Moment Magnification ACI 1

[slickdeals]

Folks,
There have been quite a few threads here that have discussed about dividing a member into finite number of pieces and run a P-delta analysis to capture both the Global and local P-delta effects.

I want to know if using such a procedure of breaking up a member into finite pieces to capture the moments due to member curvature between bracing locations due to slenderness is accepted by ACI as an analysis method?

 

[BAretired]

Why wouldn't it be?

BA

 

[Lion06]

It is, but be sure to model an appropriately reduced EI to account for cracking and the increased second order effects that will result from the reduced stiffness.

 

[asixth]

It is called a structural analysis that incorporates secondary bending moments, like StructuralEIT has indicated above the stiffness of the columns need to be reduced to account for cracking and vertical loads must be modelled.

 

[slickdeals]

This brings me to another question. Do you folks model a column with a finite number of pieces with pre-displaced nodes, meaning do you assume some sort of out-of-plumbness in the model prior to the start of analysis?

 

[ishvaaag]

I have done such thing a number of times for different reasons. If the column is a standard shape you may not need to. Other thing is if you build or fabricate it, then you may set some standard about out of straightness, as in chords of pony bridges (where it can be made by placement of the nodes of the lateral trusses). Review what the mandatory code says.

 

[Lion06]

slickdeals-

I don't model columns with an initial curvature. The reason is that there are minimum end moments to design for (which will give it an initial curvature) - whether this curvature from end moments accurately captures the column being cast with some curvature as well is another discussion, but given the fact that the EI used in the analysis is such a gross estimation it doesn't seem unreasonable to let the end moments do the work for initial curvature.

I have used initial curvature (a half-sine wave) trying to get a FEM program to match the theoretical elastic buckling load of a steel column (it didn't work like I'd hoped), but that's something completely different.

 

[JAE]

slickdeals,

I don't include any initial curvature.

I depend upon ACI's initial paragraph (section 10.10 I think..or maybe 10.10.1 - away from my desk).

Basically, ACI states something to this effect (my paraphrase): Use the moment magnifier method that we present here in Chapter 10, OR...you can use any rational analysis - [blue]provided it can be show to represent structural behavior that is verfied with full scale tests[/blue].

So the powers that be (hmmmm - are they research professors do you think) - say you can use a second order method as long as you can prove it with tests. So technically, the multi-node column method that many of us do use isn't valid unless backed up with tests - but we do use it anyway.

 

[JoshPlumSE]

If you want to copy someone else's validation of the "breaking up the member" method, then try the "benchmark" tests that AISC uses for P-little delta verificaton (see the commentary of Appendix 7 for the AISC 13th edition).

While originally assembled for the AISC specification, the concept remains essentially the same for ACI. Except, of course, for the cracking issues that other folks have already mentioned.

If you're using the RISA program then we've already done this for you. Therefore, you can just download the example problems (and some graphs that compare the results with the theoretically correct solutions):

http://www.risatech.com/documents/pdfs/RISADirectAnalysis.zip

If, however, your analysis program uses a different method of calculating P-Delta effects then you might have to run these sorts of tests yourself.

Josh