Stability Analysis - Direct Analysis Method

[mark519]

I am looking for some clarification on a stability analysis and the Direct Analysis method. Is the DAM a type of stability analysis, or is the DAM just one step in the stability analysis?

Additionally, it seems that everyone I talk to has a different interpretation of the process for completing the DAM. I have always used a K=1.0, applied a notional load equal to 0.003Yi (with tau=1.0) for ALL load combinations, used a reduced stiffness = 0.8EI, and applied B1 (not B2) factors. Does this seem correct? Does it matter what the lateral system is that you are using (i.e. moment frames, braced frames, etc.)?

 

[WillisV]

mark519 - your method is a correct way of doing it (not the only correct way), though applying the notional load to all load combinations is generally unnecessary (just the gravity only load combinations will usually be acceptable).

 

[WillisV]

I should have said your method is correct IF you are using a program to account for large P-delta effects explicitly (which elminates B2). You could also elminiate the use of B1 if you subdivided the members adequately between bracing points to account for small p-delta within the analysis.

 

[MrHershey]

DAM is a specific type of stability analysis. I agree with your application. Have to be careful omitting the B2 factors, as Willis said.

Shankar Nair has a bunch of good articles explaining the rationale for direct analysis method and the application of direct analysis method. Should be able to find them on AISC's website.

 

[structSU10]

I had stumbled upon this article from modern steel a while back discussing stability analysis:

http://www.modernsteel.com/Uploads/Issues/September_2008/092008_steelwise_welding.pdf

And I agree with above, DAM is an approximate method for stability analysis, among many options.

 

[JoshPlumSE]

Mark -

I agree with the other posters. Your analysis seems reasonable provided your software is already doing P-Big delta as part of the analysis.

On your last question: The type of lateral system doesn't really matter. Both braced and moment frames can be analyzed and designed using the DA Method.

However, you will probably find that the method is more useful for moment frames than braced frames. I tend to use it for braced frames as well because I think it is simpler to treat all of my structures the same. But, some folks will want to run a simpler analysis (with no stiffness reductions) for braced frames.