Design of a Embedded Steel Pile. Unbraced length and LTB modification factor, Cb

[structuralAutoMatt]

I am designing a steel embedded pile, I am in charge of the pile design given vendor supplied loading at the top of pile. The top of pile loading consists of a shear, axial, and negative moment, as shown in the attached sketch. Currently I am using the unbraced length of the pile to be 12" below grade, using LPile to get the design moment as this location, and checking the unbraced portion of the pile for LTB, flange buckling and combine force per AISC 360, chapter H.
1) This 12" is an assumption is there a better way to calculate the location of this point?
2) The maximum moment occurs below this point, I am assuming the pile is fully braced below this point, so am only checking plastic bending at this for this maximum moment demand. For these checks I am using the effective length method with K = 2.1, assuming the pile top is free to rotate and translate. My question is that if a negative moment is being transferred to the top of pile is this assumption correct, can I instead use a K = 1.2? Does this depend more on the geometry of the connection or is knowing that that connection transfer this negative moment into the pile enough to say that the pile is free to translate but not rotate? In LPile the output gives me a curvature of -0.00002 rad/in at the top of pile. A curvature of 0 represents a fixed connection, correct?
3) The next question is that is, should Cb = 1.0? or since the a moment is transferred into the pile can I assume it is braced and calculate Cb per F1-1?

Thanks in advance.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1648491055/tips/EmbeddedPileSketch_lxhha7.pdf[/url]

 

[CMsq]

KootK appears to have a good understanding of this.
So the top rail (torque tube) provides "some" restraint to the top of the piles from moving in the weak axis - there is a bearing at the top of pile but due to friction with the dead weight it should provide 50 lb of lateral restraint - do you think that is enough to provide lateral bracing (assume load in strong axis is 2,000 lb)?

The top rail (torque tube) will restrain the top of pile from rotating somewhat so I think the K value in somewhere between 1.2 & 2.1 - I assume an exact value of K can be worked out based on the stiffness of the pile and the top rail?

I am having these exact some problems/questions designing piles for Nextracker/ATI single axis trackers - which I have been doing for many years now

 

[CMsq]

So we did some analysis here and have worked out that Cb of 1.67 appears to be appropriate as the restraining force required is relatively small (1% of applied load).
We also did an analysis using the AISC alignment chart and have worked out that for W6 pile sizes with 4" wide flanges the K is around 1.2 for typical column heights. For W6 with 6" wide flanges K is around 1.5 but since much larger r than the 4" flange columns this does not affect the overall capacity since bending is the main concern.

 

[atrizzy]

If you have a geotechnical engineer that can provide soil 'spring' values with depth, you can model the full pile and run a buckling analysis, take the analyzed buckling load and back calculate the effective length.

 

[KootK]

@CMsq: thanks for reporting back with your results. Sorry we didn't get around to answering your questions before you got it sorted on your own.