but I don't think that time will allow, as I know that time is really, really tight on the exam.

Would this be wrong? I appreciate anyone's feedback.

 

[kevo_55]

OR-EIT-

You may use any method found in chapter C or appendix 7 in the 13th edition.

Why not simply use your B1 and B2 (approximate 2nd order method from 1st order results) to solve?

No matter what you do, if you have some sort of drift you will have to account for 2nd order effects. You simply using the unmagnified 1st order effects will get you an incorrect answer very quick.

 

[OR-EIT]

Got it. Yes I see now to calc B1 and B2 to account for second order effects is the thing to do.

Not quite as cumbersome as I first thought.

Thanks.

 

[kevo_55]

No problem.

If things weren't wierd enough, using the "B1 and B2" method is what you were doing all along in the green & dark blue (or silver) books. My ASD 9th edition and silver book are at work now but I believe that if you look in the bending and axial interaction section you'll see something very much like the B1 and B2 equations there. I know if the ASD 9th edition, it is physically integrated into the interaction equation.

It turns out we've been doing it this way all along!

I hope this helps!