PCA Rentangular Tanks Design - Question about Example(SIMPLE)

[KOJUNGMO]

I have a question about the PCA Rentangular Tank Example.

In the First Example

Design Information
Dimension : 30ft X 20ft X 10 ft
Wall thickness : 18 in.

There is Shear Strength (Vc)
Vc = 2 X [fc][/0.5] X b X d -> I belive that it has came from ACi 350-06 11.3.1.1
d = effective depth = thickness wall - cover - db/2 = 18 - 2 - 5/16 = 15.7 in.
Vc = 2 X [4000][/0.5] X 12 X 15.7

What I'd like to ask you is "12". Where does this value(12) come from???

Thank you for reading this.

 

[CELinOttawa]

I'd say they are doing the calculation for a one foot strip.

 

[KOJUNGMO]

CELinOttawa
Ah!!! Thanks! I'm not quite used to use US unit. So sorry for the silly question.

 

[CELinOttawa]

To paraphrase the wise: There are no silly questions, there are only unasked technical questions which then morph into legal ones...