Uplift and flexural reinforcement in auger cast piles 1

[BAGW]

Hi,

I have a 18" auger cast piles with 7 #8 vertical rebars. The vertical rebars extend upto 25'-0" based on the deflection/moment curve. The total length of the pile is 60'-0". The piles are subjected to considerable uplift and moments (100k and 90k-ft strength loads). I am designing the pile as a column and 7#8 bars work for combined tension and flexure.

My question is,
1) Is there a minimum spacing requirement between the vertical bars for auger cast piles? Can I go with 8 #8 bars in 18" pile?
2) I have a #8 vertical bar at the center of the pile going all the way down. Typically this bar is used to resist uplift. In my case I am using the main vertical bars to resist uplift in combination with flexure (column interaction). Based on the soil report, I need 15'-0" of pile length to resist 100k of uplift from skin friction. The vertical bars I have extends 25'-0" which is beyond the required length of the pile to resist uplift. This means I dont need to extend the rebar all the way down to the pile to resist uplift as the uplift reduces down to zero at 15'. Any concerns with the way I am approaching the design? I will still keep the #8 center bar going all the way down from detailing perspective.

Thanks

 

[Ron]

Your design is conservative and should work fine as you have it

 

[JLNJ]

I would be partial to 8 bars just for the symmetry of it. Seven bars must be a pain in the neck to lay out.

For consolidation reasons you don't want the bars too close, especially since the cage gets lowered in once the flights are withdrawn. Your bars are something like 5 or 6 inches on center, which seems like plenty.

 

[BridgeSmith]

I'm not sure of the differences between auger-cast piles and drilled shafts, but the latest guidance from AASHTO for drilled shafts is the clear spacing between bars, both transverse and longitudinal, should be 5 times the max aggregate size. This allows the concrete to flow into the annular space without the benefit of vibration.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[wannabeSE]

BridgeSmith,
Auger-cast piles have grout placed through a hollow stem auger as it is withdrawn from the hole. The reinforcing is placed into the wet grout/concrete after the auger is removed.

 

[BridgeSmith]

In that case wannabeSE, they probably wouldn't have the issue I was concerned about. Thanks for straightening me out.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Suggest to terminate the bars in staggered manner, rather than cutting at the same depth.

 

[BAGW]

Thanks all