Minimum Pier/Pedestal Reinforcing (ACI)

[buck1017]

Hello all,
Since I started working at this company 11 years ago, my boss always stressed to us that concrete columns need 0.01Ag minimum steel and pedestals/piers need 0.005Ag minimum steel. This engineer is no longer with the company and our new boss is a little less "old school" and continues to look things up and continue learning instead of being stuck in his ways. That being said, we recently discovered that the 0.005Ag does not appear to be for the reinforcing of the pier at all, but rather only at the interface between the pier and the footing, i.e. the dowels, and that we still need to be including 0.01Ag even for piers. So for example, a 24x24 pier needs (8) #8 vertical bars to meet 0.01Ag, but only needs (8) #6 vertical dowels to meet 0.005Ag at the interface. Can anyone please confirm that we are in fact reading the code correctly here and that 0.005Ag is not enough even for piers. I have gone through a few threads and it seems to me that there are a lot of engineers on here that use the 0.005Ag for reinforcing in the pier even though I don't think the code technically allows it.

Thank you,
Brandon

 

[Rabbit12]

Take a look at section 10.3.1.2 in ACI318-14. If your pier is bigger than it needs to be to fit anchor rods or some other reason you can reduce your reinforcing to 0.005Ag.

 

[WinelandV]

Buck1017,

In ACI318-08 (not the current code, I know), 10.8.4 allows the steel area to be calculated off of a reduced Ag "not less than one-half the total area".

I'm not sure what provision that translates to in the ACI318-14. However, if you can show that half of your provided pedestal area will carry the compressive load, then you can base minimum steel off of the halved area - effectively giving you 0.005Ag. At least, that's how I interpret it.

 

[buck1017]

You are both pointing to the same provision, just in different codes. This helps. Thank you. As a follow-up, so if I have a pier that is 30x30 (900 sq in) because it has a steel column on it and I need that much area for the anchors, I can design an imaginary pier that is Ag_new = 450 sq in, or 21.2x21.2 and reinforce it as much as I need to and whatever reinforcing I come up with, that's what I use in the larger pier? I presume that I still then need to include a minimum of 0.01Ag_new(which is .005Ag_original) in the new imaginary pier?

 

[JLNJ]

Sometimes I justify to myself that if I could make the pier "hollow" given that my loads are small, I will use less reinforcement because my fictitious hollow pier has a reduced cross-sectional area. The is not so much for columns and the like, but for large piers carrying large equipment where the cross-sectional area is a function of equipment size and not required pier strength.