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cuels (Civil/Environmental)
7 Jul 10 21:12
I have a question about the difference between (concrete) piers, pedestals, and short columns.  I have a colleague that wants a foundation pedestal and/or a shallow pier to be designed as a short column (large reinforcement ratio).  I don't know whether I am right, but I think that there is a difference.  I tried to explain that a short column is intended for a structural component of building framing (not the foundation).  I want to know whether I am right in my thinking or whether I have been steered wrong in my education.  

We are constantly designing square footings with pedestals, shallow piers (sonotube cast 4'-5' deep).  

Any comments?
slickdeals (Structural)
7 Jul 10 21:23
The pedestal could probably be designed for a 0.05% reinforcement ratio.

We are Virginia Tech
Go HOKIES

sdz (Structural)
8 Jul 10 3:31
I think a pedestal acts as a cantilever column. Then according to AS3600 Sect 10.3 it can be considered short if the height H is H<=3D for a square pedestal, or H<=2.5D for a circular pedestal. Also if the axial force N*< 0.1 f'c Ag then it can be designed for bending only. (Precise figures may vary according to your code).

Otherwise it should be designed as a column.
asixth (Structural)
8 Jul 10 4:23
I think pedestals should be designed by strut tie. Detailing-wise, I detail pedestals in the same manner that I would detail a column, the last one was 400x600 (16"x24") with 12-12mm diameter bars (No.4's) which is a reinforcement ratio of 0.005%. The longitudinal reinforcement was tied with 10mm bars at 300crs. The design loads where quite low, it was supporting a steel structure so N*<0.1*f'c*Ag. The shear forces applied to the top of the pedestal where present but not a substantial force.
ToadJones (Structural)
8 Jul 10 9:01
Guess it all depends on loads....
Axial, moment, & shear.
I've had "piers" that had very high moments, shears and axial and all combination's thereof. I usually even consider shear at the base of the column as bending at the base of the pier where it ties into footing even though that is probably conservative because of lateral soil pressures.
Basically what I am getting at is I design the pier for whatever part(s) of the code I believe it falls under, make sense?
Lion06 (Structural)
8 Jul 10 20:34
Why would a pier not be a short column?  What else would it be?
ToadJones (Structural)
9 Jul 10 8:50
EIT-
I agree, but sometimes loading can dictate that they act more like a cantilever beam...albeit rarely.  
Lion06 (Structural)
9 Jul 10 9:05
Toad-

Agreed.  I guess I was missing the point of the OP.

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