What Factor to Use for Drilled Shaft Interaction Capacity?

[EngDYIGuy]

Hi,
I am running Ensoft LPILE, to check the structural strength capacity and to design the reinforcement in a non-cased round bored shaft. I'm running the "Non-linear EI Only Mode" and creating a interaction diagram. The program can provide a graph of the interaction curve (Axial thrust force vs Factored Bending Moment Capacity Phi*Mn), with factors varying from 1.0 to 0.65. My question is what is the appropriate RS factor for the capacity, to check against my LRFD factored loads. My entire drilled shaft is below grade and sufficiently braced by soil. I have checked ACI 318-14 sections below, but haven't found the factor I'm looking for under the building code.

13.4.3 Deep foundation members
13.4.3.1 Portions of deep foundation members in air,
water, or soils not capable of providing adequate restraint
throughout the member length to prevent lateral buckling
shall be designed as columns in accordance with the applicable
provisions of Chapter 10.


R14.1.2 Structural elements, such as cast-in-place plain
concrete piles and piers in ground or other material sufficiently
stiff to provide adequate lateral support to prevent
buckling, are not covered by the Code. Such elements are
covered by the general building code.

 

[EngDYIGuy]

I may have answered my own question with TABLE 21.2.2 of ACI 318-14

Output file of LPILE gives tensile strain values suggesting compression controlled, and I'm not using spiral reinforcement. So Phi = 0.65 seems appropriate?

Still interested in feedback.

Thanks

 

[HENRYZAU]

If the moment is high enough to create yield points in your pile, why not increase your pile diameter? If your pile is still in elastic range and your only concern is reduced EI due to cracking, I would suggest 0.7EI and 0.5EI for short term and long term respectively and Phi=1.0

For piles in very soft clay with undrained shear strength of 10kPa or less, your piles are not fully braced and thus your effective length for buckling should be longer.

Hope the above helps