Auger Cast Pile - Structural Design

[bmetro]

I am designing an 18" diameter auger cast pile with a center rod that as fy=120ksi and a rebar cage with an fy=60ksi. The rebar cage is made up of 8-#7 bars with hoop ties. I am analyzing the pile as a short column. However, what is the proper way to input the center rod into software that does not allow multiple yield strengths (i.e. pcacol)? I don't want to multiple the cross-sectional area of the center bar by the ratio of the yield strengths unless I can verify that the center bar yields at a give location on the curve. Is this correct? Are there any good manuals or books that describe in detail the structural design of auger cast piles?

 

[JAE]

Why do you have a center rod with fy = 120 ksi?

Is this some sort of prestressing bar?

In the center, the fy may not ever be reached anyway so entering it as fy = 60 may be OK. Both have E = 29000 ksi so that won't change. So basically the bar behaves exactly like a fy=60 bar until fs = 60 ksi, right?

If you enter it as fy = 60, then at the end verify that the stress in that center bar never went past 60 might be a workable solution within PCA COL.

 

[bmetro]

Thanks for your comments.

The center bar is not for prestressing. Per ACI 543 a center bar is used the length of the pile. In this case the bar is used to care the tensile load that is induced due to seismic loading. The use of the 120ksi bar is a common practice.

 

[structuresguy]

I would also question the use of a center bar, along with a rebar cage. We design auger cast pile foundations quite frequently, and have never used a center bar along with a rebar cage. However, our designs are for wind and wave loading, not seismic. We design the piles as fixed at the top into the pile cap, as we frequently have significant lateral loads from high winds (140mph+) and quite often wave loading. Many times our piles have to be designed with the top 10 feet or more as freestanding cantilever piles, due to soil erosion, with wave loads on the cantilever portion.

ACI 543 does not require a center bar, only says that one may be used. If the rebar cage is hooked into the pile cap, I don't see what the center bar is doing structurally. Though I suppose you may want to avoid the fixity of a developed rebar cage in some cases. Can you explain a little more how you use the rebar cage vs center bar? Is the rebar cage full height? Does it develop into the pile cap?

 

[bmetro]

The rebar cage is only from the top of the pile to half-way down the pile. The rebar cage extends above the top of the pile (into the foundation - full development length). The center bar extends the full length of the auger cast pile. The rebar cage will handle the flexure + axial compression loading (column loading) while the center rebar will be used to handle the tension loading. I hope this clarifies the situation.

 

[structuresguy]

Yes, thanks. I can see why you may want a higher strength bar for tension, if only one bar is carrying it.

I guess one question I would have is how much tension exists where the rebar cage terminates. Is there still net tension that deep into the pile? If the pile is skin friction pile, with uniform soils over the length, then at half the length, you should be able to develop half the capacity. Since in my experience (again without seismic) the max tension is less than 1/2 the max compression loads, I would think that you would have almost no net tension remaining once you were at that depth.

Either way, I would ignore this single rod when designing the pile with the rebar cage. The cage bars are so much more effective than a single centered bar, since they are much further from the neutral axis. I can't see this center bar contributing more than 5% to the total capacity.

BTW, what I have done where I don't need full rebar cage to go full depth, is just stop half the cage, and continue the remaining 3 or 4 bars full depth. This way I still have some lateral confinement, in case any voids or soft layers are encountered.

 

[bmetro]

Thank you for thoughts. I like your idea of running the remaining 3 or 4 bars of the rebar cage the full length of the pile.

 

[escrowe]

FYI, we generally spec a center bar with 'centralizers' to ensure that auger cast pile holes have remained open to depth after grouting. We also mount strain gages for pile load testing on the center bars of the test piles.

For uplift resistance, the central bar is assumed to 'tie' the pile together from top to bottom: uplift resistance for a cracked, unreinforced pile is drastically reduced, and that's just not acceptable in wind country.

 

[haynewp]

I would just ignore the center bar in the PCA model and figure it into the tension requirements by hand calculation if the entire length of the pile is required to be engaged to resist uplift.