AISC Direct Analysis Method - Increase notional load vs tau-b

[Settingsun]

Hi all,
I've recently some across the Direct Analysis Method of Design (DAM) and have a couple of questions. The clauses below are from ANSI/AISC 360-16.

When using extra notional load instead of the tau-b factor [C2.3(c)], the code gives a factor of 0.001 for the notional load which is half the 'main' notional load for out-of-plumb [C2.2.2b(a)]. If I have a structure where the out-of-plumb is more than 1:500 so the main notional load needs to be larger than given in the code, does the 'tau-b notional load' also need to be increased in proportion?


In Appendix 1, advanced analysis outlaws use of notional loads - geometric imperfections must be modelled. What is the reason for this?

 

[JoshPlumSE]

The reason for the extra notional load is to account for the reduction in buckling strength of the members due to inelastic (rather than elastic) buckling. We are using an elastic, 2nd order analysis to approximate the behavior of the structure.... also know as geometrically non-linear, but materially linear analysis.

So, for the Direct Analysis Method to adequately approximate the buckling behavior of the structure, we need to make sure the structure obeys the assumptions inherent in the method. Most of the method, I believe, was developed under the assumption of 1:500 out-of-plumbness and a tau_b adjustment. The code explicitly allows you to use the method when more out-of-plumbness is present. Great, that part becomes clear. I don't believe there is anything explicit that prevents you from trying the extra notional load in lieu of the tau_b. But, it is something of an outlier. Best to be conservative.

Appendix 1 is a totally different animal. Different goals, different procedures. Don't get it confused with Direct Analysis method. The safety / accuracy of this Appendix 1 method is much, much more dependent on the quality of the analysis. It's a much higher analysis burden to meet. Whereas the Direct Analysis method was made to be simple for engineers to perform with current / typical analysis programs.

 

[Settingsun]

Thanks, Josh. I'll stick with using tau-b initially - I don't actually have a job starting soon where I could use DAM. Just thinking ahead for the moment.

Re advanced analysis: don't worry, I'm not mixing it up with DAM. Since I was thinking about increased out-of-plumb, I started thinking about increased out-of-straightness and whether notional loads at mid-height of columns would address it in DAM. Tbe advanced analysis appendix doesn't permit this but, on reflection, notional anything isn't appropriate in advanced analysis. This question was just academic.