Solution to Problem 138 Pressure Vessel

Problem 138
The strength of longitudinal joint in Fig. 1-17 is 33 kips/ft, whereas for the girth is 16 kips/ft. Calculate the maximum diameter of the cylinder tank if the internal pressure is 150 psi.
 

138-construction-joints-cylindrical-tank.gif

 

Solution 138

 

Why do you need to assume 1ft of length when the length is left out from the formula? Also, can I ask if the "ft" from the "kips/ft" the thickness of the thin-walled vessel? I'm only assuming from the formula.