Ties in Auger-cast piles

[BAGW]

Hi all,

I have always seen #3 ties in auger-cast pile. Is #4 acceptable? I dont see a reason why we cant go #4 ties instead of #3.

Thanks

 

[BAretired]

#4 ties should be okay. Some piling contractors prefer to use continuous helical reinforcement which tends to hold the cage together a little better than separate ties.

BA

 

[BAGW]

Thanks

 

[MOJOJOHN]

I design them as columns. Same rules for compression and shear and #4 ties for #11 bars or larger.

 

[jdonville]

@MOJOJOHN,

Designing as columns can be overly conservative as the earth provides generally increasing confining pressure as a function of depth, except for any portion of the pile anticipated to be exposed/unsupported during service (e.g., due to scour or very poor soil conditions). IIRC, the IBC (normally referenced by the state code) provides guidance on mimimum % of vertical steel for cast-in-place foundation elements.

Due to challenges inserting a relatively flexible cage into the fresh grout, recommend that you keep the cage as short as required to resist moment. Unless the pile is short (about 30ft or so - your local contractors can give you more specific local guidance on their preferences), if you need full-length steel, then I suggest looking at rolled structural shape reinforcement or constructing the foundation element using a drilled shaft technique.

 

[MOJOJOHN]

Jdonville,

Thank you for your suggestions.

When we design as columns we take into consideration soil support using the Lpile program or by estimating the unsupported length from soil borings. While we limit the cages to as short as reasonable, our piles are often under tension resulting in longer cages or stinger bars within the cages. We have gone to up to 140 feet with 36” diameter auger piles without problems and are considering longer piles for a potential job.

Regards,

Mojojohn

 

[r13]

Design the steel cage for bending, and gradually reduce vertical bars after the peak demand. We usually provide a full length bar in the middle for tension. As suggested by Jdonville, a rolled shape maybe considered for large tensile force, but it's a rare event (we've done that before for hydraulic structure to counter uplift).

 

[jdonville]

I only suggested the rolled shape for full-length flexural or axial reinforcing. (We do this using our ACP rigs for earth retention where vibration concerns prevent driving piles.) For tension resistance, all you need is a threaded bar centered that can take the tensile loading. These can be spliced as long as needed, but can be challenging to install - it's like pushing on a strand of spaghetti so you need open centralizers that are a goodly %age of the hole diameter. Usually best to install the center bar first and then the (hopefully much shorter) cage.

My point though was to caution against conservatively using ACI column design requirements/approach for ACP vertical steel as columns are typically laterally unbraced/unsupported whereas ACPs are, in satisfactory soil conditions, braced continuously by the soil.