ACI 10.10.2.1 Second order/First order Moment ratio

[radiocontrolhead]

Hi All,

Few basic assumptions:

-Non -Sway
-Shear wall system
-Deflections are assumed small enough that PDelta effects are NOT considered.
-SP Column slenderness feature used to design columns that are considered slender per ACI Slenderness criteria.

My question:

Does the second order/first order ratio need to be satisfied even if considering a simple pin pin concrete column? In other words, to ignore of effects of building deflections which may result in additional moments? With building deflections ignored, my understanding is that second order moments are derived from deflections of first order moments which are magnified resulting in the second order moments.

SP column seems to be doing exactly the way I have interpreted the code in that it will apply a minimum amount of primary moment (if smaller than the primary moment from the slab analysis) per ACI 10.10.6.5 and will magnify that minimum moment for design and check for the 1.4 slenderness ratio.

Thank you all in advance,

 

[JoshPlumSE]

I'm not looking at the code, but my impression has been the same as yours.

Since there is essentially no moment in the column, you use a minimum moment, then apply the amplification procedure to it. Then make sure it satisfies the maximum 2nd/ 1st ratio.

 

[radiocontrolhead]

I was told that the 1.4 ratio was for big Delta deflections and not little delta deflections from member bending.

Intuitively this does not make much sense to me. This ratio is the only stability check for slender columns that I am aware of in the code. To find the primary moments, and then magnify, and design for it is okay with me but this is usually not what governs my design, it becomes the 1.4 stability check criteria that generally fails my sections.

I ask because I wanted to make sure i wasn't missing something here and over designing my columns if I could ignore the 1.4 ratio if it didn't apply to my case.

 

[JoshPlumSE]

I look at it a little differently. I'm concerned about the degree of 2nd order effects. If you're getting a 40% amplification of moment purely due to slenderness, then you're probably approaching the euler buckling load for this column....