Micropile Design per ACI 318-25

[dblnkl]

I'm reading through the foundation provisions of the new ACI 318-25 to understand changes from 318-19.

Can anyone help me understand the intent of the minimum reinforcing steel provisions of 13.4.4?

If I have a cast-in-place pile with either uplift or M_u ≥ 0.4M_cr, I need to provide reinforcing steel, which must meet either 13.4.4.1.1 or 13.4.4.1.2.

13.4.4.1.1 clearly applies to conventional piles that are reinforced like a column (longitudinal bars with transverse reinforcing). However, 13.4.4.1.2 appears to provide a "carve-out" that allows members with P_u < 0.75ϕP_n to be designed with a single longitudinal bar at the center (to me, this sounds like a micropile).

Does this imply that micropiles should only be designed to resist 75% of ϕP_n? If so, do the specialty engineers who design micropiles take this into account?

 

[HDStructural]

I do not have access to ACI 318-25, but to me this is saying that you typically need multiply longitudinal bars and a cage. If you want to use a single longitudinal bar at the center, the pile must be over designed (not under-designed). There is essentially an additional phi factor of 0.75 to further reduce the capacity.

put another way, micropiles should be designed to resist 133% of P_u

 

[dblnkl]

I do not have access to ACI 318-25, but to me this is saying that you typically need multiply longitudinal bars and a cage. If you want to use a single longitudinal bar at the center, the pile must be over designed (not under-designed). There is essentially an additional phi factor of 0.75 to further reduce the capacity.

put another way, micropiles should be designed to resist 133% of P_u

Cool, it sounds like we came to the same understanding.

Definitely something I'll keep in mind for the next time I work with micropiles.