Circular footing with 5 feet diameter carrying 100kips what

Circular footing with 5 feet diameter carrying 100kips, what is the average stress increase at point 5 feet below the bottom of the footing?

Given load on fooring=100 kips

diameter of footing=5 feet

radius=a=5/2=2.5 feet

Uniform pressure on fooring=100/(pi*4²/4)=7.96 kip/ft²

We can use Charts to determine A and B to determine the stress increase at 5 feet below footing

z/a=5/2.5=2

r/a=0

A=0.106

B=0.179

vertical stress increase at 5 feet below=7.96*(A+B)=7.96*(0.106+0.179)=2.269 kip/ft²