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RE: Sound barrier foundation

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> I am designing a sound barrier, which consists of precast
> concrete panels spanning horizontally between steel columns
> @ 12'-6". The columns are founded on 30" diam x 12'-0" deep
> cast-in-place caissons. Of course the vertical load is almost
> zero, but there is a moment and a horizontal force applied at
> top of the footing. The question is about the minimum
> reinforcement allowed for this type of foundation. Is the
> usual Rho = 0.01 also applicable in this case, or is it
> possible to use a lower value considering the size of the
> caissons? Thanks in advance.

ACI 318 (I'm assuming that's where you got the 0.01 value) does not govern
the design of caissons or other deep foundations per section 1.1.5, unless
you're in a high-seismic area.

But if you want to use 318 as a guide, the sections on beams would control
since you don't have much axial load.  The orientation of the member doesn't
matter.  Therefore you'll be limited by rho > 0.0033 * Ac or 4/3 of the
steel required by analysis.  The 0.0033*Ac equation will probably not save
you any steel because you can only count steel on the tension side, but the
4/3 could help quite a bit.  

On a different track, my deep foundation stuff is at home, but I seem to
remember a lower limit of rho > 0.005*Ac for drilled shafts with light axial

Jason Kilgore
Leigh & O'Kane, LLC
Kansas City, Missouri

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