Unreinforced Concrete footing

[Tdog67]

Fellers,

can someone point me in the right direction for info on designing unreinforced concrete footins for a steel column load?

thanks

Tracy

 

[Lion06]

size the footing like you would a reinforced. Then just check the shear like you would a reinforced footing since they are typically unreinforced for shear. The check the bending moment in the footing, just like you would to design the reinforcing, only make sure that the factored moment, Mu, is less than phiMn per chapter 22 of ACI 318 (at least it's chapter 22 in ACI 318-05).
Note, phi for plain concrete is 0.65.

 

[Tdog67]

Is the formula still Mn=5 (f'c)**1/2 S ?

thanks!

 

[youngstructural]

I don't know the US codes well, and I won't guess that you're using one...

I'd size my thickness based on a 45degree spread of the load from the plate through the footing, and I'd check the bending as for mass concrete, with root Fc' (square root of my strength of concrete) being my limiting strength for the tension capacity of concrete.

I think I've got a (very) old paper on this from the family library; I'll see if I can dig one up.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[spats]

ACI318 Chapter 22 tells you how to design plain concrete footings. Basically you do an M/S, and the overall design thickness is taken as 2" less than actual thickness, since it's cast against soil. Shear can be checked, but should basically never control.

StructuralEIT says to use phi=0.65 for plain concrete. Not sure if that is right. In ACI318-02, it's 0.55. In ACI318-08 it's 0.6. I don't have ACI318-05, so he may be right for -05.

 

[Lion06]

Yes, that is the correct formula, however, the phi factor is actually 0.55, not 0.65.