concrete tank design tables

[stjoeengineer]

(I also posted this in the Civil/Environmental, but wasn't sure where I would get a response.)
I am designing a rectangular concrete tank using the pca design guide. I would like a little more explanation on the design table in the appendix. If I go in with a certain Mu/Phi(f'c)bd2, which sigma do column do I use to find omega? Do I assume a sigma first? It appears that their examples use a sigma of 0.001.

 

[stjoeengineer]

I realized after I posted that I confused sigma with rho. I was referring to the steel ratio. Sorry for the confusion. Also, sorry about posting twice. I've never used this forum before.
thanks

 

[minorchord2000]

There is a good explanation at the bottom of Appendix A to which you are referring.

If you are designing, enter the table with Mu/(phi*f'c*b*d**2); find omega and compute the steel percentage required, rho, from rho=omega*f/c/fy

If you are investigating a design, enter the table with omega; find value of Mn/(f'c*b*d**2) and solve for the nominal strength Mn.

 

[stjoeengineer]

When finding omega, I enter with the value on the left side of the chart. As I travel right across the columns, how do I know which column to pull omega from (i.e. .000, .001, .002, etc.)? It appears the examples use .001, but I am not sure if that is always typical or not.

 

[minorchord2000]

The left to right values across the top are further refinements to the vertical columns. The left to right values allow you to go to additional accuracy. The chart does not explain that at all but if you look at it you will see what I mean.