aci 318-08 21.6.4.4

huangyhg

aci 318-08 21.6.4.4
i'm attempting to wrap up my design on a drilled shaft in an sdc d design area.
its a 48" diameter shaft pretty much everything is done aside from fishing through ch 21...i'm finding this section a little difficult to interpret so i want to make sure i'm doing it correctly.
for background
shaft is 48" diameter
ties are 60 ksi #4
verts are 60 ksi #10
so eqn 21-3 specifies:
ps = 0.12*f'c/fyt
ps = 0.008
ps is defined as the ratio of volume of spiral reinforcement to total volume of core confined by the spiral (21.6.4.4 a calls for ties or spirals so still applies it seems)
so area confined is:
48-3-3 = 42
a = 1385
ps = 0.008
steel area req'd = 11.08 in^2
so i'm confused as to what this number means...since the section cut is taken from the top is it a comparison to the area from a section cut from the top?
so with a #4...
c=pi*d=3.14*42=131.88
diameter #4 = 0.2
ps = 0.2 inches * 131.88 inches = 26.38 in^2 > 11.08 in^2
so should be ok...but the intent of this check is confusing to me, i mean if you had a really big shaft it would probably make #3 ties difficult.
does that seem correct or am i missing something?
boy its almost funny, according to my calcs i have enough strength in the concrete to not even need ties (for design anyway, construction would require them) and by the time ch 21 is through with it i'm going to have ties as tight as 4.5" oc...
thanks folks
will

so i think i took another step in figuring this out...
its a volumetric comparison so you have to assume something for the length of the shaft as well...so cut and pasting from above...
a = 1385
v = 8312.4
ps = 0.008
steel area req'd = 66.5
so with a #4...
c=pi*d=3.14*42=131.88
area #4 = 0.2
ps = 0.2 inches^2 * 131.88 inches = 26.38 << 66.5 hence ng
at 4" oc and #5 ties...
so with a #5...
c=pi*d=3.14*42=131.88
area #5 = 0.307
ps = 0.307 inches^2 * 131.88 inches = 40.5 << 66.5 ng
so something is wrong here i think, #5 bar is the max size possible for this bend i think...
i'm still trying to work through this, there is a fella upstair who i'm going to talk with but if anyone can spot whats going wrong here i'd appreciate it.

wbuw,
i don't have time to get into your issue right now but you might want to check out aci section 1.1.5.  drilled piers typically don't fall under aci 318 requirements.  there's a separate aci code for piers...can't re  
jae, much appreciate the reply.  
it is an oddball sort of thing since seems to me its about 90% geotechnical and 10% structrual.
i think you are probably referring to aci 336.3r-93, i grabbed a copy of that a while ago.  its somewhat helpful but more of a pointer than a guide.  
under 336.3r-93 3.3 it calls for use of 318 ch7 and 10...not specifically 21, but if you check r21.12.1 "...assigned to sdc d, e or f were added to the 1999 code.  they represent a consensus of a minimum level of good practice in designing and detailing concrete foundations including piles, drilled piers and caissons."
so while i think its greatly overly conservative it does appear to be part of the recommendations.
i've also been using fhwa-if-99-025 as a general guide.  i'm not doing highway work but the text is well written.
regardless i appreciate the reply, let me know if i missed something.
thanks,
will
will,
i'm headed out of the office right now but see attached.  the ties have to go 5x the diameter not 3x as my calc indicates.  also, if you use spiral ties, i believe the code allows you to reduce the ps number to 1/2 of what this calculation shows.
hope it helps
chip
that take care of it for you will?
chip

hi chip,
i think so....it looks similar to the numbers i'd run, only a little slicker where you solved for as directly where i was doing iteration type stuff.
seems to confirm the requirement, pretty darn tight spacing!
thanks really appreciate the reply, helps me feel better about the end result.
thanks
will
yep, pretty tight.  as i said, if you use spiral ties, you can reduce ps by 50%.  read section 1810.1.2.2 in ibc 2006 for clarification.