Concerns of rebar buckling for end-bearing CFA piles

[whollycow]

Hello,

For a CFA pile designed as end-bearing on rock, is a full rebar cage necessary for the full length of the pile or is it okay to use the typical CFA reinforcement scheme of a full cage in the upper 1/3 of the pile and only extending a full-length center bar to the bottom of the pile?

Is there any concern for the center bar buckling due to fact that there is no lateral confinement reinforcement i.e. spirals or ties? I know the FHWA micropile manual recommends limiting the steel yield strength to 87 ksi due to the issue of strain compatibility between the grout and the rebar. I believe this is to prevent the grout from cracking due to excessive compressive strain but I don't know if this necessarily addresses the issue of rebar buckling. The scenario I'm envisioning in my head is that the grout will not provide sufficient confinement to the center bar, and the grout will crack and the center bar will buckle, i.e. local buckling of the rebar as opposed to global buckling of the pile. Is there any legitimacy to this concern or is it safe to assume that as long as the soils above rock have sufficient strength to prevent global buckling of the pile then there is no need to worry about local buckling of the rebar?

Are there any other concerns with only using a single center bar for an end-bearing drilled pile?

Thanks!

 

[Retrograde]

Normally I have only used the full-length central bar to deal with tension loads - and if it is in tension there is no buckling issue. For compression loads you ignore the central bar and design it as unreinforced.

 

[whollycow]

Thanks for the response Retrograde. My approach would be to ignore the center bar when designing for compressive loads as well. However, even if the center bar is ignored for design, it is still present in the pile, and will attract compressive load purely due to the relative stiffness between the concrete and the steel. Consider the following example: 18" diameter CFA with a #28 bar in the center for relatively large uplift loads. Assuming 6 ksi concrete, 75 ksi steel, and an allowable pile capacity of 450 kips, the center bar will take 92 kips of that load. The stress in the steel is still very low at about 10 ksi, but for say a 50' length of pile with just concrete and a center bar, would you be concerned at all about buckling?

 

[GC_Hopi]

The general approach is to keep the stresses in the concrete and steel to a low level which would negate your concern about bursting of the concrete. The following link discusses the allowable piles stresses used in several design standards/codes. Link

 

[Retrograde]

would you be concerned at all about buckling

No I would not be concerned. If the design works as an unreinforced section, the stresses will never be high enough to cause buckling.

 

[ATSE]

Maybe not concerned for buckling, but it gives most structural engineers a bit of heartburn to design using the ACI plain concrete provisions. You wouldn't ever design a slender column as unreinforced plain concrete, right?
Long center bar extensions common in ACIPs are more typical for skin friction piles. I'm sure you can get an experienced engineer to argue otherwise, but I would suggest a full cage full depth for end bearing piles - just like what you'd do for precast piles or CIDH piles.