Column stability factor, Cp for timber pile

[Dusni]

Hi,

I'm struggling to understand AASHTO's (Stnd Specs) guidance, which I have to follow instead of NDS.

I understand NDS's approach:


The derivation for equation H-2 above is as follows:


Then if you have a circle instead of a square cross-section, d gets replaced with r * sqrt(12), where r = radius of gyration. This I can also derive:


But AASHTO's (Stnd Specs) equation for Fce is different, and I can't understand it:




Basically 0.822 from the NDS formula, which can be derived mathematically, is replaced by the factor KcE, which varies based on type of lumber (and is significantly lower)? And how do I adjust this for a circular cross-section? Do I still just replace d with sqrt(12) * r?

 

[SWComposites]

Could there be different assumed end conditions buried in the Fce equation?

 

[bugbus]

My guess is it is could be related to creep. Any initial imperfections or curvature in the columns will be magnified by creep, which would have the effect of reducing the effective buckling load over time. The qualifier of Le/d < 50 in NDS might be a hint that for stockier columns, it is maybe not so significant a factor.

There is a similar provision in the Australian standards (and I assume others) for slender concrete columns, where the moment magnifier includes a factor relating to creep.

Again, just a guess

 

[Dusni]

Could there be different assumed end conditions buried in the Fce equation?

I don't think so, since le should account for the end conditions: