Moment Inertia Reducing

[SPR Baker]

Dear everyone
If I do run non-linear static analysis for capture P-Delta and then let’s the program (Etabs) design/check the capacity of column, instead using Moment Magnifier. Should I reduce the moment inertia of members as per ACI?
Another question
In Global Analyses (Drift Check), Should I reduce the moment inertia?
Thank in advance

 

[KootK]

Should I reduce the moment inertia of members as per ACI?

I would think so. Otherwise, you're failing to capture some of the things that affect stiffness materially. I'd start with AISC's design guide on the Direct Design Method and take it from there.

I don't believe that the moment or inertia reduction is necessary for a global drift check. I'll typically go with just stock EI values and my best guess at an honest appraisal of connection stiffnesses. These stiffness reductions are really about capturing issues that will exacerbate stability issues local at ULS load levels.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[Agent666]

I'd disagree in part with what Kootk has said, if you're dealing with a reinforced concrete structure the ACI stiffness reductions are about directly accounting for the effect of the reduction in the stiffness of your members once they are have cracked. So if your design actions are greater than the cracking moment (which is almost always the case for a well proportioned structure) then you should consider the stiffness reductions for both strength and global lateral drift checks.

Pretty sure ACI says this type of thing explicitly, that any modelling should take account of the degree of cracking expected at that limit state.

 

[KootK]

Oops, I thought we were talking steel.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[JoshPlumSE]

Note: I believe the reference to ACI by the OP was a mistake / typo and that he meant to say AISC.

If you're talking about reducing the stiffness of steel members, use the provisions of AISC. If you're talking about concrete then use the ACI provisions.

 

[SPR Baker]

KootK,Agent666,Agent666,JoshPlum
how much we reduce for ULS and Drift Check? Or follow Code?
Please Attachment
thank you.

 

[JoshPlumSE]

Okay, so the ACI reference was NOT a mistake.... So, yes you adjust your member stiffness per ACI.

Either method works. I usually just use the (a) values. We're really trying to linear-ize a non-linear behavior in a way that provides acceptable analysis. Using method (b) is more complex and involves evaluating the moment of inertia under a number of different load combinations. Then you'd have to re-iterate your analysis to see if the forces changes enough to alter the moment of inertia further.