Second Order Analysis of Pier Columns

[OSUCivlEng]

We use RC Pier to design concrete piers in our office. When I use the word pier, I am referring to a concrete cap (beam) on top of two or more round concrete columns. Traditionally we have used the moment magnification option for second order analysis (AASHTO 5.7.4.3). Compared to the P-delta analysis, moment magnification seems to be overly conservative. I think that makes sense since it is an approximate method. This has led me to question if P-delta is the better way to go.

Another engineer in our office is questioning if we should take cracked section properties into account when performing the p-delta analysis. The commentary in 5.7.4.3 just states that some kind of second order analysis should be used. I am unsure if the cracked section properties or the gross section properties should be used. The only way to do this in RC Pier is to reduce the moment of inertia by a factor less than 1 (i.e. 50%,80%,etc). If the gross section properties shouldn't be used, how would you go about determining how much to reduce the moment of inertia? Icrack/Igross is the only thing that comes to mind, but is that to conservative?

 

[Qshake]

You typically use the cracked section properties and a ratio of Icracked = 0.5Iuncracked.

Regards,
Qshake

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[OSUCivlEng]

Thanks. Guess I'll just have to reduce the moment of inertia to 0.5 because that's all I can change. I'll see how it changes the results and compare it to the moment magnification results.