AISC Chapter C-Stability analysis methods 1

[palves]

So, Chapter C gives 3 analysis methods and I'm wondering if anybody out there has experience with the 2 alternative methods. I usually use the direct analysis method, but am curious when it might be useful to use the effective length or first-order analysis methods. When are those more advantageous than the DDM? What are the pros and cons of each? Let me know your thoughts because I'd like a little practical advice to make sure I'm using the correct analysis method in the appropriate situation. Thanks everybody,

 

[TehMightyEngineer]

They're only advantageous if you don't want to use a computer to calculate second order effects. If you look at the assumptions involved you'll find that they're not very accurate for a typical multi-story building and really should only be used for simple structures. That said, I use the effective length method for single story structures all the time as it is nice and fast and second order effects can almost always be all but neglected. With modern software though it's pretty much no advantage over the direct analysis method in terms of speed.

Here's a great, but brief, introduction to the effective length method: [URL unfurl="true"]http://media.aisc.org/NASCC2014/140327-N40A.mp4[/url]

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[JoshPlumSE]

One thing I notice is some people have a hard time understanding what to do when they get divergence for a DA Method Analysis. In those cases, it is often useful to switch back to something like those other two methods. They will usually find that they need to reduce drift or better brace some columns or the like.

I guess what I'm saying is that a first order analysis or an effective length method analysis can be easier to investigate if the DA method has you stymied.

 

[TehMightyEngineer]

Josh does make a good point; it's often a lot harder to verify by hand that second order effects are being applied appropriately by the "black box". Running an effective length analysis can make a good double check to ensure a feasible design.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com