Spiral Reinforcing in Seismic Pile

[WAstruc10]

I have a fixed pier in a high seismic zone, for which the geotech has specified 16" square prestressed piles. For the spiral reinforcing, I need 2" cover (marine environment) so I am left with a core diameter (Dc) of 12". Works fine with equation 21-2 (ps=0.12f'c/fyt), which calculates to #3 spiral at 3" pitch. But then ACI states "shall not be less than req'd by eq. 10-5"...and that equation seems to give huge unrealistic values for the spiral reinforcing - #5 spiral at 1.81" pitch! Seems like this is the case due to the ratio of Ag to Ach being much larger than it would be with square ties in a square column, but 16" square piles with spiral cage are very common, no? Am I missing something, or does anyone know how this is commonly handled? Much thanks...

 

[miecz]

You must be missing something. #5 should calculate at 3.25" pitch. You would then have A_c=132 in[²], A_g=256 in[²], volume of spiral reinf. = 12 in[³], and a ratio of .028. vs. a required ratio of 0.021. Problem is, you may not be able to get #5 bent to 13 inches. Try #4 at 2" pitch.

 

[WAstruc10]

Not sure I follow that logic. My core diameter can only be 12" (16"-2"-2"), which gives Ac=113.1 in^2. So equation 10-5 gives rho_s = 0.45((256/113.1)-1)(6/60) = 0.057. And then I'm using pitch = (4*Asp)/(rho_s*Dc) = (4*0.31)/(0.057*12) = 1.81". Can you clarify where this might be going wrong? Thanks,

 

[miecz]

I was using f'c of 4000 psi, and a 2" clear spacing to the main reinforcing. At 6000 psi and 2" clear to the spirals, you're correct. If you have ACI SP17A, Table Reinforcement 7.2 shows required pitch for various conditions.

 

[miecz]

Formula 10-5 is quite sensitive to concrete strength and cover to the spirals. Is it possible to lower the concrete strength? Also, you referenced ACI318 for the reinforcing ratio, but I don't think the 2 inch cover comes from 318. Does the code that specifies the 2 inch cover to the spirals have the same reinforcing ratio provision? Is that cover because you are designing to ACI357 and are in a splash zone subject to salt spray?

 

[civilperson]

Ach is area to outside edges of transverse reinforcement, 16" minus two covers of 2" is 12"<2=144 sq. in.
Using 4000 psi for concrete and 60 ksi for spiral ties gives 0.0233 for ratio of spiral steel volume to total volume of the core, (12" x 12"x 12" length). Thus 40.3 cubic inches of spiral steel. 201.6 inches of #4 spiral length in 12" of column length at pitch of 2.93 in.