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RE: UBC 1808.2.2

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Mark:

Totally off the cuff and perhaps way off base ( I think about these things from time to time when designing tract housing.)  This provision dates back to the 1927 UBC when the value was 20 percent of concrete strength.  Later it was changed to .225 f’c.  Finally changed to .33 in 1970 code.  Likely a response to the fact that it was impracticable to consider the degree to which soil provided lateral support to a column (pier) so everyone then assumed no buckling and kept stresses low.  If you assume end bearing and a full unbraced pier length, maybe increase cover to 4 inches, then you may be justified in going to higher column stress.  The provision makes little sense in light of the LRFD design method used just about everywhere else in the code.  My opinion is that the provision is a leftover from another era in engineering practice that no one has cleaned out.  Likely anyone who wrote it is or was involved with it has packed up his slide rule and moved on.  Hopefully someone who designs real foundations will pipe in.  (At least that is what ACI told me when I asked them about using ACI 318 for house foundations.  "No, the 318 code is for REAL buildings.")

George Richards, P. E.

-----Original Message-----
From: Mark Baker [mailto:shake4bake(--nospam--at)earthlink.net]
Sent: Monday, February 04, 2002 4:28 PM
To: seaint(--nospam--at)seaint.org
Subject: UBC 1808.2.2

I have posed this question before to the list, received some input, but am asking again.
 
UBC 1808.2.2 limits compressive stress of concrete to .33f'c and compressive stress of steel to .34fy for drilled, cast in place piles. I do not know from where this requirement comes but would surely like to know.
 
ACI 318-99, Chapter 21 also has requirements for drilled, cast in place piles, some similar to UBC although no limitation on compressive stresses in dcip piles.
 
I have a condition where I am stuck with the loads, pile diameters, pile quantities, compressive strength of concrete, etc.. My compressive stress is ranging from .4f'c to .76f'c for concrete (similar numbers for steel). Aside from this, pile design is adequate.
 
Question 1: Does anyone know why this limitation is in the code? The leading response to this in the past has been "it is a factor of safety for pile capacity due to unknown pile quality down a 50' deep hole".
 
Question 2: What could I do to the pile to compensate for not meeting the compr. stress limitation. Higher level of quality control during concreting....provide transverse reinf. full length of pile....something.....anything....
 
Question 3: Is there anyone out there on a code writing committee who would have insight to this issue?
 
Thanks,
 
Mark