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Steel reinforcing in cast-in-place concrete pile foundations

PostFrameSE (Structural) (OP)
20 May 09 9:42
I read with great interest thread256-192290: Drilled Pier Minimum Steel as I had similar questions.  My question will go a step further.  

If this "foundation" is going to extend above grade any amount, does the portion above grade need to be considered a "column" and use a minimum steel/concrete ratio of .01 rather than .0025 or .005 as used for the portion in the ground?  If indeed there is a transition, at what point does that transistion occur?  If I'm 24" above grade, do I need to use a minimum of .01?  That would seem excessive.  I'm working on one that is 8'-6" above grade.  I could see that being considered a column.

Lastly, in that referenced post, DaveAtkins reported using .005 for his reinforcing ratio because of the oversized concrete section.  Are there limitations on height above grade that I could use that same logic?

Thanks.   


Thanks.
kslee1000 (Civil/Environmental)
20 May 09 10:06
If you design the pile upper end as fixed support for structures above, then you would need to design it as beam column, and reinforced accordingly. The transition occurs at where the rotational resistance is no longer required. Otherwise, previous recommendation stays.
hokie66 (Structural)
22 May 09 7:36
Dave's suggestion that you can use as little as .5% reinforcement (if the column is oversized enough) applies to any column.  However, you still have to provide bending reinforcement as required.  
PostFrameSE (Structural) (OP)
22 May 09 8:23
Thanks to both of you for your responses.  The structures I design are very light single story buildings but can often times have very high laterally loaded beam-columns.  For that reason, my concrete piers are sized for lateral soil bearing rather than axial or bending forces in it and of course are bigger than they need to be.  

Does it make sense, for simplicity sake, to look at my bending reinforcement as though I had a rectangular shaped pier (one that would fit inside of my circular pier) and design it in both directions?  At that point, I can see whether or not I have 50% more concrete than is really required and then lower my reinforcement ratio.

Lastly, I haven't seen that provision in the ACI about lowering the reinforcement ratio.  Am I missing something, or is this just a general rule that people have come up with because theoretically it makes sense?  I see ACI 318-05 Section 10.9.1 speak of ratios between 1% - 8%.  I don't see exceptions.

Thanks.
kslee1000 (Civil/Environmental)
22 May 09 9:43
For your last question, if I remember correctly, the reduction is for "Architecture Columns", and "Pedestals", usually do not deal with significant lateral load. For considerable lateral load, I suggest to design it as beam-columns, and reinforce accordingly.
BAretired (Structural)
22 May 09 10:25
Article 10.10.5 of CSA A23.3-94 states:

Quote:

Columns with rho smaller than 0.01 but larger than 0.005 may be used provided the factored axial and moment resistances are multiplied by the ratio pho/0.01.

rho = area of steel/gross area of concrete.

BA

asixth (Structural)
25 May 09 8:35
Depending on what code you are using, in Australia we are required to provide 0.005 longitudinal steel for cast-in-place piles and 0.01 for any portion of the pile which protrudes above ground.

I was recently pulled up by my local department of transportion for provided cast-in-place piles for a bridge pier of a overpass with 0.005 longitudinal steel. Their claim was that a cast-in-place pile must be designed as a column to the concrete code and must satisfy the 0.01 minimum reinforcement. I had no hesitation in providing the additional steel.  
JAE (Structural)
25 May 09 11:07
PostFrameSE,

What hokie66 suggested is correct and the need for "column" design will kick in when the pier (I call them piers) extends up near the ground surface.  ACI 318 excluedes drilled piers from that code.  

The column design provisions within 318 would apply to a column which wouldn't have the continuous constraint provided by the surrounding earth - thus the lateral ties are less in drilled piers as is the vertical reinforcement since buckling of the pier is impossible in the ground.

Near the surface (perhaps the top 4 feet and up, I'd go with column design provision - and yes, a reduced amount of steel under the 1% limit is acceptable.
hokie66 (Structural)
25 May 09 17:41
asixth,

There is also the provision in Paragraph 10.7.1(a).  If the column is larger than it needs to be for structural purposes, the steel only has to be capable of taking 15% of the compression force.  This sometimes allows the use of less than 1% steel.

You were right not to argue about the bridge piles.
dik (Structural)
25 May 09 17:59
for piles totally below grade, for 16" dia. I generally use a single 15M (#5) x 19'8" long (cut from 18m bars).

for 24" dia, I generally use 6-15M x 19'8" long with 10M (slightly >#3) ties at 36" o/c and 3 ties @6" at the top.

Dik

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