acip pile reinforcing design

huangyhg

acip pile reinforcing design
if the geotech. engineer is designing the acip(augered cast-in-place) piles, who determines if steel reinforcing is required and who designes the depth of the cage and size of the reinforcing.  if structural designs the reinforcing, what methods are used.  are there any guidelines or references.  looking for how this is normally handled.
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typically, we (the structural engineer) usually call out the steel reinforcing in auger cast piles. we have some standard pile reinforcing details that we use unless there is something unusual about the pile ie: it is taking unusually high lateral loads or eccentricities.
for example: for 16" diamter piles we have a cage of 4-#6 bars around the perimeter with #3 ties. this cage extends 20 into the pile and 2'-6" into the pile cap. we also place 1 full length (usually a #9) bar down the entire length of the pile. the exterior cage is to handle any accidental or incidental bending on the pile from lateral loads or eccentricities. the center bar is to provide ductile continuity down the entire length of the pile and to engage the entire pile when we are counting on it to resist uplift.
others, i'm sure have different standards and opinions on the subject. just how our office does this.
the geotech should provide you with deflection and moment graphs for the different load conditions assumed and the head restraint (fixed or free).  the pile reinforcing should be designed for the moments in the pile.  you need the max steel at the max moment.  at some point along the pile there is a point of fixity...no moment.  usually the reinforcing cage will extend past this point a short distance.  don't know exactly what this distance is.  i have only seen center bars placed in a pile for uplift only and they have been full length like lkjh345 said.
lkjh345 you mentioned 16" diameter piles.  do you know off hand what capacity a 16" is normally used for,.....i guess i am asking what is a common size pile diameter that you would expect for say, 40 tons, 12", 14", 16"?
barring something unusual in the geotech report or in the loading of the pile, we usually assume a 16" auger pile is good for 100 tons (200 kips). see section 1810.3 of ibc 2006. assuming 3,000 psi grout, this works out to 200 kips or 100 tons.
(full disclosure: our local building department has modified 1810.3 to allow for the maximum stress to be only 25% of the f'c. we therefore typically use 4,000 psi grout to get back to the 100 ton capacity for a 16" diameter.)
we estimate the length based on parameters of skin friction and end bearing set by the geotech. final length is determined in the feild by the geotech rep.
uplift resistance is based on parameters given by the geotech.
i have seen 14" diameter acip piles. i personally have never called for or seen anything smaller than a 14" dimater piles. doesn't mean they are not out there, just have never seen them used around my area.
might check with a local installer to see what the smallest they typically do in your area.
lkjh345 thanks for your help.  i have only specified wood piles where the diameter is about 12", where i spaced them 2'6" or 3'-0" apart for a two-pile  pile cap.  what is the minimum recommended spacing for 16" diameter acip piles.
talk to the geotech for spacing.  he will probably tell you 3 times the diameter..that is usually standard.  any closer and you start to get group action, which reduces the capacity of the piles.